This document was developed for electrical technical personnel of electrical laboratories, electrical technical sections of industrial facilities, carrying out work to measure the insulation resistance of electrical equipment, wires and cables in existing and reconstructed electrical installations for all electricity consumers, regardless of their departmental affiliation.
NORMATIVE REFERENCES
This document uses references to the following regulatory documents:
Rules for the technical operation of consumer electrical installations, 1992;
Safety regulations for the operation of consumer electrical installations, 1994;
Electrical Installation Regulations 1986;
Standards for testing electrical equipment and devices of consumer electrical installations, 1982;
Electrical Test Code 1978;
GOST 26567-85. Semiconductor power converters. Test methods;
GOST 3345-76. Cables, wires and cords. Method for determining electrical insulation resistance;
GOST 3484-88. Power transformers. Electromagnetic testing methods;
GOST 3484.3-83. Power transformers. Methods for measuring dielectric parameters of insulation.
DEFINITIONS
3.1. This methodology uses the terms established in GOST 3345-76, GOST 3484.3-83, GOST 3484.1-88, GOST 16504, GOST 23875.
- Switchgear - a switchgear of the generator voltage of a power plant or the secondary voltage of a step-down substation of a district (enterprise) to which the networks of the district (enterprise) are connected.
- Designations and abbreviations:
HV - high voltage windings;
MV - medium voltage windings;
LV - low voltage windings;
NN1, NN2 - low voltage windings of transformers with split windings;
R15 - fifteen second value of insulation resistance in MOhm;
R60 - one-minute insulation resistance value in MOhm;
PEEP - operating rules for consumer electrical installations;
PTBEEP - safety regulations for the operation of consumer electrical installations;
PUE - Rules for electrical installations.
4. MEASUREMENT PROCEDURE
- Measurable Indicators
Insulation resistance is measured with megohm meters (100-2500V) with measured values in Ohm, kOhm and MOhm.
- Measuring instruments
Insulation measuring instruments include megohmmeters: ESO 202, F4100, M4100/1-M4100/5, M4107/1, M4107/2, F4101. F4102/1, F4102/2, BM200/G and others, produced by domestic and foreign companies.
4.3 Qualification requirements
- Trained electrical personnel who have a certificate of knowledge testing and a qualification group for electrical safety of at least 3rd, when performing measurements in installations up to 1000 V, and not lower than 4th, when measuring in installations above 1000 V, are allowed to perform insulation resistance measurements.
- Persons from electrical engineering personnel with secondary or higher specialized education may be allowed to process measurement results.
- Analysis of measurement results should be carried out by personnel involved in the insulation of electrical equipment, cables and wires.
5. SAFETY REQUIREMENTS
- When performing insulation resistance measurements, safety requirements must be met in accordance with GOST 12.3.019.80, GOST 12.2.007-75, Rules for the operation of consumer electrical installations and Safety rules for the operation of consumer electrical installations.
- The premises used for measuring insulation must meet the explosion and fire safety requirements in accordance with GOST 12.01.004-91.
- Measuring instruments must meet the safety requirements in accordance with GOST 2226182.
- Megger measurements may only be carried out by trained electrical personnel. In installations with voltages above 1000 V, measurements are carried out by two persons at a time, one of whom must have electrical safety ratings of at least IV. Carrying out measurements during installation or repair is specified in the work order in the line “Entrusted”. In installations with voltages up to 1000 V, measurements are carried out by order of two persons, one of whom must have a group of at least III. An exception is the tests specified in clause BZ.7.20.
- Measuring the insulation of a line that can receive voltage from both sides is permitted only if a message is received from the responsible person of the electrical installation that is connected to the other end of this line by telephone, by messenger, etc. (with a reverse check) that the line disconnectors and switch are turned off and a poster “Do not turn on. People are working” is posted.
- Before starting tests, it is necessary to make sure that there are no people working on that part of the electrical installation to which the test device is connected, to prohibit persons located near it from touching live parts and, if necessary, to set up security.
- To monitor the insulation condition of electrical machines in accordance with methodological instructions or programs, measurements with a megger on a stopped or rotating, but not excited machine can be carried out by operational personnel or, by their order, in the order of routine operation by electrical laboratory workers. Under the supervision of operating personnel, these measurements can also be performed by maintenance personnel. Insulation tests of rotors, armatures and excitation circuits can be carried out by one person with an electrical safety group of at least III, stator insulation tests - by at least two persons, one of whom must have a group of at least IV, and the second - not lower than III.
- When working with a megger, touching the live parts to which it is connected is prohibited. After completion of work, it is necessary to remove the residual charge from the equipment being tested by briefly grounding it. The person removing the residual charge must wear dielectric gloves and stand on an insulated base.
- It is prohibited to take measurements with a megger: on one circuit of double-circuit lines with a voltage above 1000 V, while the other circuit is energized; on a single-circuit line, if it runs in parallel with a working line with a voltage above 1000 V; during a thunderstorm or when it is approaching.
- Measuring the insulation resistance with a megger is carried out on disconnected current-carrying parts from which the charge has been removed by first grounding them. Grounding from live parts should be removed only after connecting the megger. When removing grounding, you must use dielectric gloves.
6. CONDITIONS FOR PERFORMING MEASUREMENTS
- Insulation measurements must be carried out under normal climatic conditions in accordance with GOST 15150-85 and under normal power supply conditions or as specified in the manufacturer's data sheet - technical description for megohmmeters.
- The value of the electrical insulation resistance of the connecting wires of the measuring circuit must exceed at least 20 times the minimum permissible value of the electrical insulation resistance of the product under test.
- The measurement is carried out indoors at a temperature of 25±10 °C and a relative air humidity of no more than 80%, unless other conditions are provided for in the standards or technical specifications for cables, wires, cords and equipment.
- PREPARATION FOR MEASUREMENTS
- In preparation for performing insulation resistance measurements, the following operations are carried out:
- They check the climatic conditions at the place where the insulation resistance is measured with the measurement of temperature and humidity and the compliance of the room with explosion and fire hazards to select a megger for the appropriate conditions.
- The condition of the selected megohmmeter, connecting conductors, and the operability of the megohmmeter are checked by external inspection in accordance with the technical description for the megohmmeter.
- Check the validity period of the state verification on the megohmmeter.
- The preparation of measurements of cable and wire samples is carried out in accordance with GOST 3345-76.
- When performing periodic preventive work in electrical installations, as well as when performing work on reconstructed facilities in electrical installations, the preparation of the workplace is carried out by the electrical technical personnel of the enterprise where the work is performed in accordance with the rules of PTBEEEP and PEEP.
- TAKING MEASUREMENTS
- The reading of the electrical insulation resistance values during measurement is carried out after 1 minute from the moment the measuring voltage is applied to the sample, but not more than 5 minutes, unless other requirements are provided for in the standards or technical conditions for specific cable products or other equipment being measured.
Before re-measurement, all metal elements of the cable product must be grounded for at least 2 minutes.
- The electrical insulation resistance of individual cores of single-core cables, wires and cords must be measured:
for products without a metal sheath, screen and armor - between the conductor and the metal rod or between the conductor and grounding;
for products with a metal shell, screen and armor - between the conductive conductor and the metal shell or screen, or armor.
- The electrical insulation resistance of multi-core cables, wires and cords must be measured:
for products without a metal sheath, screen and armor - between each current-carrying conductor and the remaining conductors connected to each other or between each conductive conductor; residential and other conductors connected to each other and grounding;
for products with a metal shell, screen and armor - between each current-carrying conductor and the remaining conductors connected to each other and to the metal shell or screen, or armor.
- If the insulation resistance of cables, wires and cords is lower than the normative rules of PUE, PEEP, GOST, it is necessary to perform repeated measurements by disconnecting the cables, wires and cords from the consumer terminals and separating the current-carrying conductors.
- When measuring the insulation resistance of individual samples of cables, wires and cords, they must be selected for construction lengths, wound on drums or in coils, or samples with a length of at least 10 m, excluding the length of end cuts, if in the standards or technical specifications for cables, wires and cords are not specified in other lengths. The number of construction lengths and samples for measurement must be specified in the standards or technical specifications for cables, wires and cords.
9. CONVERTER ISOLATION MEASUREMENT
9.1. Measurement of electrical resistance and insulation of converters is carried out in accordance with the requirements of this standard, and when exposed to climatic factors, measurement of insulation resistance is carried out taking into account GOST/16962-71.
Measuring instruments: megohmmeters and ohmmeters in accordance with GOST 16862-71. Electrical insulation resistance is measured:
in normal climatic conditions; at the upper value of the ambient temperature after thermal equilibrium has been established in the converter;
at the upper value of relative humidity.
Insulation resistance is measured between electrically unconnected circuits;
electrical circuits and housing. In the specifications or design documentation for converters of specific series and types, the terminals between which the resistance should be measured and the value of the direct voltage at which this measurement is carried out are indicated. If one of the terminals or elements according to the circuit is connected to the housing, then this circuit must be disconnected for the duration of the tests.
When measuring the insulation resistance of converters, the following conditions must be met:
Table 1.
before testing, the converter must be disconnected from external power supplies and load;
the input (output) terminals of the converter, capacitors connected to power circuits, as well as anode, cathode and control terminals of power semiconductor devices must be connected to each other or shunted;
contacts of switching equipment of power circuits must be closed or bypassed;
electrical circuits containing semiconductor devices and microcircuits must be disconnected and, if necessary, tested separately;
The voltage of the measuring device when measuring insulation resistance, depending on the nominal (amplitude) value of the circuit voltage, is selected according to the table. 1.
If necessary, the insulation resistance is measured at higher voltages, but not exceeding the test voltage of the circuit.
Measuring the insulation resistance of converters consisting of several cabinets can be carried out separately for each cabinet.
If the insulation resistance of each cabinet and (or) structural unit of the converter is measured, then the value of the insulation resistance of each cabinet and (or) structural unit must be indicated in the specifications for converters of specific series and types.
The values of the minimum permissible insulation resistance for power cables, switches, load switches, disconnectors, valve arresters, dry-type reactors, instrument transformers, 6-10 kV indoor switchgear, AC motors, stationary, mobile and complete testing devices are given in Table. 2.
10. PROCESSING OF MEASUREMENT RESULTS
10.1. If the measurement for cable products was carried out at a temperature different from 20 °C, and the value of electrical insulation resistance required by standards or technical specifications for specific cable products is normalized at a temperature of 20 °C, then the measured value of electrical insulation resistance is recalculated to a temperature of 20 °C according to formula:
R20=KRt,
Rt - electrical insulation resistance at measurement temperature, MOhm;
K is the coefficient for bringing the electrical insulation resistance to a temperature of 20 °C, the values of which are given in the appendix to this standard.
In the absence of conversion factors, the arbitration method is to measure the electrical resistance of the insulation at a temperature of (20±1)°C.
10.2. The recalculation of the electrical insulation resistance R for a length of 1 km should be carried out according to the formula:
R=R20L,
where R20 is the electrical insulation resistance at a temperature of 20 °C, MOhm;
L is the length of the tested product without taking into account the end sections, km.
Coefficient K for reducing the electrical resistance of insulation to a temperature of 20 °C.
The error in the insulation resistance value is calculated according to the recommendations specified in the technical descriptions and operating instructions for megohmmeters, taking into account external influencing factors.
11. REGISTRATION OF MEASUREMENT RESULTS
The measurement results are included in test reports for cables up to and above 1000 V, as well as in protocols for preventive adjustment work on relay protection and electrical equipment.
Table 2.
Name of insulation resistance measurements |
Standardized value, Mohm, not less |
Megger voltage, V |
Directions |
Power cables above 1000 V |
Not standardized |
When testing with increased voltage, the insulation resistance R60 must be the same before and after the test |
|
Power cables up to 1000V |
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Oil switches: |
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1. Movable and guides |
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parts made of organic material. 3-10kV, |
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2. Secondary circuits, including |
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H.Load switches: measuring the insulation resistance of the closing and disconnecting coils |
The insulation resistance of the power section is not measured, but is tested with increased power frequency voltage |
||
4. Disconnectors, short circuiters and separators: |
Produced only at positive ambient temperatures |
||
1.Leash rods made |
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from organic materials |
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Element resistance measurement |
Arrester resistance or |
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above 3 kV and above |
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less than 3 kV |
at the manufacturer or previous measurements during operation |
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Dry reactors. Winding resistance measurement relative to |
After major renovation. |
||
In use |
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Instrument transformers |
Not standardized. |
When assessing the condition of the secondary windings, you can focus on the following average resistance values of a serviceable winding: for built-in CTs - 10 MOhm, |
|
primary windings, |
Not lower than 1 together with connected |
||
Switchgear 3-10 kV: primary circuits |
The measurement is performed at |
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AC electric motors |
Must be taken into account when drying is required. |
||
normalized |
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exchange stator. up to 660 V |
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The stator windings of the electric engines |
Produced at synchronous |
||
I don’t standardize |
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Rotor windings |
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Stationary, mobile, portable complete testing installations. |
Not standardized |
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Circuit insulation measurements and |
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Circuits and equipment for voltage |
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DC machines: |
Winding insulation resistance |
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measurement of insulation of windings and bands up to 500V, |
measured relative to the body, and bandages - relative to the body and |
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windings held by it together with the circuits and cables connected to them |
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Power and lighting wiring |
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Distribution devices, |
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Secondary control circuits, |
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Each connection of secondary |
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Control, protection, automation, telemechanics, excitation circuits |
Circuit insulation resistance |
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Based on the article "Measurement of insulation resistance (IR) - 2", http://electrical-engineering-portal.com
1. Insulation resistance values for electrical equipment and systems
(PEARL/NETA MTS-1997 Standard Table 10.1)
|
Rated maximum equipment voltage |
Megger class |
|
1 MΩ Rule for Equipment Insulation Resistance Value
Depending on the rated voltage of the equipment:
< 1 кВ = не менее 1 МОм
> 1 kV = 1 MΩ per 1 kV
According to IE Rules - 1956
When 1000 V is present between each live conductor and earth for one minute, the insulation resistance of high voltage installations shall be not less than 1 MΩ or as specified by the Bureau of Indian Standards. Medium Voltage and Low Voltage Installations - If 500 V is present between each live conductor and earth for one minute, the insulation resistance of Medium Voltage and Low Voltage Installations shall be not less than 1 MΩ or as specified by the Bureau of Indian Standards. According to CBIP specifications, acceptable values are 2 MΩ per kV.
Medium Voltage and Low Voltage Installations - If 500 V is present between each live conductor and earth for one minute, the insulation resistance of Medium Voltage and Low Voltage Installations shall be not less than 1 MΩ or as specified by the Bureau of Indian Standards.
According to CBIP specifications, acceptable values are 2 MΩ per kV
2. Insulation resistance value for transformer
Insulation resistance testing is necessary to determine the insulation resistance of individual windings to ground or between individual windings. In this type of testing, insulation resistance is usually either measured directly in MΩ or calculated from the applied voltage and the magnitude of the leakage current.
When measuring insulation resistance, it is recommended to always ground the frame (and core). Short-circuit each transformer winding to the bushing terminals. After this, measure the resistance between each winding and all other grounded windings.
Insulation resistance testing: between high voltage side and ground, and between high voltage side and low voltage side.
HV1 (2, 3) - Low voltage 1 (2, 3); LV1 (2, 3) - High voltage 1 (2, 3))
When measuring insulation resistance, never leave the transformer windings ungrounded. To measure the resistance of a grounded winding, it is necessary to remove solid grounding from it. If it is not possible to remove the ground, as is the case with some windings with solidly grounded neutrals, the insulation resistance of such a winding will not be measurable. Consider them part of the grounded section of the circuit.
Testing must be done between windings and between winding and ground (E). On three-phase transformers, it is necessary to test the winding (L1, L2, L3) minus the ground for transformers with a delta connection or the winding (L1, L2, L3) with ground (E) and neutral (N) for transformers with a star connection.
Insulation resistance value for transformer
Where C = 1.5 for oil-filled transformers with an oil tank, 30 for oil-filled transformers without an oil tank or for dry transformers.
Temperature correction factor (relative to 20°C)
Example for a three-phase transformer 1600 KVA, 20 kV / 400 V:
- insulation resistance value on high voltage side = (1.5 x 20000) / √1600 = 16000 / 40 = 750 MOhm at 20°C;
- insulation resistance value on low voltage side = (1.5 x 400) / √1600 = 320 / 40 = 15 MOhm at 20°C;
- insulation resistance value at 30°C = 15 x 1.98 = 29.7 MOhm.
Transformer winding insulation resistance
Transformer insulation resistance value
|
Voltage |
Test voltage (DC), low voltage side |
Test voltage (DC), high voltage side |
Minimum insulation resistance value |
|
6.6 kV - 11 kV |
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11 kV - 33 kV |
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33 kV - 66 kV |
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66 kV - 132 kV |
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|
132 kV - 220 kV |
Measuring the insulation resistance of a transformer:
- turn off the transformer and disconnect jumpers and lightning rods;
- discharge the interturn capacitance;
- completely clean all bushings;
- short-circuit the windings;
- Protect the terminals to prevent surface leakage across the terminal insulators;
- record the ambient temperature;
- connect test leads (avoid additional connections);
- Apply test voltage and record readings. The insulation resistance value 60 seconds after applying the test voltage is taken as the insulation resistance of the transformer at the testing temperature;
- The neutral terminal of the transformer must be disconnected from ground during testing;
- Also, during testing, all connections to the ground of the lightning rod on the low voltage side must be disconnected;
- due to the inductive characteristics of the transformer, insulation resistance readings must be taken only after the test current has stabilized;
- Do not take resistance readings while the transformer is under vacuum.
Transformer connections when testing insulation resistance (at least 200 MOhm)
Transformer with two windings
2. High voltage winding - (low voltage winding + ground)
3. Low voltage winding - (high voltage winding + ground)
Transformer with three windings
1. High voltage winding - (low voltage winding + tap winding + ground)
2. Low voltage winding - (high voltage winding + tap winding + ground)
3. (High voltage winding + low voltage winding + tap winding) - ground
4. Branch winding - (high voltage winding + low voltage winding + ground)
Autotransformer (two windings)
1. (High voltage winding + low voltage winding) - ground
Autotransformer (three windings)
1. (High voltage winding + low voltage winding) - (tap winding + ground)
2. (High voltage winding + low voltage winding + tap winding) - ground
3. Branch winding - (high voltage winding + low voltage winding + ground)
For any insulation, the measured insulation resistance should not be less than:
- high-voltage winding - ground 200 MOhm;
- low-voltage winding - ground 100 MOhm;
- high-voltage winding - low-voltage winding 200 MOhm.
Factors affecting the transformer insulation resistance value
The insulation resistance value of transformers is affected by the following:
- condition of the surface of the terminal bushing;
- oil quality;
- winding insulation quality;
- oil temperature;
- duration of use and test voltage value.
3. Insulation resistance value for output winding switch
- insulation resistance between high-voltage and low-voltage windings, as well as between windings and ground;
- The minimum resistance value for the output winding switch is 1000 ohms per volt of operating voltage.
An insulation tester is used to measure the grounded motor winding resistance (E).
- for rated voltages below 1 kV, measurement is carried out with a 500 V DC megger;
- for rated voltages above 1 kV, measurement is carried out with a 1000 V DC megger;
- In accordance with IEEE 43, Article 9.3, the following formula should be applied:
minimum insulation resistance value (for a rotating machine) = (Rated voltage (V) / 1000) +1.
In accordance with IEEE 43 1974, 2000
Example 1: For a three-phase electric motor 11 kV
- insulation resistance value = 11 + 1 = 12 MΩ, but according to IEEE43 should be 100 MΩ.
Example 2: For a 415V three-phase motor
- insulation resistance value = 0.415 + 1 = 1.41 MΩ, but according to IEEE43 should be 5 MΩ;
- as per IS 732 minimum insulation resistance value for electric motor = (20 x Voltage (p-p)) / (1000 + 2 x kW).
Motor insulation resistance value according to NETA ATS 2007 Section 7.15.1
|
Motor nameplate (B) |
Test voltage |
Minimum insulation resistance value |
|
500 V DC |
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1000 V DC |
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1000 V DC |
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1000 V DC |
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2500 V DC |
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2500 V DC |
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2500 V DC |
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|
5000 V DC |
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|
15000 V DC |
Submersible motor insulation resistance value
5. Insulation resistance value for electrical cables and wiring
Insulation testing requires disconnecting cables from the panel or equipment and from the power source. Wiring and cables should be tested relative to each other (phase to phase) with the ground cable (E). The IPCEA (Insulated Power Cable Engineers Association) offers a formula for determining the minimum insulation resistance values.
R = K x Log 10 (D/d)
R= Insulation resistance value in MOhm for 305 meters of cable
TO= Constant of the insulating material. (Electrical insulating lacquer fabric = 2460, thermoplastic polyethylene = 50000, composite polyethylene = 30000)
D= Outer diameter of conductor insulation for solid wire or cable (D = d + 2c + 2b diameter of solid cable)
d= Conductor diameter
c= Conductor insulation thickness
b= Thickness of insulating sheath
High voltage testing of new XLPE cable (according to ETSA standard)
11 kV and 33 kV cables between core and earth (according to ETSA standard
Measuring the insulation resistance value (between conductors (cross insulation))
- The first conductor to be measured for cross-insulation must be connected to the Line terminal of the megger. The other conductors are connected together (using alligator clips) and connected to the Earth terminal of the megger. At the other end, the conductors are not connected;
- then turn the knob or press the megger button. The meter display will show the insulation resistance between conductor 1 and the remaining conductors. Insulation resistance readings should be recorded;
- then connect another conductor to the Line terminal of the megger, and connect the other conductors to the ground terminal of the megger. Take a measurement.
Measuring insulation resistance value (insulation between conductor and ground)
- connect the conductor under test to the Line terminal of the megger;
- connect the Earth terminal of the megohmmeter to ground.;
- turn the knob or press the megohmmeter button. The meter display will show the insulation resistance of the conductors. After maintaining the test voltage for a minute until a stable reading is obtained, record the insulation resistance value.
Measured values:
- If, during periodic testing, the insulation resistance of an underground cable at the corresponding temperature is from 5 MΩ to 1 MΩ per kilometer, this cable must be included in the replacement program;
- if the measured insulation resistance of an underground cable at the corresponding temperature is from 1000 kOhm to 100 kOhm per kilometer, this cable should be replaced urgently, within a year;
- If the measured cable insulation resistance is less than 100 kOhm per kilometer, this cable should be replaced immediately as an emergency cable.
6. Insulation resistance value for transmission line/distribution line
7. Insulation resistance value for panel bus
Insulation resistance value for panel = 2 x rated voltage of panel in kV
For example, for a 5 kV panel the minimum insulation resistance is 2 x 5 = 10 MOhm.
8. Insulation resistance value for substation equipment
Typical resistance values for substation equipment are:
Typical insulation resistance value for substation equipment
|
Equipment |
Megger class |
Minimum insulation resistance value |
|
|
Circuit breaker |
(Phase - Earth) |
||
|
(Phase - Phase) |
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|
Control circuit |
|||
|
(Primary - Earth) |
|||
|
(Secondary - Phase) |
|||
|
Control circuit |
|||
|
Insulator |
(Phase - Earth) |
||
|
(Phase - Phase) |
|||
|
Control circuit |
|||
|
(Phase - Earth) |
|||
|
Electric motor |
(Phase - Earth) |
||
|
Switchgear LT |
(Phase - Earth) |
||
|
Transformer LT |
(Phase - Earth) |
||
Insulation resistance value of substation equipment in accordance with DEP standard:
|
Equipment |
Measurement |
Insulation resistance value at the time of commissioning (MOhm) |
Insulation resistance value at the time of service (MOhm) |
|
Switchgear |
High voltage bus |
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|
Low voltage bus |
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|
Low voltage wiring |
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|
Cable (minimum 100 meters) |
(10 x kV)/km |
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|
Electric motor and generator |
Phase - Earth |
||
|
Transformer immersed in oil |
High voltage and low voltage |
||
|
Transformer, dry type |
High voltage |
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|
Low voltage |
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|
Stationary equipment/tools |
Phase - Earth |
5 kOhm per volt |
1 kOhm per volt |
|
Removable equipment |
Phase - Earth |
||
|
Distribution equipment |
Phase - Earth |
||
|
Circuit breaker |
Power circuit |
2 MΩ per kV |
|
|
Control circuit |
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|
DC Circuit - Ground |
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|
Circuit LT - Ground |
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|
LT - DC circuit |
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9. Insulation resistance value for household/industrial wiring
Low resistance between phase and neutral conductors or between live conductors and earth will result in leakage current. This leads to deterioration of insulation, as well as energy losses, which will result in increased operating costs for the installed system.
At normal supply voltages, the phase-to-phase-neutral-to-earth resistance should never be less than 0.5 MΩ.
In addition to the leakage current due to the active resistance of the insulation, there is also a leakage current due to its reactance, since it acts as the dielectric of a capacitor. This current does not dissipate any energy and is not harmful, but we need to measure insulation resistance, so DC voltage is used to prevent reactance measurement from being included in the test.
Single phase wiring
Insulation resistance testing between phase-neutral and ground should be performed on the entire installation with the power switch turned off, with the line and neutral connected together, with lamps and other equipment disconnected but with circuit breakers closed, and with all circuit breakers closed.
If bidirectional switching is used, only one of the two wires will be tested. To test a different wire, you must operate both bi-directional switches and retest the system. If necessary, the installation can be tested as a whole, but then a value of at least 0.5 MΩ must be obtained.
Three-phase wiring
In the case of a very large installation with many parallel connections to ground, lower readings can be expected. In this case, it is necessary to repeat the testing after partitioning the system. Each of these parts must meet minimum requirements.
Insulation resistance testing should be performed between phase-phase-neutral-ground. The minimum acceptable value for each test is 0.5 MΩ.
Low Voltage Insulation Resistance Testing
Minimum insulation resistance value= 50 MOhm / number of electrical outlets (all electrical points with installation elements and plugs)
Minimum insulation resistance value= 100 MOhm / number of electrical outlets (all electrical outlets without installation elements and plugs)
Safety precautions when measuring insulation resistance
High test voltage can cause damage to electronic equipment such as electronic fluorescent lamp starters, touch switches, dimmer switches, and power controllers. Therefore, such equipment should be disconnected.
Capacitors and indicator or test lamps should also be disconnected because they may cause inaccurate test results.
If any equipment is disconnected for testing, it must be subject to its own insulation test using a voltage that will not damage them. The result must be as specified in the UK standard or be at least 0.5 MΩ if not specified in the standard.
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Method for measuring insulation resistance
The purpose of this methodology is to ensure high-quality and safe work during the electrical laboratory (hereinafter referred to as EL) tests (measurements).
The methodology is based on:
- GOST R 8.563-96 “Measurement methods”;
- interindustry rules on labor protection (safety rules) during the operation of electrical installations. POT R M-016-2001;
- documentation from manufacturers of instruments used in the work.
Purpose
The purpose of the methodology is to describe procedures for the organization, implementation and registration of work carried out by electric power plants to measure insulation resistance.
Name and characteristics of the measured quantity
The measured value is the insulation resistance. DC insulation resistance is the main indicator of the insulation condition and its measurement is an integral part of testing all types of electrical equipment and circuits.
Composition of instruments used in measurement
Insulation resistance is measured with a megger. Currently, the most common types of megohmmeters are M-4100, ESO202/2G, MIC-1000, MIC-2500.
Description of megohmmeters
A megohmmeter is a device consisting of a voltage source (a constant or alternating generator with a current rectifier) and a measuring mechanism.
Meggers are divided according to rated operating voltage up to 1000 V and up to 2500 V.
Megohm meters are equipped with flexible copper wires up to 2-3 m long with an insulation resistance of at least 100 MOhm. The ends of the wires connected to the megohmmeter must have terminations, and the opposite ends must have alligator clips with insulated handles.
Measurement procedure
The procedure for carrying out measurements with megohmmeters type M-4100 and ESO202/2G. Before starting measurements you must:
- Before starting the measurement, the megohmmeter must be subjected to a control test, which consists of checking the readings of the device with open wires (the arrow of the device should be at the infinity mark -?) and closed wires (the arrow of the device should be at the 0 mark).
- Make sure that there is no voltage on the cable under test (it is necessary to check the absence of voltage using a tested voltage indicator, the serviceability of which must be checked on parts of the electrical installation that are known to be under voltage - clause 3.3.1 of the “Inter-industry rules on labor protection” POT R M-016-2001) .
- Ground the current-carrying conductors of the cable being tested (grounding from live parts can only be removed after connecting the megger).
The connected megohmmeter wires must have clamps with insulated handles; in electrical installations above 1000 V, in addition, dielectric gloves should be used.
When working with a megger, touching the live parts to which it is connected is not allowed.
As a rule, the insulation resistance of each phase of the cable is measured relative to the remaining grounded phases. If measurements using this abbreviated version give an unsatisfactory result, then it is necessary to measure the insulation resistance between each two phases and each phase to ground.
When making measurements on cables above 1000 V (when the measurement results can be distorted by leakage points along the insulation surface), an electrode (screen rings) connected to terminal “E” (screen) is placed on the insulation of the measurement object (end funnel, etc.).
When measuring the insulation resistance of cables for voltages up to 1000 V with zero cores, remember the following:
- neutral working and protective conductors must have insulation equal to the insulation of phase conductors;
- Both on the power supply side and on the receiver side, the neutral conductors must be disconnected from the grounded parts.
Scheme for measuring insulation resistance: a - electric motor; 6 - cable; 1 - terminal board; 2 - coil terminals; 3 - metal protection (shell); 4 - insulation; 5 - screen; 6 - conductive core.
Measurement (taking readings) should be carried out with the instrument needle in a stable position. To do this, you need to rotate the device handle at a speed of 120 rpm.
The insulation resistance is determined by the reading of the instrument arrow 15 seconds and 60 seconds after the start of rotation. If determination of the cable absorption coefficient is not required, the readings are taken after the pointer has calmed down, but not earlier than 60 seconds from the start of rotation.
If the measurement limit is incorrectly selected, you must:
- remove the charge from the test phase by applying grounding;
- switch the limit and repeat the measurement at the new limit.
When applying and removing grounding, you must use dielectric gloves
At the end of the measurements, before disconnecting the ends of the device, it is necessary to remove the accumulated charge by applying grounding.
Measuring the insulation resistance of lighting networks is carried out with a 1000 V megohmmeter and includes:
- Measuring the insulation resistance of main lines - from 0.4 kV assemblies (main switchboards, ASU) to automatic switchboards (SC) or group circuit breakers (depending on the circuit);
- Measurement of insulation resistance from distribution (floor) switchboards to local control (apartment) group switchboards.
- Measuring the insulation resistance of the lighting network from local circuit breakers (fuses), group control panels (SC) to lamps (including the insulation of the lamp itself). At the same time, in lighting networks in luminaires with incandescent lamps, insulation resistance is measured with the voltage removed, switches turned on, fuses removed (or switches turned off), neutral working and protective wires disconnected, electrical receivers turned off and electric lamps turned out. In lighting networks with gas-discharge lamps, measurements can be taken both with the lamps installed and without them, but with the starters removed.
- The insulation resistance value in each section of the lighting network, starting from the circuit breaker (fuse) panel and including the luminaire wiring, must be at least 0.5 MOhm.
Processing and registration of measurement results
Data on the instruments used in the process of measuring work, as well as the measurement results, are recorded in protocols.
Requirements for safe work performance
Table 1. Permissible distances to live parts that are energized.
In accordance with Chapter 12 of the Interindustry Rules for Labor Protection (Safety Rules) for the Operation of Electrical Installations. POT R M-016-2001" EL workers (as employees of organizations sent to perform work in existing, under construction, technically re-equipped, reconstructed electrical installations and who are not on the staff of the organizations that own the electrical installation) are classified as seconded personnel.
Seconded workers must have certificates of the established form for testing knowledge of norms and rules of work in electrical installations with a mark on the group assigned by the commission of the sending organization. The sending organization is responsible for the compliance of the groups assigned to posted workers, as well as for the staff’s compliance with regulatory documents for the safe performance of work.
The organization of work for traveling personnel involves the following procedures performed before the start of work:
- notifying the organization that owns the electrical installation in a letter about the purpose of the business trip, as well as the composition and qualifications of the traveling electric personnel;
- determination and provision by the owner organization to seconded workers of the right to work in existing electrical installations (as issuers of work orders, responsible managers and work producers, team members);
- Conducting introductory and initial briefings on electrical safety with seconded personnel upon their arrival;
- familiarization of seconded personnel with the electrical circuit and features of the electrical installation in which they will work (moreover, the employee who is granted the right to perform the duties of a work supervisor must undergo training on the electrical installation's electrical supply diagram);
- preparation of the workplace by employees of the owner organization and admission of seconded personnel to work.
The organization in whose electrical installations work is carried out by seconded personnel is responsible for the implementation of the prescribed safety measures and permission to work.
Work is carried out on the basis of a permit, order or in the order of routine operation in accordance with the requirements of Chapter 5 of the Interindustry Rules for Labor Protection (Safety Rules) for the Operation of Electrical Installations. POT R M-016-2001". In addition, when carrying out tests and measurements you should:
- Be guided by the instructions in the passports (operating instructions) of the instruments used and safety instructions (in force at the enterprise where the measurements are performed), as well as additional safety requirements specified in permits, orders, and briefings.
- Check the absence of voltage (it is necessary to check the absence of voltage with a tested voltage indicator, the serviceability of which must be checked on parts of the electrical installation that are known to be energized - clause 3.3.1 of the “Inter-industry rules on labor protection” POT R M-016-2001). The absence of voltage should be checked both between all phases and between phase and ground. Moreover, in electrical installations with the TN-C system, at least six measurements should be made, and in electrical installations with the TN-S system - at least ten measurements.
- Connect and disconnect everything with the voltage removed.
- Ensure the use of protective equipment and tools with insulating handles, tested in accordance with the “Instructions for the use and testing of protective equipment used in electrical installations”, approved by Order of the Ministry of Energy of Russia dated June 30, 2003 No. 261.
The team performing the work must consist of at least two people, including a work performer with an electrical safety group of at least IV and a team member with an electrical safety group of at least III. When carrying out measurements, it is prohibited to approach live parts at distances less than those specified in Table 1.
The most important reason for the increased attention to cable and wire products is this: we are completely dependent on electricity. Everything in our lives - from children's toys and computers, to the work of factories and factories - continues to operate thanks to electricity. And since there is no other way to transmit electricity other than wires, their stable and trouble-free operation is a task of paramount importance.
And if we compare the requirements directly for current-carrying conductors with the requirements for insulation, then the latter will be an order of magnitude greater. By and large, the conductor has only two tasks: to transmit electricity, and not to “lose” it along the way. Cable insulation has, of course, more tasks.
Firstly, the insulation protects the conductors from mechanical damage, as well as from environmental influences, because cables are laid in water, in the ground and in wall grooves. Of course, for such special installation methods, the rules establish additional requirements for protecting cables and wires from damage (trays, pipes, etc.). But the cable itself and its insulation must be resistant to external influences. Therefore, there are cables on the market with multilayer and multi-component insulation, as well as armored wires.
Secondly, the insulation must be an insurmountable barrier to the conductors inside the cable itself. It's no secret that shorting current-carrying wires will not lead to anything good. And since most cables carry both phase and zero loads, the insulation between them must be especially reliable.
Third, as we have already indicated above, insulation protects a person from electric shock. Of course, this does not mean that electricians can work with their bare hands when working with insulated cables. No! In this case, the cable insulation is designed primarily to eliminate accidental contact. The cable is protected from such accidents by insulation, and the person is protected by rubber gloves and a mat, the “right” tool, safety glasses, and so on, in accordance with the Interindustry Safety Rules.
Another important requirement concerns the durability of the cable. This, of course, is also a task of isolation. First of all, this means maintaining the tightness of current-carrying conductors. If, for example, water gets on them, it will very quickly cause corrosion and negatively affect the operation of the cable as a whole. To meet this requirement, oiled paper insulation is used.
This list can be continued for quite a long time. There is an incredible variety of cables, wires, cords with a wide variety of insulation, designed to meet specific requirements. We only note that whatever the insulation, it must remain moderately flexible so as not to break during production, packaging, transportation and installation.
Frequency of insulation resistance measurements
Another reason why cable insulation resistance testing is so popular is the need for continuous testing. The fact is that cable insulation loses its properties over time. Despite the fact that it is made from materials that can serve as reliable protection for many years, it is still necessary to check its condition from time to time. In addition to this, during operation the current load on the cable may increase because the number of energy consumers is growing every day.
If we look at residential buildings that were built several decades ago as an example, it is easy to guess that today the number of electrical appliances in apartments is incomparably greater. And at the time of construction, the electrical wiring inside the building, as well as the cross-section of the input cable, were not designed for such loads. The result is increased load on the cable, cable heating, premature wear and inevitable replacement.
To avoid these troubles, the condition of cables and cable insulation must be constantly monitored. Essentially, this is electrical wiring maintenance, which includes a set of cable capacity measurements and insulation resistance measurements.
- Production.
Before the cable finds its place (is laid and installed), it has already been checked several times and its technical properties have been measured.
As a rule, modern lines for the production of cable and wire products are full cycle lines. That is, at the input all necessary materials are loaded, and at the output there is a cable coil or a drum ready for transportation. But before you send the finished product to the warehouse or sell it, you need to make sure that the cable meets all the requirements. To do this, the electrical laboratory carries out a set of measurements, including mandatory measurement of insulation resistance. If a cable drum or coil does not pass the tests, it means that the technological process has been disrupted somewhere, and the produced cable cannot be used. - Installation.
During electrical installation work, the cable insulation must also be checked for its integrity and readiness for installation. Insulation testing is mandatory, both before and after cable installation. It should be noted that checking the condition of the cable insulation should be carried out before and after each operation with the cable.
We delivered the cable drum to the construction site and took measurements.
If the cable on the drum needs to be heated, then measurements must be taken after it.
We unwound the cable before laying it and took measurements.
We laid a cable from the source to the consumer and took measurements.
Only after measuring the insulation resistance at all stages of installation with a positive result can permission to supply electricity be given. - Exploitation.
As we wrote above, during the operation of any power system, monitoring the condition of the cables is a priority task. Cable insulation dries out over time and loses its insulating properties. In addition, cables can heat up due to excessive loads, which also negatively affects the insulation. In new buildings, the cable may be negatively affected by shrinkage. And in general, cables are very often exposed to influences that do not have the best effect on their performance: soil, water, sea air, rodents, in the end! Therefore, it is very important to constantly monitor the insulation of cable routes. For general purpose cable lines, such checks must be carried out at least once every three years, and for cables located in an aggressive or hazardous environment - at least once a year.
Cable insulation testing equipment
Probably everyone at school, in physics lessons, saw and tried to work with such devices as an ammeter, a voltmeter and an ohmmeter. The first was to measure current, the second was to measure voltage, and the third measured the resistance of the conductor.
In the case of insulation, an ohmmeter is also used. But since the insulation must withstand an increased current load, its resistance is measured in megaohms. Hence the name of the measuring device - megohmmeter (or megometer).
Today there are three varieties of this device on the market.
- Megaohmmeters manufactured before the 2000s (analog). They are a box, approximately the size of a two-liter tetrapack, with plug-in terminal blocks and a rotating handle. The main component of such a device is a dynamo. After connecting the device to the cables, by twisting the handle, the dynamo pumps up the required level of excess voltage at a constant current in the conductors.
Despite the fact that such devices have a fairly large mass and dimensions, they are still popular and are in service in many electrical laboratories. - Modern megohmmeters (digital) are measuring instruments that eliminate the most important disadvantages of their predecessors: excess weight and large dimensions. In terms of their weight and size, they can be compared with a regular notebook, A5 format. Very often, such devices are equipped with a rubberized body, so they are very comfortable to hold in your hand. Moreover, there are no “knobs” on modern megohmmeters, and the process of measuring cable insulation resistance is as automated as possible. The current source in them is galvanic cells or batteries. Moreover, since the device is digital, it is equipped with many useful functions: automatic setting of the required current parameters for various categories of energy consumers, the ability to remember and save measurement results, and others.
- In recent years, measuring systems – multimeters – have become very popular. That is, several devices are contained in one housing; for example, a voltmeter can also work in conjunction with a megohmmeter. For technicians who constantly take measurements, this technical solution is very important. At the same time, neither the size nor the weight of such a device prevents it from being carried in a pocket of overalls.
And of course, one cannot fail to mention that any measuring device must undergo annual verification. Such verification is carried out by specialized metrological and testing centers. The result of the verification is a conclusion about the condition of the measuring device and a special holographic sticker on the body, indicating the date of the last verification.
To carry out just one measurement, along with a megohmmeter, a number of auxiliary instruments and devices are used in the electrical laboratory. All of them must also undergo verification and have accompanying permits.
The essence, standards and technology for measuring insulation resistance
So, we got to the most important thing - the technological part of the work. And before we begin to describe the intricacies of measuring the insulation resistance of various cables, it is necessary to explain the physical essence of this process.
During the same physics lessons at school, they explained to us that in nature there are materials that, according to their physical properties, can be either conductors of electricity, or semiconductors, or dielectrics. The first ones conduct electric current, and they do it very well and with minimal losses. The latter also conduct electric current, but do so less willingly. The latter type of materials does not conduct electricity at all. These properties of materials are given by such a parameter as resistance. The relationship between the conductivity of materials and their resistance is inversely proportional. That is, the lower the resistance of a material, the better it conducts electricity, and vice versa.
Now let's return to our sheep, or rather, to cable insulation. It is clear that cable cores are made of conductors that are capable of transmitting electric current very well, with minimal losses, even over long distances. It is also clear that the insulation of current-carrying conductors (and the cable as a whole) is made of dielectric materials. Thus, the insulated cable cores will never intersect, and, therefore, there will be no power leakage or short circuit. It seems that everything is logical and understandable.
But, if the cable cores are completely isolated from each other and do not interact with each other in any way, then how and due to what is the insulation resistance measured? What parameter does a megohmmeter measure if during measurements all the cable cores are separated and do not come into contact with each other? Likewise, the voltage generated by the megohmmeter is constant, therefore, the cables do not experience any interference with each other.
To answer this question, you need to remember that any dielectric insulation base loses its properties over time.
And this process is accelerated due to the fact that the insulating material is in constant contact with the metal base of the cable, which is energized. In addition, shell wear occurs for many reasons. For example, rubber insulation is more susceptible to drying out than others, and as a result, it not only becomes more rigid and brittle, it becomes thin. Plastic insulation also does not last forever and deteriorates over time. And if the cable is located in an aggressive or dangerous environment, its protective life may expire after just a few years.
And what happens to the electric current that is passed through conductors with a poor protective layer? The insulation begins to let it through, and the current-carrying wires of the cable begin to interact with each other. Of course, in such small doses this interaction cannot be seen by the human eye, but the megohmmeter certainly detects these changes. To put it simply, the insulating layer changes over time from the state of a dielectric to a semiconductor. And as long as this transition remains within the permissible values, the cable can be used.
In addition, electric current leakage can pass through microcracks in the cable insulation, and also until such time as this leakage remains within acceptable limits. And if the insulation is not sealed, then moisture and dust can get inside the cable, making the process of insulation wear more rapid and inevitable.
When the cable is completely new, the result of measuring the insulation resistance will tend to infinity, because there is no current leakage, and the conductors of the cable do not interact with each other in any way. But as the insulation ages, the measurement results will get worse and worse. When the cable is very old, even a short circuit may occur during measurement. Therefore, experienced technicians never apply a full load to the cable under test, but do it gradually, as written in IEC 364-6-61.
In general, speaking about regulatory documents in the field of electrical measurements, it should be noted that in addition to an impressive list of various rules and regulations for carrying out measurements, each electrical laboratory must have its own methods and instructions intended for instrumentation and automation technicians and engineers who directly carry out measurements. These documents are developed at the stage of laboratory formation, approved by Rostechnadzor, and serve exclusively for internal use in each electrical laboratory. We will analyze the basic principles and stages of measuring cable insulation.
Preparatory work
Any work in the construction industry begins with a study of operational documentation and the facility as a whole. Technicians should carefully review single-line cabinet layouts and floor plans for cabling. Moreover, since the resistance value of the dielectric part of the cable is not constant and depends on several factors (for example, ambient temperature, cable service life, etc.), specialists also need to study the test object in detail. All this is necessary for more accurate final test results.
Any testing of cable products involves the supply of electricity to the conductors. In this regard, it is necessary to protect people and electrical appliances from damage. First of all, the object is completely de-energized. Next, you need to disconnect the circuit breakers, RCDs, protective inserts and other devices.
The process of protecting energy consumers (lamps, electrical equipment, etc.) involves disconnecting them from the network. The work is quite simple, but time- and labor-intensive. After disconnecting the conductors from the power consumers, the process should be completed by grounding all cables that are planned to be tested. This should be done without fail, since the cables may retain a residual electrical charge.
Protection against injury to people is carried out by fencing the test sites and installing warning signs and plates. If necessary, security can be posted in front of the place where measuring work is performed.
Measuring the insulation resistance of two-core cables
The simplest, most understandable and visual example of measuring insulation resistance is a cable consisting of two cores - a pair. The megohmmeter probes are attached to each core and voltage is applied. The insulation resistance level for all cables, wires and cords designed for operating loads up to 220V must be at least 0.5 MOhm. If the cable consists of several pairs (for example, a trunk telephone cable), then measurements must be taken both between the cores of each pair and between the cores of different pairs.
Measuring the insulation resistance of three-core cables
In this case we are talking about power and some control cables. The insulation resistance is measured here in a circle, in pairs. First, between the cores there is “phase” - “zero”, then “zero” - “earth”, and finally, “earth” - “phase”. Since all conductors must have the same insulation, the megohmmeter readings must be the same. The insulation of three-core power cables designed for operating voltages up to 1000V must have a resistance of at least 0.5 MOhm. And if the measurement is made on the control cable, then its insulation resistance should not be less than 1 MOhm.
Measurement of insulation resistance of multi-core cables
Measuring insulation resistance for multi-core cables has the same structure as for paired cables. For example, to measure the insulation resistance of a four-core cable (three “phases” and “zero”), six measurements must be made. Five-core cable - ten measurements.
Power cables designed for a rated operating load of more than 1000V must have insulation whose resistance cannot be less than 10 MOhm.
At the end of this section, it is also necessary to pay attention to test voltage, which certainly differs from the nominal one.
- If the cable is designed for everyday use under voltage up to 100 V, then the maximum voltage at which insulation resistance is measured is 100 V;
2. If the cable is live from 100 to 500 V, then the insulation resistance is measured under voltage from 250 to 1000 V;
3. Cable lines designed for rated load from 500 to 1000 V must be tested with voltage from 500 to 1000 V;
4. Well, if the rated operating voltage of the cable exceeds 1000 V, then the resistance is measured with a load of 2500 V.
Results of measurements: technical reports, protocols, acts
To ensure that the measurements do not remain in the memory of the people who carried them out or in the memory of the digital megohmmeter, their results are recorded in a special document - protocol. The protocol itself can consist of either one type of test or be a combined document after a set of measurements. Initially, the protocol form is developed by each laboratory independently and approved by the Rostechnadzor authorities along with methods and instructions.
The protocols are combined into technical report, are placed in a folder, supplied with a title page and a list of measurements that were carried out at the site. Also, electrical laboratories complete the folder with the technical report with other necessary documents: ETL Certificate, passports and instrument verification certificates, documents for the specialists who carried out the measurements, etc. The documentation is drawn up in such a way that supervisory authorities do not have additional questions about the work done at the site during the inspection.
If measurements were carried out as part of the construction or reconstruction of a facility, then the technical report must be included in the as-built documentation. And if the cable system tests were planned, then the technical report is transferred to the customer.
The protocols themselves are a summary table that reflects absolutely all test results of insulation resistance measurements of each tested cable. This is the most convenient and compact form of recording a large amount of information. The header of each protocol indicates the name of the measurement, the date of the measurement, as well as the name of the company and the assigned number of the electrical laboratory. On the last page of each protocol, in addition to the signatures of the persons responsible for carrying out the measurement, the name of the measuring device and the date of the last verification are indicated.
Mobile electrical laboratory: features of cable testing
Any mobile electrical laboratory, of course, can measure cable insulation resistance. Moreover, if there is an electric current generator on board the mobile ETL, the laboratory will be able to test the insulation resistance even of cables designed for very high operating voltages.
The peculiarity of such work is that the mobile laboratory operates outside buildings, therefore, it deals with main cables that can stretch from one substation to another over a distance of several tens of kilometers. Therefore, even to carry out preparatory work, you need to spend some time.
Distance is the most important feature of testing trunk cables. For example, if the test results inside the building do not meet the standard indicators, the cable route is divided into small sections along the cable connections, and each section is checked individually. Thus, it is possible to identify a section of the cable where the insulation does not meet the established standards and replace it, while material and labor costs will be minimal. If such an insulation defect is detected on the main cable, then its elimination will require many times more costs. But this is a topic for the next article.
Insulation resistance monitoring
So, we need to summarize all of the above. First of all, it is worth mentioning that the method for measuring insulation resistance is not as simple and unambiguous as described above. All the intricacies of this work are, of course, very well known to professionals, who daily subject the insulation of cable lines to tests. And such responsible work should only be entrusted to true gurus in this field, who will not leave a single detail unattended.
It must be remembered that the reliable and stable operation of any power system directly depends on the technical condition of the cable system included in its composition. Therefore, in order for factories to work, for the streets to be illuminated with lanterns at night, for children to rejoice at the lights on New Year’s trees on New Year’s Eve, for the lights to be on in every house and (more importantly!!!) for the Internet to work, it is necessary to maintain all the components of this huge system in proper order. condition.
Commissioning work on testing and measuring the insulation resistance of wires, cables, power electrical equipment and devices is carried out when putting electrical installations into operation, during periodic audits and emergency checks.
Work on checking the insulation resistance value of electrical appliances, cables, equipment, input distribution devices, apartment and floor panels, as well as equipment of consumer transformer substations and protective equipment in order to assess the quality of insulation and compare with current standards, is carried out on the basis of current methods of commissioning laboratories , drawn up taking into account the requirements of current GOST, PUE, PTEEP, POT, instructions and accompanying documents of manufacturers.
Organizational and technical measures for security
Insulation resistance measurements with a megohmmeter can be carried out in electrical installations with voltages above 1000 V by a team of at least two people, one of whom must have an electrical safety group of at least IV.
In electrical installations with voltages up to 1000 V, measurements are carried out by order of two workers, one of whom must have an electrical safety group of at least III.
In electrical installations located in premises, except for those that are particularly dangerous in terms of electric shock, having a voltage of up to 1000 V, an employee with group III, who has the right to be a work performer, can carry out measurements alone.
To measure insulation resistance, megaohmmeters of the following types are used: ESO 202/1, ESO 202/1-g, PSI-2500, etc., with an output voltage of 500, 1000, 2500 V, M4100 meters and their modifications, F4100 meters, etc.
Features of measurements
If the circuit contains electronic devices, then only the insulation resistance measurement should be made between the phase and neutral conductors connected together and to ground.
This precaution is necessary because performing tests without connecting live conductors may cause damage to electronic devices.
In accordance with GOST R 50571.3-2009, insulating (non-conducting) rooms, zones, areas are intended to prevent simultaneous contact with parts that are at different potentials in the event of damage to the main insulation of live parts. The requirements are considered met if the floor and walls of the room are insulating and one or more of the conditions below are met:
- open conductive parts and third-party conductive parts, as well as open conductive parts, are separated from each other by at least 2 m, and outside the reach - 1.25 m;
- effective barriers are installed between exposed conductive parts and third-party conductive parts;
- third-party conductive parts are insulated.
Resistance of insulating floor and walls, measured ineach point must be at least:
- 50 kOhm at a rated voltage of electrical installations not higher than 500 V;
- 100 kOhm at rated installation voltage above 500 V.
Three measurements must be taken in each room and for each surface in accordance with clause 612.5 of IEC 364-4-61. One measurement should be taken approximately 1 m from any external conductive parts located in the room. Other measurements must be made at a greater distance.
When measuring the insulation resistance of cables and electrical wiring, the following must be taken into account:
- measurement of the insulation resistance of cables (except for armored cables) with a cross section of up to 16 mm 2 is carried out with a 1000 V megohmmeter, and above 16 mm 2 and armored ones - with a 2500 V megohmmeter;
- The insulation resistance of wires of all sections is measured with a 1000 V megohmmeter.
In this case, it is necessary to make the following measurements:
- on two- and three-wire lines - three measurements: L-N; N-PE; L-PE;
- on four-wire lines - four measurements: L 1 -L 2, L 3; L 2 -L 3 L 1 PEN ; L 3 -L 1 L 2 PEN; PEN-L 1 L 2 L 3 or six measurements: L 1 -L 2; L 2 -L 3 ; L 1 -L 3 ; L 1 -PEN; L 2 -PEN; L 3 -PEN;
- on five-wire lines - five measurements: L 1 -L 2 L 3 NPE; L 2 -L 1 L 3 NPE; L 3- L 1, L 2 NPE; N-L 1 L 2 L 3 PE; PE-NL 1 L 2 L 3 or 10 measurements: L 1 -L 2 ; L 2 -L 3 ; L 1 — — L 3 ; L 1 -N; L 2 -N; L 3 -N; L 1 -PE; L 2 -PE; L 3 - PE; N-PE.
If electrical receivers in operation have an insulation resistance of 1 MΩ, then a conclusion about their suitability is made after testing with alternating current of industrial frequency, voltage 1 kV.
The insulation resistance of electrical machines and devices depends largely on temperature. Therefore, the insulation resistance should be measured at its temperature not lower than +5°C, except for cases specifically specified in the attached instructions. At lower temperatures, reliable measurement results are difficult to obtain.
The degree of insulation moisture is determined by the absorption coefficient, which is calculated based on two resistance measurements: one measurement obtained 60 seconds after applying the megohmmeter voltage (R 60), to the measured state of the insulation after 15 seconds (R 15)
When measuring the insulation resistance of power transformers, megohmmeters with an output voltage of 2500 V are used. Measurements are taken between each winding and the housing and between the windings of the transformer. In this case, the R 60 value must be adjusted to the results of factory tests depending on the temperature difference at which the tests were carried out. The value of the absorption coefficient should differ (downwards) from the factory data within 20%, and its value should not be lower than 1.3 at a temperature of 10-30°C. If these conditions are not met, the transformer is dried. The minimum permissible insulation resistance for installations in operation is given in the appendix based on current data.
Measuring the insulation resistance of AV and RCD
Measurement of insulation resistance of AV and RCD is carried out:
- Between each pole terminal and the pole terminals connected to each other (when the AB or RCD is open).
- Between each opposite pole and the remaining poles connected to each other (in the closed state of AB or RCD).
- Between all poles connected to each other and between the body wrapped in metal foil.
At the same time, for household AVs (GOST R 50345-2010) and RCDs when measuring points. 1.2 The insulation resistance is allowed to be at least 2 MOhm, and according to clause 3 - at least 0.5 MOhm.
When measuring insulation resistance, it is necessary to use minimally short wires with insulated handles at the ends in front of the contact terminals and insulation of at least 10 MOhm to connect the megohmmeter to the test object. Before carrying out work, it is necessary to install the megohmmeter almost horizontally, away from powerful power transformers.