Short Answer: How to Check Insulation
Measuring insulation resistance with a megohmmeter is a check of the ability of the cable insulation to prevent current leakage. The instrument applies a DC voltage to the conductor (usually 2500 V for power cables up to 1 kV) and measures the current flowing through the insulation. According to Ohm’s law, the resistance is calculated. For power cables up to 1 kV, the insulation resistance must be not less than 0.5 MΩ, for control cables — not less than 1 MΩ.
The measurement is carried out before and after testing the cable with increased voltage, as well as before commissioning the line. Before starting work, the cable is disconnected from the network, the residual charge is removed, and earthing is installed. After each measurement, the conductor is discharged to earth. The result depends on temperature and humidity, therefore measurements are carried out at positive ambient temperature.
What Is Checked and Why
Physical meaning of insulation resistance. Insulation separates the current-carrying conductors and protects against current leakage to earth or between conductors. Ideal insulation has infinitely high resistance. In practice, the resistance is finite: the lower its value, the greater the leakage current and the higher the risk of breakdown. Measuring insulation resistance reveals defects: moisture, contamination, mechanical damage, ageing of the material.
Why the measurement is carried out. Measuring insulation resistance is a mandatory part of the acceptance tests of cable lines. It makes it possible to:
- make sure the insulation is intact before applying voltage;
- identify defects that occurred during installation or transportation;
- assess the condition of the insulation during operation;
- confirm the cable’s compliance with the requirements of regulatory documents.
For cables with copper and aluminium conductors, the measurement method is the same. The differences concern only the standardised values of DC conductor resistance, not insulation resistance. The choice between copper and aluminium conductors affects other line parameters. The design and characteristics of a power cable are described in a separate material: power cable: design, types, characteristics.
When the Check Is Carried Out
Measuring insulation resistance is carried out at different stages of the cable line life cycle.
Production. Factory acceptance tests include checking the electrical insulation resistance. Cables are presented for acceptance in batches. The measurement is carried out according to GOST 3345-76 at a voltage from 100 to 1000 V, unless other conditions are specified in the standards or technical specifications for specific types. A cable that has not passed factory tests is not allowed for delivery.
Acceptance at the site. Incoming inspection includes checking the accompanying documentation and visual inspection. Electrical measurements during incoming inspection may be carried out selectively. More about this procedure — in the article incoming cable inspection: what to check when receiving a batch.
After installation. Insulation resistance measurement is carried out before commissioning the cable line. This is a mandatory test provided for by the PUE RK. For power cables up to 1 kV, the measurement is carried out with a 2.5 kV megohmmeter. The measurement is carried out before and after testing the cable with increased voltage, if such a test is provided.
Operation. Periodic tests are carried out in accordance with the schedule of planned preventive repairs. For cable lines in operation, insulation resistance measurement is carried out at least once every three years, unless a different frequency is established by the technical documentation.
Troubleshooting. When cable damage is suspected, insulation resistance measurement is the first diagnostic step. If the resistance is below normal, they proceed to more precise methods: determining the damage location with a reflectometer, acoustic or induction method.
How It Is Carried Out
Preparation for Measurement
Before measurement, the following actions are performed:
- Disconnect the cable from the power source and remove the voltage.
- Make sure there is no voltage on the conductors using a voltage indicator.
- Remove the residual charge from the conductors by earthing for several minutes.
- Install earthing on the cable conductors.
- Post warning signs: “Stop”, “Voltage”, “Testing is dangerous to life”.
- Inspect the end terminations: the surfaces of the conductors must be clean, without cracks. If cracks are detected, re-termination is performed.
The measurement is carried out at positive ambient temperature. At negative temperatures, water in the insulation is in a frozen state and is not detected, since ice is a dielectric. Measurements with a 2500 V megohmmeter are carried out by order with an entry in the operational log by two people with qualification groups IV and III.
Measurement Procedure
Insulation resistance measurement is performed alternately for each conductor. The other two conductors, screens and the earthing conductor at the moment of testing are connected to each other and earthed. The order of actions:
- Connect the megohmmeter to the conductor under test and to the earthed conductors.
- Apply the test voltage. For power cables up to 1 kV — 2500 V.
- Take the reading 60 seconds after the start of the measurement. During this time, the absorption current stabilises.
- Remove the test voltage.
- Discharge the conductor under test to earth using an earthing rod.
- Repeat the procedure for the next conductor.
For cables with cross-linked polyethylene (XLPE) insulation and other insulation types, the measurement conditions may differ. The test voltage and hold time depend on the cable type and are specified in the standards or technical specifications for specific types.
Equipment Used
Insulation resistance measurement is performed with a megohmmeter. The instrument must be calibrated and have a valid calibration certificate. The accuracy class of the instrument must ensure an error of no more than 10% for measured values from 1·10⁵ to 1·10¹⁰ Ω, no more than 20% for values above 1·10¹⁰ to 1·10¹⁴ Ω and no more than 25% for values above 1·10¹⁴ Ω (according to GOST 3345-76).
How to Evaluate the Result
The measurement result is compared with the standardised value. The norms depend on the cable type and the network voltage.
| Cable type | Megohmmeter voltage | Lowest insulation resistance | Source |
| Power cables up to 1 kV | 2500 V | 0.5 MΩ | PUE RK, clause 2 (insulation resistance measurement) |
| Power cables above 1 kV | 2500 V | Not standardised | PUE RK, clause 2 |
| Control cables | 1000–2500 V | 1 MΩ | PUE RK; methodological recommendations |
| Cables with rubber insulation up to 1 kV | 1000 V | 0.5 MΩ | PUE RK |
Source: Electrical Installation Code of the Republic of Kazakhstan, section 29 “Power cable lines”, clause 2.
The measured insulation resistance depends on temperature. Reduction to +20 °C is performed according to the formulas specified in the standards. If the cable temperature differs from the standardised one, the result is recalculated. For power cables above 1 kV, insulation resistance is not standardised, but the measurement is carried out to assess the condition of the insulation and identify a tendency to deterioration.
The measurement results are recorded in a report. If the insulation resistance is below normal, the cable is considered to have failed the test. In this case, measures are taken to identify and eliminate the cause.
What Can Distort the Measurement
| Factor | How it affects | What to do |
| Ambient temperature | Insulation resistance decreases as temperature rises. At negative temperatures, moisture is not detected, since water freezes. | Carry out measurements at positive temperature. Bring the result to +20 °C. |
| Air humidity | Condensation on the spark gaps and insulators of the test installation can lead to breakdown. | Wipe the end terminations from dust, dirt and moisture before measurement. |
| Condition of end terminations | Contamination, moisture or damage to the termination reduces resistance. | Inspect the terminations, re-terminate if necessary. |
| Cable length | Insulation resistance is inversely proportional to length. On short sections the result is higher. | Take the length into account when evaluating the result. |
| Contact quality | Poor contact of the measuring leads gives an overestimated resistance. | Strip the conductor ends, ensure a reliable connection. |
| Instrument error | An uncalibrated instrument gives an incorrect result. | Use a calibrated megohmmeter with a valid certificate. |
| Residual charge | Immediately after disconnection, the cable may retain a charge that distorts the measurement. | Discharge the conductors to earth before measurement. |
What to Do in Case of Deviation
When a deviation from the norm is detected, sequential diagnostic steps are performed.
Resistance below normal. The condition of the end terminations is checked: contamination, moisture, cracks. The ends are wiped, dried, and the measurement is repeated. If the resistance remains low, the insulation is defective. The cable is not subject to use without restorative repair or replacement of the section.
A sharp drop in resistance compared to previous measurements. This is a sign of a developing defect. The location of the damage must be determined. The following methods are applied: reflectometry, acoustic method, induction method. After localising the damage location, a decision is made on repair or replacement.
Unstable readings during measurement. The instrument needle fluctuates, the readings do not stabilise. The cause is poor contact, moisture or an incipient breakdown. The contacts are checked, and the insulation is inspected.
Breakdown during measurement. If a breakdown occurs when the test voltage is applied, the cable is considered to have failed the test. The breakdown location is localised, and the section is replaced.
Typical Mistakes
Measurement at negative temperature. Frozen water in the insulation is not detected. The cable is recognised as suitable, and after thawing the resistance falls below normal. Measurements are carried out only at positive temperature.
Failure to discharge the conductor after measurement. After the test voltage is removed, a charge remains on the conductor, which is dangerous to life. Discharge to earth is mandatory after each measurement.
Measurement without disconnecting the cable from the network. An attempt to measure insulation resistance on a cable that is under voltage leads to damage to the instrument and danger to personnel. The cable is disconnected, the absence of voltage is checked, and earthing is installed.
Using an uncalibrated megohmmeter. The instrument error is not taken into account, and the result is invalid. The instrument must be calibrated, and its accuracy class must correspond to the requirements of the method.
Measuring only one conductor. Insulation resistance may differ for different conductors. All conductors are measured, not just one selectively. Similarly, the resistance is checked between each conductor and the remaining conductors connected to the sheath.
FAQ
What megohmmeter voltage should be used for a power cable up to 1 kV?
For power cables up to 1 kV, insulation resistance measurement is carried out with a 2.5 kV megohmmeter. For cables with a cross-section up to 16 mm² (except armoured ones), the use of a 1000 V megohmmeter is permitted. For armoured cables and cables with a cross-section above 16 mm², a 2500 V megohmmeter is used.
What insulation resistance is considered normal for a control cable?
For control cables, insulation resistance must be not less than 1 MΩ. The measurement is carried out with a 1000–2500 V megohmmeter, depending on the cable type and operating conditions.
Why is the measurement carried out at positive temperature?
At negative temperatures, water in the insulation freezes and becomes a dielectric. The moisture defect is not detected. After thawing, the insulation resistance drops, and the cable may fail. Therefore, measurements are carried out at positive ambient temperature.
Is it necessary to discharge the conductor after measurement?
Yes. After the test voltage is removed, a charge remains on the conductor, which is dangerous to life. The conductor is discharged to earth using an earthing rod after each measurement. This is a mandatory safety requirement.
What to do if the insulation resistance is below normal?
Check the condition of the end terminations: contamination, moisture, cracks. Wipe, dry, and repeat the measurement. If the resistance remains low, the insulation is defective. The cable is not subject to use without restorative repair or replacement. To localise the damage location, reflectometry, the acoustic method or the induction method are used.
Sources
- Electrical Installation Code of the Republic of Kazakhstan 2015, as amended by the order of the Minister of Energy of the Republic of Kazakhstan dated 31.10.2022 No. 340 — section 29 “Power cable lines”, clause 2 — IPS “Adilet”, adilet.zan.kz — norms of insulation resistance, megohmmeter test voltage.
- GOST 3345-76 “Cables, wires and cords. Method for determining electrical insulation resistance” — in force — official text on elec.ru — measurement method, requirements for equipment, error, conditions of performance.
- IEC 60364-6:2016 “Low-voltage electrical installations — Part 6: Verification” — in force — official IEC website, webstore.iec.ch — requirements for insulation verification, minimum resistance values.
- Methodological recommendations for testing electrical equipment and apparatus of consumer electrical installations — section 5.1 “Measurement of insulation resistance of power cables and wiring” — in force — ruscable.ru — choice of test voltage depending on the cross-section and cable type.
- GOST 31996-2012 “Power cables with plastic insulation for rated voltages of 0.66; 1 and 3 kV” — in force — official text of the standard — requirements for cable design.
