Insulation resistance shows how well wires and cables keep the current where it belongs. The lower it is, the higher the risk of leakage, short circuits and fire. Below are the limits for installations up to 1 kV, the test method and the reasons resistance drops.
- For wiring, panels and power cables up to 1 kV, the minimum insulation resistance is 0.5 MΩ (PUE table 1.8.34 and clause 1.8.40).
- Wiring is tested with a 1000 V megohmmeter, and power cables up to 1 kV at 2500 V.
- The test is done with the circuit de-energised and sensitive devices disconnected.
What is insulation resistance?
It is the resistance of the insulation between a live part and earth or an adjacent phase. It is measured with a megohmmeter, an instrument that applies a high DC voltage. The result is given in megohms (MΩ).
Limits table (installations up to 1 kV)
Under the electrical installation rules (PUE clause 1.8.37, table 1.8.34 and clause 1.8.40), the acceptance test limits are:
| Item | Megohmmeter voltage | Minimum resistance |
|---|---|---|
| Wiring, including lighting circuits (up to 1 kV) | 1000 V | 0.5 MΩ |
| Switchgear, panels, busways (up to 1 kV) | 500–1000 V | 0.5 MΩ |
| Power cables up to 1 kV | 2500 V | 0.5 MΩ |
Each section of switchgear is tested separately. For cables above 1 kV, insulation resistance has no set limit: their condition is checked by high-voltage testing.
How is it tested?
- The circuit is de-energised and checked dead.
- Sensitive devices are disconnected: electronic meters, drives, LED drivers, UPS units.
- Measurements are taken phase–phase, phase–N and phase–PE.
- The reading is recorded once stable, usually after 60 seconds.
- The circuit is discharged afterwards, because a cable holds a charge.
We test cables to the GOST 3345-76 method. The results and test conditions go into the report.
Why does insulation resistance drop?
- Moisture: basements, flooded ducts, condensation.
- Ageing and overheating: insulation hardens and cracks.
- Mechanical damage: nails, excavation, rodents.
- Dirt and dust, especially in panels and on terminals.
When should insulation be tested?
- At handover of a new installation.
- After repairs, flooding or fire.
- At periodic inspections, at intervals set by the operating rules and the site owner.
- When a breaker or RCD trips for no obvious reason.
The test report
An accredited laboratory's report lists the measured value for each circuit, the test voltage, the instrument's calibration date, the limit and a conclusion. The Enerji Test Servis LLC laboratory is accredited to AZS ISO/IEC 17025:2020.
Insulation tests — Enerji Test Servis
FAQ
Is 0.5 MΩ a lot or a little?
It is the minimum acceptance limit. A new, dry installation usually reads far higher. A value close to the limit is a sign of a problem.
Can a megohmmeter damage equipment?
Yes, which is why we disconnect electronics before testing.
Are insulation and earthing tests the same thing?
No, they are different tests, but they are usually done together and recorded in one report.
How long does testing take?
It depends on the number of circuits. A flat usually takes a few hours, a large site a few days.