Insulation resistance testing: which test voltage and limit values apply?
Insulation resistance shows how well the insulation of cables, motors and installations keeps current away from the protective earth. It is measured with an insulation tester that applies a high DC voltage and reads the resistance in megohms or gigohms. The test voltage and limit value depend on the nominal voltage.
View testersHow is insulation resistance measured?
Insulation resistance is measured with an insulation tester, often called a megohmmeter. The instrument applies a high DC voltage between the conductors and the protective earth or ground and measures the small leakage current that flows. From this it calculates the resistance and displays it in megohms (MΩ) or gigohms (GΩ).
Unlike a continuity or resistance measurement with a multimeter, insulation testing uses a much higher voltage, because weaknesses in the insulation only appear under stress. Faulty insulation, moisture or ageing cause leakage currents that lower the measured resistance. Typical objects are cables and wiring, motor windings, transformers and complete electrical installations, both before commissioning and during periodic testing.
The requirements for the tester itself are covered by the IEC 61557 series; part 2 describes equipment for measuring insulation resistance. A high reading means good insulation, while a low value points to a developing or existing insulation fault.
Which test voltage and limit values apply?
The test voltage and minimum value depend on the nominal voltage of the circuit under test. For low-voltage installations the basis is IEC 60364‑6 (verification of electrical installations). The higher the operating voltage, the higher the test voltage and the required minimum resistance.
| Nominal circuit voltage | Test voltage (DC) | Minimum resistance |
|---|---|---|
| SELV and PELV | 250 V | 0.5 MΩ |
| up to 500 V (incl. FELV) | 500 V | 1 MΩ |
| above 500 V | 1000 V | 1 MΩ |
The rule of thumb for the minimum value is 1 MΩ for most low-voltage circuits up to 500 V. These figures are lower limits for acceptance; new, dry installations often reach values in the high MΩ or GΩ range in practice. Motors and machines may have their own recommendations, for example from IEC 60204‑1 or manufacturer limits, which can be considerably higher.
Condensation or ingress of water often lowers the insulation resistance only temporarily.
Brittle or cracked insulation gives permanently low values and should be investigated.
If the value falls below the minimum resistance, the circuit is considered a fail.
Falling values over several tests reveal deteriorating insulation at an early stage.
What matters for safety and influencing factors?
Before the measurement the circuit must be safely isolated and de-energised. During the test a hazardous DC voltage is present; afterwards, cables and windings store charge like a capacitor. The test object must therefore be discharged after the test before it is touched.
Modern insulation testers discharge the object automatically through an internal resistor once the test button is released. Wait until the display drops to 0 V, and only then disconnect the test leads. On long cables or large machines the discharge can take several seconds.
Temperature and humidity strongly influence the result: insulation resistance falls as temperature and humidity rise. For comparable values, measure under similar conditions on a dry surface and record the temperature. For related topics see our guides on earth resistance testing and choosing a multimeter.
Frequently asked questions
Why is DC used instead of AC for the measurement?
DC stresses the insulation without capacitive reactive current and gives a stable, repeatable reading. This lets the pure insulation resistance be determined without the influence of cable capacitance.
What minimum value must the insulation resistance reach?
To IEC 60364‑6 the minimum is 1 MΩ for circuits up to 500 V, 0.5 MΩ for SELV and PELV and again 1 MΩ above 500 V. New, dry installations are usually well above this, in the MΩ to GΩ range.
Do I really need to discharge after the test?
Yes. Cables and windings hold a hazardous residual charge after the test. Most instruments discharge automatically; wait until the display shows 0 V and only then disconnect the test leads.
Find the right insulation tester
From a compact insulation tester to an installation tester with 250, 500 and 1000 V - we help you choose and apply it to IEC 61557.
Standards
Limit values to IEC 60364-6, testers to IEC 61557-2.
Neutral advice
Recommendation by application and voltage level, independent of brand.
Matching instruments
Insulation testers and installation testers for 250 to 1000 V DC.
Expertise
Related reading: our earth resistance testing guide at /en/ratgeber/earth-resistance-test.