What is an insulation resistance measurement?

Explanation of insulation resistance measurement (ISO)

The insulation resistance measurement is an important measurement used to assess the quality of the insulation of electrical equipment.

It measures the resistance in kilohm, megaohm or gigaohm between the (live) conductors and the insulation of a piece of equipment or installation.

The insulation resistance measurement is mainly intended to assess the condition of the insulation and determine whether there may be problems with the device.

The voltage applied during the measurement is always a DC voltage, and the insulation resistance is calculated using Ohm’s law (R = U / I). The insulation resistance test is used for test objects with protection class I or II. Products with protection class I are equipped with an earth conductor (PE*). Products with class II are equipped with additional protective insulation and do not have a PE conductor.

What does an insulation resistance measurement measure?

Insulation resistance measurement is used to test whether the ohmic insulation resistance is above the limit value specified in the standard. The test is performed to check whether excessive leakage currents occur with respect to earth or between electrical lines.

If the insulation resistance is too low, an excessively high touch current occurs in the metal parts of the product under test (DUT) and may pose a hazard.

When touching these metal parts, the so-called touch current will flow through the person to earth. Especially for test objects with protective class II, insulation faults on metal parts can be dangerous, as the current cannot be dissipated by the grounding.

If the insulation resistance is high and the leakage current is low, this indicates that sufficient insulation is present from the current-carrying parts of the test object. The tester then indicates a value that is, for example, in the kilohm, gigaohm or teraohm range.

In standard norms, for example, for lighting, the minimum limit is > 1 Mohm.

If the earth connection is not properly connected, the touch voltage at the switchgear/casing of the DUT (Device Under Test) may become too high. When touching conductive parts of the metal enclosure, a dangerous touch current flows through the person to earth.

In the example below, you can see an example where the earthing is not connected correctly, it has too high a resistance value of the earthing (> 0.2 ohms) This fault current will eventually flow through the person to earth and not through the earth.

Definitions
Utility : Network operator / grid operator
DUT : Device Under Test | test object
IL : Current through test object
IAbl : leakage current
UN : Supply voltage
RPE : Earthing resistance
UB : Possible contact voltage

Below is a picture of a class II device where the fault current flows directly through the person to earth. The reason is that earth is not present in a class 2 device.

The image below shows a DUT connected to a safety tester, here you can see that the the test voltage on the L and N (short-circuited) with respect to the earth connection

The test voltage in an insulation resistance measurement is typically 500 volts DC.

The test is often performed with shorted power cables with respect to the earth (PE) cable. There are different standards and rules for different test objects. The voltage level is defined in these standards.

The insulation resistance measurement can only be carried out with a direct voltage (DC), the capacitive part of the test object is only affected during the DC voltage application/build-up. If the capacitance is “charged” one only measures the pure leakage current through the ohmic part of the test object to earth.

Insulation resistance calculation is based on R = U/I, where U is the applied test voltage, I is the leakage current and R is the calculated insulation resistance value.

If a test object has a high capacity, it is mandatory to discharge the test object, our test systems automatically check if there is a residual voltage and how many Volts are present during discharge.

*The abbreviation PE stands for Protective Earth

Electrical test systems that can be fitted with insulation resistance measurement:

Electrical safety testers

Electrical safety tester Handheld
Electrical safety tester GLP1-g
Electrical safety tester GLP2-Basic
Electrical safety tester GLP2-Modular
Electrical safety tester GLP3

Electric motor testing

Electric motor tester MotorAnalyzer 1
Electric motor tester MotorAnalyzer 3
Electric motor tester MTC2-R7

Portable Insulation Resistance Meters

1 kV portable insulation resistance meter
5 kV portable insulation resistance meter
10 kV portable insulation resistance meter
15 kV portable insulation resistance meter
MotorAnalyzer 3

Cable harness testers

Wire harness tester Automeg
Cable tester portable Automeg

Test voltages per standard

The test voltage for an insulation resistance measurement depends on the applicable standard. Below is an overview of commonly used standards and their corresponding test voltages:

EN 60204-1 (machinery directive): 500 V DC, minimum insulation resistance 1 MΩ
EN 60598-1 (luminaires): 500 V DC, minimum insulation resistance 2 MΩ (class I) or 4 MΩ (class II)
EN 61010-1 (measurement and laboratory equipment): 500 V DC, minimum insulation resistance depends on working voltage
EN 60335-1 (household appliances): 500 V DC, minimum insulation resistance 1 MΩ (class I) or 2 MΩ (class II)
EN 61439 (switchgear assemblies): 500 V DC or 1000 V DC, minimum insulation resistance 1 MΩ per circuit
IEC 60034 (electric motors): test voltage depends on nominal voltage, typically 500 V – 5 kV DC

Always consult the specific standard for exact test conditions. If in doubt, IONIO can help you with the right advice.

Difference with the high voltage test

The insulation resistance measurement and the high voltage test (HV) both assess insulation quality, but in a fundamentally different way. During an insulation resistance measurement, you apply a relatively low DC voltage (typically 500 V – 5 kV) and measure the resistance of the insulation. During a high voltage test, you apply a much higher voltage (often several kV) to verify whether the insulation can withstand an overvoltage without breaking down.

In short: the insulation resistance measurement checks the “quality” of the insulation, the high voltage test checks the “strength”.

Factors influencing the measurement

The measured value of an insulation resistance measurement is influenced by several factors. The most important are:

Temperature – A higher temperature reduces insulation resistance. A rise of 10 °C can reduce the measured value by a factor of 2. Take this into account when comparing measurements over time. See also: resistance and temperature.

Humidity – Moisture on or in the insulation significantly lowers the resistance value. Preferably measure under dry conditions, or allow the test object to warm up first.

Contamination – Dust, oil or other contaminants on the insulation material create conductive paths that lower the measured value.

Test leads – Always use properly insulated test leads. Damaged cables or poor contacts give unreliable results.

Insulation resistance testers and safety testers from IONIO

IONIO supplies a complete range for insulation resistance measurements, from portable meters for field use to integrated safety testers for the production line.

Portable insulation resistance testers (Megabras)
For standalone insulation measurements on cables, motors, transformers and installations. Available in 1 kV, 5 kV, 10 kV and 15 kV versions. All models are battery powered, IP65 rated and suitable for field work. View the complete overview.

Safety testers (Schleich)
For end-of-line testing of electrical products, the GLP2-Modular and GLP2-Basic HV combine insulation resistance measurement with PE measurement, high voltage test and functional test in one system. For compact applications, the GLP1-g is available.

Motor test systems (Schleich)
The MotorAnalyzer 3 and MTC2-R7 combine insulation resistance measurement with surge test, PI/DAR, resistance measurement and partial discharge for complete motor diagnostics.

Rental
Need temporary extra capacity? IONIO rents out both insulation resistance testers and safety testers.

Need advice on the right measurement solution? Schedule a consultation – our specialists are happy to help.