Insulation Resistance Test
The insulation resistance (IR) test (also commonly known as a Megger) is a spot insulation test which uses an applied DC voltage (typically either 250Vdc, 500Vdc or 1,000Vdc for low voltage equipment <600V and 2,500Vdc and 5,000Vdc for high voltage equipment) to measure insulation resistance in either k[math]\Omega[/math], M[math]\Omega[/math] or G[math]\Omega[/math]. The measured resistance is intended to indicate the condition of the insulation or dieletric between two conductive parts, where the higher the resistance, the better the condition of the insulation. Ideally, the insulation resistance would be infinite, but as no insulators are perfect, leakage currents through the dielectric will ensure that a finite (though high) resistance value is measured.
Because IR testers are portable, the IR test is often used in the field as the final check of equipment insulation and also to confirm the reliability of the circuit and that there are no leakage currents from unintended faults in the wiring (e.g. a shorted connection would be obvious from the test results).
One of the advantages of the IR test is its non-destructive nature. DC voltages do not cause harmful and/or cumulative effects on insulation materials and provided the voltage is below the breakdown voltage of the insulation, does not deteriorate the insulation. IR test voltages are all well within the safe test voltage for most (if not all) insulation materials.
The Megger company were the original manufacturers of IR test equipment over 100 years ago and have become synonymous with insulation resistance testing. Most modern IR testers are digital, portable / handheld units and some have multi-functional capabilities (e.g. built-in continuity testing).
Firstly ensure that the equipment to be tested and the work area is safe, e.g. equipment is de-energised and disconnected, all the relevant work permits have been approved and all locks / tags in place.
Next, discharge capacitances on the equipment (especially for HV equipment) with static discharge sticks or an IR tester with automatic discharging capabilities.
The leads on the IR tester can then be connected to the conductive parts of the equipment. For example, for a three-core and earth cable, the IR test would be applied between cores (Core 1 to Core 2, Core 1 to Core 3 and Core 2 to Core 3) and between each core and earth. Similarly for three-phase motors, circuit breakrs, switch-disconnectors, etc the IR test can be applied at the equipment terminals (and earth connection).
Note that when applying an IR test to earth, it is good practice to connect the positive pole of the IR tester to earth in order to avoid any polarisation effects on the earth.
Once connected, the IR tester is energised for a typical test duration of 1 minute. The IR test measurements are recorded after 1 minute.
When the IR test is finished, discharge capacitances again for a period of 4-5 times the test duration.
Interpretation of Test Results
The minimum values for IR tests vary depending on the type of equipment and the nominal voltage. They also vary according to international standards. Some standards will define the minimum IR test values for the general electrical installations.
For example, for low voltage installations in the IEC world, IEC 60364-6  Table 6A gives the minimum IR values and also suggests test voltage, i.e.
|Nominal Circuit Voltage (Vac)||Test Voltage (Vdc)||Insulation Resistance (M[math]\Omega[/math])|
|Extra low voltage||250||[math]\geq[/math]0.5|
|Up to 500V||500||[math]\geq[/math]1.0|
In the ANSI/NEC world, the standard ANSI/NETA ATS-2009  provides test procedures and acceptance levels for most types of electrical equipment. Table 100.1 provides representative acceptance values for IR test measurements, which should be used in the absence of any other guidance (from the manufacturer or other standards):
|Nominal Equipment Voltage (Vac)||Min Test Voltage (Vdc)||Min Insulation Resistance (M[math]\Omega[/math])|
|34,500 and above||15,000||100,000|
NFPA 70B  also provides some guidance on insulation resistance testing for different types of equipment.
Factors Affecting Test Results
There are two main factors that will affect IR test results:
Electrical resistance has an inverse exponential relationship with temperature, i.e. as temperature increases, resistance will decrease and vice versa. Since the minimum acceptable IR test values are based on a fixed reference temperature (usually 20oC), the measured IR test values must be corrected to the reference temperature in order to make sense of them.
As a rule of thumb, the resistance halves for every 10oC increase in temperature (and vice versa). So if the measured IR test value was 2M[math]\Omega[/math] at 20oC, then it would be 1M[math]\Omega[/math] at 30oC or 4M[math]\Omega[/math] at 10oC.
ANSI/NETA ATS-2009 Table 100.14 provides correction factors for IR test measurements taken at temperatures other than 20oC or 40oC, which were in turn based on the correction factors in the freely available Megger book "A stitch in time..." .
The presence (or lack) of moisture can also affect the IR test measurements, the higher the moisture content in the air, the lower the IR test reading. If possible, IR tests should not be carried out in very humid atmospheres (below the dew point). While there are no standard correction factors or guidance for humid conditions, it is good practice to record the relative humidity of each IR test so that they can be used for baseline comparisons in future tests. For example, having past data on the IR test values for dry and humid days will give you a foundation for evaluating future test values.
For equipment maintenance, the Dielectric Absorption Test is normally performed in conjunction with the IR test using the same testing equipment. The results are either in the form of a Dielectric Absorption Ratio (DAR) or a Polarisation Index (PI). Refer to the Dielectric Absorption Test article for more details.
- 1. IEC 60364-6, "Low voltage electrical installations - Part 6: Verification", 2006
- 2. ANSI/NETA ATS, "Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems", 2013
- 3. NFPA 70B, "Recommended Practice for Electrical Equipment Maintenance", 2013
- 4. Megger, "A stitch in time - The Complete Guide to Electrical Insulation Testing", a free book which is an excellent resource on IR testing