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 kΩ, MΩ or GΩ.
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.
IR test set (courtesy of Megger)
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).
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.
For example, for low voltage installations in the IEC world, IEC 60364-6 [1] Table 6A gives the minimum IR values and also suggests test voltage, i.e.
In the ANSI/NEC world, the standard ANSI/NETA ATS-2009 [2] 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):
NFPA 70B [3] also provides some guidance on insulation resistance testing for different types of equipment.
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 2MΩ at 20oC, then it would be 1MΩ at 30oC or 4MΩ 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..." [4].
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.
Test Equipment
Test Procedure
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 [1] Table 6A gives the minimum IR values and also suggests test voltage, i.e.
| Nominal Circuit Voltage (Vac) | Test Voltage (Vdc) | Insulation Resistance (MΩ) |
|---|---|---|
| Extra low voltage | 250 |  0.5
|
| Up to 500V | 500 | 1.0
|
| Above 500V | 1,000 | 1.0
|
| Nominal Equipment Voltage (Vac) | Min Test Voltage (Vdc) | Min Insulation Resistance (MΩ) |
|---|---|---|
| 250 | 500 | 25 |
| 600 | 1,000 | 100 |
| 1,000 | 1,000 | 100 |
| 2,500 | 1,000 | 500 |
| 5,000 | 2,500 | 1,000 |
| 8,000 | 2,500 | 2,000 |
| 15,000 | 2,500 | 5,000 |
| 25,000 | 5,000 | 20,000 |
| 34,500 and above | 15,000 | 100,000 |
Factors Affecting Test Results
There are two main factors that will affect IR test results:Temperature
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 2MΩ at 20oC, then it would be 1MΩ at 30oC or 4MΩ 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..." [4].
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