What is a megger or insulation resistance test?
Headcore
Megger test: the everyday name, taken from an instrument brand, for an insulation resistance test, in which a high DC voltage is put across cable, board or winding insulation and its resistance read in megohms (MΩ).[5][6]
Also called Insulation resistance test, Megohmmeter test
What is a megger test?
A megger test is an insulation resistance test: a tester puts a DC voltage across the insulation, between conductors or from conductors to earth, and reads its resistance in megohms (MΩ).[5]
Megger is one instrument maker's registered trademark, and it has become the everyday site word for the test. The IET writes of "insulation resistance testing, or 'megger' testing as it is more commonly known".[5][6]
A low or falling reading shows insulation weakened by moisture, dirt, oil, heat or mechanical damage before it fails in service.[5]
In South Africa, a Certificate of Compliance (CoC) covers the electrical installation up to the point of consumption, which for a hard-wired machine is its supply terminals. The test report that goes with a CoC "covers the circuits for fixed appliances but not the actual appliances", in the words of the Electrical Conformance Board's template. In practice, insulation testing of motor and generator windings is done as preventive maintenance.[1][2][4][5]
Is an insulation resistance test required in South Africa?
Yes, whenever a CoC is needed. A completed or partly completed installation may not be connected to the supply "unless it has been inspected and tested by a registered person and a certificate of compliance for that electrical installation has been issued" (Electrical Installation Regulations, regulation 8(2)), and an addition or alteration needs a CoC "for at least the addition or alteration" (regulation 7(4)).[1]
The test follows SANS 10142-1, the wiring code the regulations incorporate, "or any updated revision of the standard" in the Department of Labour's words. The standard sets the test voltage and the minimum insulation resistance, and the registered person applies them. The current edition is 3.03, approved on 24 June 2026.[2][3][1]
- Only a registered person may issue a CoC (regulation 9), and must show the certificate of registration on request (regulation 11(4)).[1]
- An electrical tester for single phase may certify only installations supplied single-phase at the point of control; an installation electrician may certify any installation. Neither may certify installations in explosive atmospheres, the petroleum industry, hazardous locations or medical locations. A master installation electrician may certify any installation.[1]
What is a good megger reading?
A good reading clears the minimum for the job and holds steady between tests: "what really matters is the trend", in the words of Megger's guide. A flooded motor reading 1.5 MΩ after clean-up is fine if it always read 1 to 2 MΩ, and still wet if it used to read 10 or 20 MΩ. Readings roughly halve for every 10°C rise, so compare them at one temperature.[5]
| What is tested | Test voltage | Good reading | Basis |
|---|---|---|---|
| Installation wiring, for a CoC | Set by SANS 10142-1 | The SANS 10142-1 minimum, applied by the registered person | SANS 10142-1 and the Electrical Installation Regulations |
| Equipment rated 440 to 550 V | 500 V or 1,000 V DC | At least 1 MΩ; sound insulation reads far higher | Megger's guide |
| Equipment rated 2,400 V and above | 1,000 V to 2,500 V DC or higher; 1,000 V to 5,000 V DC or higher from 4,160 V | About 1 MΩ per 1,000 V of operating voltage, never below 1 MΩ | Megger's guide |
| Motor or generator winding, 10-minute test | As for its rating | Polarisation index (10-minute reading divided by 1-minute reading) of 2 to 4 is good, above 4 excellent; below 1 is dangerous | Megger's guide |
What should an insulation resistance test report show?
In practice, an insulation resistance test report shows the reading in MΩ for each distribution board, the instrument, whether the reading complies, and who did the test.[4][1]
- A reading for each distribution board, with the circuits on annex pages attached to the CoC test report.[4]
- The test voltage, the instrument and a compliant or not compliant mark against each reading.[4][5]
- What was disconnected for the test, and the parts of the installation the report does not cover.[5][4]
- The registered person's name, registration number and category, and the CoC number written on the test report.[1]
- For motors and cables on a maintenance plan: temperature, humidity and test duration, with the same connections and voltage as last time, so readings can be trended.[5]
Questions buyers ask
Can a megger test damage equipment?
Megger's guide calls the DC test basically non-destructive. It says to take the equipment out of service and disconnect it from other equipment and circuits first, and never to connect a megger to live equipment or use one in an explosive atmosphere.[5]
How long does a megger test take?
A spot reading takes a minute: Megger's guide recommends taking it at a fixed 60 seconds. A polarisation index test on a motor or generator runs for 10 minutes, with readings at 1 and 10 minutes. Isolating and disconnecting the equipment comes on top of that.[5]
How often should motors and cables be megger tested?
Megger's guide suggests every 6 to 12 months for motors, generators and other working equipment, depending on size and surroundings, and once a year for wiring unless plant conditions are unusually severe. Experience of the plant then sets the interval. Test after flooding too, which drops resistance suddenly.[5]
Sources
- Department of Labour, Electrical Installation Regulations, 2009 (GN R.242), 6 March 2009.
- Department of Labour, Explanatory notes on the Electrical Installation Regulations, 2009 (General Notice 258 of 2012), 26 March 2012.
- SABS, SANS 10142-1:2026 (Ed. 3.03), The wiring of premises Part 1: Low-voltage installations, 24 June 2026.
- Electrical Conformance Board of South Africa, Test report for all general electrical installations to SANS 10142-1 (template), undated, read November 2026.
- Megger, A Stitch in Time: The Complete Guide to Electrical Insulation Testing, 2006.
- IET Wiring Matters, History of insulation resistance testing, July 2020.