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MEGGER TEST – When is a fail not a Fail?
Most electricians understand a Megger Test is a test of the insulation properties in devices, such as electric motor windings and high power antenna mounts, using much higher voltages to check resistance than a normal Volt-ohm meter.
Basically one plugs in a product into an electrical device and if the light comes on it “fails”, right? Possibly not…….Some power protection products that contain voltage limiting components, like MOV (metal oxide varistors) or Filter RFI (Radio Frequency Interference) or EMI (Electro Magnetic Interference) or contain an earth detection circuit may register a fail but in fact be perfectly fine.
The reason is that there is a relaxation in the AS/NZS 3760 Standard for the test voltage to be reduced to 250V DC instead of the 500V DC for products that contain current limiting components, filtering and earth detection circuits.
Alternatively there is the option of carrying out a leakage current test as explained in the extract from the Standard below…
The AS/NZS 3760 Standard states Appendix B: Insulation Testing of AS/NZS 3760:2003, in section 2.3.3.2, states the leakage current test should be performed at rated voltage with values not exceeding 5mA for Class I appliances or 1mA for Class II appliances.
“Alternatively, measure insulation resistance values do not exceed 1MΩ for Class I and Class II appliances at 500VDC or alternatively, to avoid the equipment apparently failing the test because the MOV or EMI suppression has triggered, for equipment containing voltage limiting devices such as MOVs, or EMI suppression at 250VDC.
“Note: Insulation resistance tests shall be performed with the switch in the ‘on’ position. If the equipment must be energised to close or operate a switching device, then a leakage current test shall be performed.
“2.3.3.2.1: Where equipment contains, by design, resistors installed between live conductors and earth, values of insulation resistance not less than the effective value of the resistors are acceptable provided the manufacturer can prove that the resistors are part of the design.”
Thor products have all of the mentioned component and circuitry that qualifies the products for leakage testing instead of the megger test. Please keep this in mind especially when dealing with power protection products.
For more information feel free to contact George Forster-Jones from Thor Technologies Pty Ltd, an Australian Manufacturer of Power Filtration and Protection products on 1300 766 140 or visit them www.thortechnologies.com.au
Date
Monday 09 Feb 2009 - Tuesday 09 Feb 2010
