Extech Instruments MG500 User Manual page 12

Transformer Testing
Transformer tests are performed with the transformer completely disconnected from the line
and the load. Note that the case ground should not be removed.
The five tests listed below will completely test a single-phase transformer. Note that at least 1
minute should be allowed between each test.
1. High voltage winding to low voltage
winding and ground
2. Low voltage winding to high voltage
winding and ground
3. High voltage winding to low voltage
winding (pictured at right)
4. High voltage winding to ground
5. Low voltage winding to ground
Notes on Measuring the Resistance of a Contactor or Circuit breaker
A Contactor or Circuit breaker must provide a low resistance path to electricity when closed but
should provide a high resistance when open. As they age, their characteristics may deteriorate
because of heavy use, operation in a dirty environment, or other factors.
Unusual Insulation Resistance Measurement Applications
o Special paint or varnish may be used as an insulator on transformers and other electrical
applications
o Insulating blankets, suits and gloves can also be tested
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