An insulation resistance test is usually performed with a megger, an instrument that is not only capable of reading high resistance values, but is also able to produce the necessary currents and voltages to obtain the readings. Megger test potentials are usually applied at 500, 1,000, 2,500, and 5,000 volts DC. These potentials are obtained by using a motor driven or hand-crank operated magneto. The hand crank units are both lightweight and portable, and because they require no batteries or external source, they are also extremely dependable. Motor-driven units, on the other hand, are capable of achieving higher and more constant test voltages, but are practically useless without batteries or a external source. Both units are available in models capable of producing accurate readings for resistance levels as high as 100,000 megohms.
The following conditions should be observed when performing an insulation resistance test: Make sure that both the tank and core iron are solidly grounded. Disconnect any systems that may be connected to the transformer winding, including high and low voltage and neutral connections, lightning arrestors, fan systems, meters, and potential transformers.
Potential transformers are often located on the line sides of breakers or disconnects; when the disconnector is opened, there will still be a path available to ground. Short circuit all high and low voltage windings together at the bushings connections; jumpers should be installed to ground, and no winding should be left floating. The ground connection on grounded windings must be removed. If the ground cannot be conveniently removed, the test cannot be performed on that winding. Such a winding must be treated as part of the grounded circuit.
Using a megohmmeter with a minimum scale of 20,000 megohms, measure the insulation resistance across the connections as shown in following figure as a maintenance and acceptance test.
The terminal markings are referenced as follows:
The L terminal is the line or “Hot” terminal of the instrument, where the test potential is generated. The E terminal is the “Earth” or ground connection. The G terminal is the “Guard” terminal, it is used to isolate a certain portion of the circuit from the test.