I nsulation Resistance to Earth Low Voltage Generators 100 – 690VAC. Note When conducting high voltage test to earth, it is advisable to either disconnect or short out any electronic devices, such as the Automatic Voltage Regulator, (AVR), and Main rotor diodes. Short circuiting the terminals can be achieved with a piece of fuse wire, which must be removed immediately after the tests are completed. Caution: Running the Generator before removing the short circuit connection could seriously damage the Generator. When Megger testing a machine, failure to protect the voltage control unit and diodes could result in permanent damage to one or more of the electronic components . The resistance of the insulation between the copper conductors and the frame of the machine, (earth or ground), is measured by means of a high voltage tester, or “Megger”, which applies a D.C. potential of 500 or 1000 volts across the winding insulation . The high voltage causes a current to ‘leak’ through the insulation system. This current produces an output reading on the Insulation tester (‘Megger’), which is measured in Megohms (resistance to earth or ground). A normal value for a low voltage Generator winding should be higher than 1 Megohm to earth. Generators with an output voltage of between 100V to 600V should be tested as above. If the output winding (stator) is lower than 1 Megohm to earth, the windings should be cleaned, dried, or removed to a workshop for complete refurbish.
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