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Phantom voltages when testing dry type transformers

Due to the high impedance of measuring instruments, a voltage reading may be detected on open conductors where there is no hard electrical connection to a voltage source. Conductors that are installed in close proximity to one another, and are capacitively coupled to each other, can cause
this a.c. voltage reading. Such a reading could be 2 or 3 volts, or it may be as high as the voltage on the adjacent conductors. This is what is referred to as a “phantom” voltage. According to Underwriters Laboratories Inc., this can be a harmless reading and can be caused by the high input impedance of the measuring instrument, which places very little loading on the circuit under test. The capacitance is increased as the length of the run is increased. A 50-foot run may produce a pronounced capacitance effect whereas a one-foot sample may not produce
any. Since the “phantom” voltage is a physical phenomenon involving very small values of capacitance, it cannot energize a load or cause physiological damage to a person. Care must be taken to be sure that the voltage reading is a phantom voltage, which is caused by improper use of high impedance multi meters, and not as a result of a cable defect or improper installation, which may result in a shock hazard. In order to help minimize the likelihood of reaching a wrong conclusion from this phenomenon, NEMA recommends the use of a Listed low impedance multi meter in place of a high impedance multi meter or other high impedance measuring device for testing on open conductors where there is no hard electrical connection. Without a low impedance measuring device, a high voltage reading is an inconclusive indication of possible faults in the cable.


NEMA Phantom Voltages Bulletin-88.pdf

For a selection of Eaton's dry type transformers please see our Catalog Volume 2 Type 2.

  1. NEMA Phantom Voltages Bulletin-88.pdf