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B Emf without load and load voltage of a generator

  1. Aug 1, 2017 #1
    Hello


    Last week I was confronted with a situation where an industrial equipment, with a large equipotential mass (metal casing) that was not grounded, had a ground tension, measured with Fluke, of 382 vac. I put a 220 Vac bulb and nothing happens, the bulb remains off and is not burned out.

    By measuring the voltage with the lamp as a charge it is a few millivolts. I even touch the carcass without feeling anything.

    My interpretation is:

    The voltage measured with the impedance of the fluke (Zfluke) >>> 20 M Ohm which is surely much higher than the internal impedance of the generator that is seen between the ground and the carcass of the equipment (E generator, Z generator), gives a high voltage (Vfluke) (voltage divider).

    As soon as a 100 watt bulb is placed with an impedance close to 500 ohm (Z bulb), this impedance is considerably less than the internal impedance of the generator seen between the ground and the carcass of the equipment, which makes That the voltage at the voltage divisor point drops to the millivolt range.


    Thank you
     
  2. jcsd
  3. Aug 1, 2017 #2
    You have the basic idea. Leakage current through the insulation system increases voltage from the electrically insulated metal case to ground, and it will approach applied line voltage. I strongly recommend adding a conductor to bond the metal case to ground. As the insulation system ages it will pass progressively more current. Once it is capable of currents of 1 ma and more it becomes a safety hazard, and not too much more leakage current can be lethal.
     
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