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Isolation transformers

  1. Feb 5, 2015 #1
    hello!

    isolation transformers are supposedly protecting from electric shock

    without them, the voltage source is connected to a circuit and the circuit is connected to earth via an earthing wire.

    if you touch the live circuit, a closed circuit will be formed that will give you electric shock: the current will flow from the live circuit through you to the earth and from the earth to the earthing cable

    so the purpose of isolation transformer is to actually isolate this earthing cable, so that there will be no path for the current to return to the live circuit and thus form a closed circuit

    am I right?

    my question is:
    after installing an isolating transformer, don't we lose the earth wiring protection of the circuit? which serves the thunder protection?
     
  2. jcsd
  3. Feb 5, 2015 #2

    sophiecentaur

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    You seem to have the right idea. When you isolate the supply and connect (via your body, perhaps) any part of the equipment to Earth, then that part of the equipment acquired Earth potential with only a very small flow of current and you do not die or even feel anything. The two supply wire potentials will 'pivot' around this point and have the same PD between them as ever. This is perfect protection for a single fault in an otherwise perfect system. However, if someone else touches a different point in the isolated system then current can flow through both of you and kill you. It need not be another person - just any path to earth. The system is no longer floating and gives you no protection at all. If there is a third, Earth cable then, of course, any exposed metal will be held at 0V and you would be protected as normal. The two safety devices are not really equivalent (and are not mutually exclusive), though and an Earth cable is much more 'fail safe' and passive than a system that you assume to be balanced. An earth network will help give lightning protection in any case.
     
  4. Feb 26, 2015 #3

    tech99

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    There is an impossible problem regarding lightning protection when using isolation transformers because the two requirements are in conflict. For instance, when a transformer is supplying lights on a high mast. There is a danger that a moderate static discharge, perhaps one induced on the power line, can flash across the transformer and destroy it.
     
  5. Feb 26, 2015 #4
    Yes, but in the most of situations, good lightning protection of primary circuit is sufficient.
     
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