Isolation Transformers

Discussion in 'Homework Help' started by elecmatters, Jun 28, 2014.

  1. elecmatters

    Thread Starter New Member

    Jun 28, 2014
    I appreciate that the transformer structure means that the primary and secondary coils are electrically insulated, but I don't see how that make it a safety device. for a 1:1 transformer with AC on the primary, you will get equal voltage and current on the secondary coil and circuit as explained by faraday's law.

    Can anyone help explain how an isolation transformer is considered a safety device in the light of that, please?

  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    hi elec,

    Its because the Neutral line of the local mains supply is connected to Earth.
    So touching the Line wire will give you a electric shock to Earth

    On the output side of the transformer, both lines are Earth free.
    To get a electrical shock you must be across both output lines of the transformer.
  3. crutschow


    Mar 14, 2008
    A related aspect would be working on an electrical circuit powered by the line voltage. Without a transformer for isolation, even touching the circuit common could give a nasty shock to earth ground if the circuit neutral and hot connections are somehow reversed on the circuit (such as a miswire on the main's plug). If you have a transformer in between that can't happen.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    I would like to expand on what Eric said.

    The case/skin of almost every appliance, test equipment, power supply, o-scope probe, radio, etc. is connected to neutral/ground. If you are working on a hot circuit that does not have a transformer to isolate it from the line, you are in an electrical minefield. Even experienced technicians/engineers reconize that this condition is a foolish deadly risk. This is where a USB port will kill you.

  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    Of course there is a further protection in using a transformer.

    The old saying is

    'It's the volts that jolts but the mils that kills'

    This means that you can get an unpleasant shock from high voltage (or even low voltage) but unless the source can supply enough current, you will not die.

    Now, whilst it is true what you say about the secondary voltage, a transformer can limit the available secondary current to a safe level by magnetic saturation.

    This is the priciple of the shaving isolation transformer built into every bathroom shaving socket.

    A final point, a variac provides low voltage, but like any autotransformer, does not provide isolation.
  6. elecmatters

    Thread Starter New Member

    Jun 28, 2014
    Thanks to all who took the time to reply. Particular thanks to Ericgibbs, I think your answer helped the most.

    Studiot, could explain what magnetic saturation is and how this limits the secondary current, please?