Grounded neutral line Vs Isolating Transformer?

Discussion in 'General Electronics Chat' started by TheWeasel, Aug 28, 2009.

  1. TheWeasel

    Thread Starter New Member

    Aug 28, 2009
    Hi all, I'm new to this forum, and it looks pretty useful.

    I've just been reading up on mains electrical safety, and two of the safety features described seem contradictory.

    First I read that the earth terminal at the consumer's socket serves as an additional neutral connected to appliance metal casings and to the earthed N at the substation. If the case becomes live, then the earth line takes the current away, keeping the case close to 0V and preventing current through the person, and also blowing the fuse.

    Then, a few pages Isolating Transformer is explained as a safety device which prevents completion of a fault circuit through a person and to earth.

    If that's the case, then why not just isolate the supply from earth at the substation end, thereby providing the same benefit as an isolating transformer, but at no cost?

    OK so a live case won't then blow the fuse, but then it wouldn't if fed through an isolating transformer anyway.

    What am I missing here please?
    Last edited: Aug 28, 2009
  2. ifixit

    Well-Known Member

    Nov 20, 2008
    The power line distribution system needs to be grounded to prevent electric charges building up and to provide a discharge path for lightning strikes to the power lines.
  3. TheWeasel

    Thread Starter New Member

    Aug 28, 2009
    Of course! Thanks Ifixit.:)
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Another important reason is that if the whole circuit is floating, you can have a single fault (a short) from one power line to eg. the casing of a piece of equipment, and nothing happens, no fuse blows and no obvious indication.

    If another fault occurs on the other leg of the power line, even in a different building, then both faulty items are accidents waiting to happen - if people touch both simultaneously, both get electric shocks.

    With one line grounded, any hazardous fault will blow a fuse or trip a breaker without people being part of the trip path.
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    all metal housings are BONDED to earth. This is not a neutral. Neutral is bonded to earth at the supply and again at the service, only. This is your exposed wiring sink.

    An isolation transformation has no reference to ground, hence no potential difference (shock value).