[Jewelry making - Article 01309] - Re: [Orchid] Anodzing Power Supply

[Brian Symons]

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>          The isolation transformer just "floats" the 240VAC away from
>     GND. This means there is no longer a connection- as there is with
>     the line fed voltage. This does nothing to protect you from getting
>     zapped by the 240VAC 

    As stated the isolation transformer "floats" the mains and "breaks"
    any ground link.  Touching a "live" mains wire will not cause a shock
    unless there is some connection to ground or to the "neutral" or
    other related phase.  I have seen films of workers safely working on
    cables energised at 100's of thousands of volts because they were
    isolated from ground.  They were actually bonded to the high voltage
    cables that they were working on for safety! 

    The reason that I specified contact to EITHER side of the winding is
    safe is that if you get hooked across it (ie both sides of the
    winding) then the transformer will just see you as another appliance
    and happily fry you.  I have a few 240 volt  isolation transformers
    (Australian mains is 240 volts) and I can assure you that they do
    work as I stated to protect you against mains connection.  I have on
    more than one occasion inadvertently touched a "live" trace when
    using an isolation transformer without receiving a shock. The old
    rule for electrical testing of live circuits was to keep one hand in
    your pocket to prevent touching BOTH sides of the mains or ground. 

    An isolation transformer works well exactly as stated if it is in
    good condition.  It is also essential that there is no ground
    connection to the output winding - I have seen units that have had a
    bond fitted for some reason.  If you are going to use one for
    isolation purposes then I would recommend getting an electrician to
    insulation test (megger test) it to ensure no leakage to earth on
    either side of the output  before trusting your life to it. 

    The GFI - Ground Fault Interrupter, is also called Safety Switch,
    RCD - Residual Current Device, ELCB -Earth Leakage Circuit Breaker,
    Core Balance Relay and other names in different places. 

    Before these devices were readily available, the isolation
    transformer was the only protective device available.  It is still
    widely used to work (repair) on circuits that have mains potential
    circuitry or to isolate test equipment earths to prevent short
    circuits when testing such circuitry. 

    I am an electrician and I fitted all of my power and light circuits
    in my house with individual safety switch protection so I don't
    discount the effectiveness of that mode of protection.  It is another
    line of protection to me in case of an isolation transformer failing
    but that hasn't ever happened. 

    It is also important that only a single appliance is plugged into a
    isolation transformer at any time.  Although the chance is remote,
    if two items had faults to opposite sides of the output winding (eg
    to the case of each) and you touched both cases, then you would be
    effectively connected across the full potential.   In Australia,
    isolation transformers with output sockets are supposed to have a
    surround (often a pvc pipe fitting) to ensure that only a plug is
    inserted and prevent double adaptors etc being used for that reason. 

    As a last point, you were to use a standard "non isolating" variac
    with GFI protection and contact something earthed (say when
    anodising), it is possible to trip the GFI. 

Regards,
Brian.

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