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Mains Isolation transformers help

Discussion in 'Electrical Engineering' started by Andy & Kim, Jun 4, 2006.

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  1. Andy & Kim

    Andy & Kim Guest

    Hi guys, maybe you can clear something up for me. I use a 230-230v isolation
    transformer between the mains socket and some sound equipment as the supply
    is on a different ring most of the time from the equipment on the stage.
    (amps etc.) I don't suffer from earth loop and also feel it makes things
    safer. A few people on a sound group have suggested this is illegal and
    dangerous as it disconnects the PE. Could somebody explain the isolation
    transformer thing and tell me if I am in deed causing safety problems.

    Regards

    Andy
     
  2. Dale Farmer

    Dale Farmer Guest

    I can't speak to the legality, as I'm not familiar with UK laws.
    Safety I can speak on. You don't mention what you do the safety ground
    connection in the electrical supply. The ground must be connected to
    the building electrical ground system, preferably the same ground system
    as the rest of the equipment.
    If you have disconnected the safety ground connection, then you have
    set yourself up to have a dandy electrocution hazard. The purpose of
    the safety ground is to provide a lower resistance path to earth for
    fault currents than the one provided by your sweaty body. This diverts
    most of the electrical shock away from you, and hopefully allows enough
    current to flow to allow protective breakers or fuses to open up,
    stopping the electrical flow before they set your gear on fire.
    Power isolation transformers are a good solution to a few rather
    uncommon problems. But what it sounds like you are doing is breaking
    the safety ground connection and adding an isolation transformer.
    Rather dangerous, and probably a waste of time and resources. You would
    have better results by putting those resources into fixing the pin one
    problem (see AES paper #48, I think) that your gear has, and don't screw
    around with the power supply side of things. Work on the signal lines
    where the fault currents are measured in milliamps, and audio level
    isolation transformers only weigh an ounce or so.

    --Dale
     
  3. Andy & Kim

    Andy & Kim Guest

    Thanks for your thoughts Dale. The transformer I use is a off the shelf unit
    and there is no disconnection of the earth in the plug top or any of the
    outlets. I think I am asking is if this gives me better safer protection.
    Rather like a building site isolation transformer.

    Andy
     
  4. Dale Farmer

    Dale Farmer Guest

    Protection from what? The main thing an isolation transformer as I
    understand your usage, would protect you from power factor problems in
    your electrical supply, or if you are in a particulary grotty area for
    electrical power. So if you are next to a welding factory or something
    like that it is worthwhile.
    Read the AES paper on the pin one problem. Learn about ground loops
    and how to troubleshoot them. Better information can be found on the
    rec.audio.pro news group.

    --Dale
     
  5. TimPerry

    TimPerry Guest

    Andy, in days of old TVs and radios were often built with "hot"chassies. it
    was intended that these chassied be connected to neutral but it was possible
    to get the hot/neutral reversed. the chasseis were "safely" protected by an
    insulated cabinet however an unwary serviceman could be killed hence the use
    of isolation transformers in the shop.

    in addition in modern equipment some switching power supplys omit the use of
    an isolation transformer thus the use of an isolation transformer is
    recomended when servicing.

    the whole purpose for this is so that if the tech is somehow grounded to
    earth he can safely touch either AC wire (but not both) and not be shocked.

    in your case, you are using a (small) line transformer at FOH (front of
    house or the mixer position) to help break a ground loop to the power amps
    and thus reduce or eliminate an unwanted "buzz". i would not think that
    there was anything illegal about this (your local laws may vary) as long ts
    the transformer is not overloaded or modified. it not much different (IMO)
    then plugging in a battery charger.

    if personal safety is an issue (remember even big transformers can short)
    just add a GFI (GFCI) breaker after the transformer.

    personally i avoid the issue by habitually running power from the amp rack
    to FOH. i have a quad box of 1:1 audio isolation transformers for
    contingencies. the move away from unbalanced amplifier sends helps too.
     
  6. "Protective earth", but same thing.
    Also called CPC "Circuit Protective Conductor".
     
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