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Isolation Transformer Earth

Discussion in 'General Electronics Discussion' started by Marius Muller, Jun 22, 2017.

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  1. Marius Muller

    Marius Muller

    Apr 8, 2017

    I want to test switch mode PSU'S and want the frames to be safe to the touch.NO nasty shocks!
    1) My question; should I only ground the primary input and NOT the secondary of the ISO TFR? I supply input power to the switch mode via the secondary without earth connected?.
    2) Second question; When I use the isolation transformer to test ANY device such as a microwave, I plug the microwave into the secondary receptacle of the iso tfr ( not grounded) would that make it relatively safe to work on that device or any other machine such as a DVD player etc?
    Do the connections on the ISO TFR look correct and fuse on the Input earth side?
    Last edited by a moderator: Jun 22, 2017
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    An isolation transformer can protect you only from a shock from mains vs. earth. As the secondary of the transformer in not connected to earth, there is no use in connecting earth as a protective measure. In fact, the isolation transformers I know do not have an earth connection to the output connector.

    As said above, an isolation transformer can protect you from electric shock from mains vs. earth. It will not protect you from an electric shock that results from touching the two output wires. There is still a 220 V (in your case) potential difference between these wires and the risk of an electric shock is present. In addition there is a risk of a thermal event (fire, explosion..) when a short circuit happens.
    Therefore using an isolation transformer makes things a bit more safe, but by no means completely safe.

    The fuse is the component in the lower right corner? From what I see this all looks o.k.
    You may want to add some sealing glue (sorry for my not having the correct English name at hand) on top of the screws to prevent them from accidentally becoming loose which may cause harm.
    Another connection that is in my opinion mandatory is to connect the metallic housing to earth (protectiv earth, green/yellow wire) such that in the event of a mains conductor becoming lose the user is still protected from harmful potential.
  3. duke37


    Jan 9, 2011
    The wireing looks correct but the fuse is NOT in the earth, it is in the neutral.

    As Harald says, it will not protect from all shocks but will only protect if you touch one place only.
    Steer away from microwave innards.

    I think the case is earthed through the transformer core but there would be no harm in checking it and duplicating this connection.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Provided the colors of the wires do have a meaning.
    I don't know anything about SA electrical standards. The plug seems to be polarized, so hot and neutral could be in defined positions (provided the outlet is correctly wired). In Germany and many otehr parts of Europe this wouldn't work as our plugs have 180 ° rotational symmetry: what's hot is neutral in the othe rposition and vice versa.
  5. Marius Muller

    Marius Muller

    Apr 8, 2017
    Ive got the fuse inline on the live wire and the transformer core and case are all earthed on the primary side.
    If I work on say something like a fridge,tv,vacuum cleaner,dvd player etc - will this transformer help to protect me from electric shock of course provided that I do not touch the neutral and live at the same time ?When touching the live only on the secondary is there No shock at all ?
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    No deadly shock if the transformer is well built.
    One cannot avoid a small coupling capacitance between primary and secondary of the transformer. SS you may experience a tickling feeling, but no harm should be done.
  7. Minder


    Apr 24, 2015
    Check out Sec V fig 6.
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