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  1. Balanced power GFCI
    Discussion of AC mains isolation transformers
    8 hours ago, ray-dude said:

    Now that I have a couple balanced isolation transformers in place, time to harden the setup against people hurting themselves by plugging things in that don't play nice with balanced power.


    My understanding from searches and this thread is that the best way to protect against the formally neutral line (now -60V) creating a short to ground is to have a ground fault interrupt breaker between the balanced isolation transformer and the things I'm plugging in.  This is guess work, since I'm not a GFI expert.




    My tentative plan is to get a couple of these, hack off the plug end, and hard wire them to the output of the BIT's (3 outlets per BIT)


    For those that have gone through this research and come up with a good solution, could you please share your findings and solution, or give feedback on the suitability of the generic GFI tap cord I linked to above?


    Thank you again to everyone here that has been so generous in sharing their experiences and expertise!


    For a balanced connection, there are two lines that are above earth (180 degrees apart not quite -60V), so a single pole GFCI cannot be used. This involves a 2 pole type, like this one at Amazon. 10kA is a real overkill. seems to be the only type they have a quick search revealed, 3kA is more than enough, please find one of these, that's why the price is a lot higher.

    That breaker needs a proper box, glands, cable, not really an audio hobbyist venture. If you can find a double pole type in a plug in arrangement, would be better.


    Where one side is grounded on the output, you can use the yellow plug in job, single pole. Grounding the output neutral is important to keep the neutral at ground potential without causing the neutral to 'go high' which is a bad thing for equipment, in a small system like this, there should be less than 1-2mV difference if that between ground and neutral. Current will NOT flow in the ground (unless the HOT touches the frame or leaks to it).  I use this system for appliances, and computers where balanced supply doesn't work due to the two HOTs not being switched by the appliance, like lights. It's technically a TN network.

  2. Wiring
    Discussion of AC mains isolation transformers
    On 1/10/2018 at 6:11 PM, One and a half said:

    Each winding is rated at 120V.

    There are 2 x120V windings on the primary and 2 x 120V on the secondary.


    Normal supply

    use an iec class D breaker, don’t know the equal in Nema std.


    120V input, bridge the two windings together on the primary 

    H1 to h3

    H2 to H4

    Repeat bridging on the secondary 120V output.

    X1 to X3

    X2 to X4

    hot output on x1

    neutral output on X4

    ground the X4 and the shield together.

    Use a standard GFCI on the output.


    Capacitance is the same regardless of the windings connections.


    Balanced mode

    Connect primary with h2 h3 shorted

    connect hot to h1

    neuteal to h2



    short x2,x3, ground this point and the shield tog

    hot 1 x1

    hot 2 x2

    use a 2 pole GFCI-important it’s a 2pole


    On 1/13/2018 at 8:28 PM, One and a half said:

    Yes, there's a screw stud on the output box on the frame of the Topaz. Wire a green ground wire with an eye type crimplug from the screw, to X4 and connect the black flying wire that comes from within the transformer also to X4. The earth point on the primary runs through to the secondary box.

    The ground to the outlet remains on the screw stud , also with an eye crimplug. 


    44 minutes ago, seatrope said:

    Hi guys, @Superdad, @One and a half, @Speedskater @Johnseye and @Jud, apologies if I forgot anyone, HUGE thanks for all the guidance and advice.


    Sorry I was missing from the thread for a bit, several late nights/early mornings at the hospital for emergencies (I'm a physician) have me beat.


    @Johnseye you have hit the nail on the head. The diagram does not explain how to attach the ground wires on the input and output side. I'm going to devote a few hours tonight in trying to figure out how things should be connected, but if anyone has a photo (maybe @Superdad or @One and a half, perhaps superdad's USA setup would be more relevant?) that would be hugely appreciated!!


    The X1-4 and H1-4 parts are pretty self-explanatory, I hope. I plan on following Gary's guide from Headfi on testing, just had a insulation tester delivered today!


    Thanks so much to all of you, once again.



    The two posts I quoted from One and a half enabled me to connect it properly and get a correct ground indicator from the outlets.

  3. Discussion of AC mains isolation transformers
    Discussion of AC mains isolation transformers

    Okay, found the plate photo (click for larger image):



    And this post by @One and a half is an invaluable resource for anyone that buys a Topaz tranny:


    Great instructions on checking/verifying that all is okay.

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