88V to earth from secondary of 12V 10A power supply....

Discussion in 'The Projects Forum' started by bowlingo, Jun 18, 2012.

  1. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    Hi all,

    I had a 10A 12V power supply today of which is switching 2 x 2A 12V pumps wired in paralel. I had a relay switching the primary side of the transformer...This was working fine until yesterday when I put my hand in the tank and got an electric shock!

    I was then measuring 30 - 100V between + or - of the 12V secondary side (+ and - was giving 12.7V) to the concrete floor of the garage I had the pump running in. When I measured to earth on the back of a socket between the + or - I was getting 88 volts.

    The meter was saying the polarity was correct if I connected the black lead to + or - of the secondary 12V side and the red leas to earth..if I connected it the other way round it was saying reverse polarity.

    I then went off and bought another 12V 10A power supply and connected it as before then after a while the 8A relay switching the primary side stuck on (burnt out)

    Should I be switching the primary or the secondary of the transformer? I think switching the primary caused the transfomer to burn out but I have no idea why it would give 88V to earth from the secondary of a 12V transformer of which was giving a steady 12.7V between + and -

    I also added an earth to the - on the secondary side as when the problem happened it bought the voltage down to 0.2 - 2V

    Anyone any ideas please?

    Thanks
     
    Last edited: Jun 18, 2012
  2. mcasale

    Member

    Jul 18, 2011
    210
    12
    On many DC bench supplies, the output is floating from ground. Usually there is an earth ground plug somewhere so you can have either a positive or negative supply. It's not a good idea to let the DC float because it can be well above ground - as you found out.

    I don't know what your pumps are hooked to, but ALWAYS be sure your voltages are ground-referenced.
     
  3. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    can anyone help me with how to find out exactly what earth potential the sockets within the garage earths are at?
    Today I did a Zs test and was getting 0.42 ohms of which is good
    I also did an RCD test and it was disconnecting the main switch RCD within the consumer unit within the house at normal times...around 25ms at 1 x 30mA rated test and around 15ms at 5 x rated test.
    As I understand it if the RCD goes within the house and I have a good Zs it should all be at the same potential?
    I even plugged in an extension lead from a socket within the house and ran it out to the garage and was still getting the 88V from the secondary side to the sockets earth...

    In effect what I am doing is relying on the earth wire that ive connected to the pumps negative...if this breaks or comes off then I could be in trouble
     
  4. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Make a real earth ground by driving a metal rod into the ground or finding a metal water pipe that is buried in the soil and use that for a voltage reference.

    PS, I don't know what an RCD is. Can you spell it out for me?
     
  5. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I have taken the old power supply.....
    http://www.maplin.co.uk/12vdc-10a-power-supply-with-2.1mm-tip-513521
    home with me (different supply) and am still getting 88V DC between the + or - to the earth on the back of a socket and its 12.7V DC between the + and - of the supply
    Could I of damaged this by switching the primary on and off with a relay?
     
  6. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Your earth ground is not connected properly. You need to fix that.

    It is not likely that switching a power supply on and off will of damaged it, but that is not the problem. The problem is an improper earth ground.
     
  7. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I posted the faulty PSU to a friend up North and he has just e-mailed me with the below...

    That power supply was an odd one... there was no earth connection. Most of that type (laptop PSU) I could find about the place did have the output connected to earth. I do know that is is not the case on more and more power supplies. The problem with that unit of yours was that the output was not isolated from the input there is some capacitor link between input and output. I connected an ampmeter between ov on the output and earth and measured a constant 2mA AC. Although not lethal it will bite you as you well know. The other problem is ... it is more than enough to kill some electronics that you may have connected to a laptop ... so be careful using such PSU's. Maybe connect the output to earth if it is like that one....
     
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