bench power supply not grounded

Discussion in 'General Electronics Chat' started by casulica, Jun 25, 2013.

  1. casulica

    Thread Starter New Member

    Apr 17, 2013
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    Hi everyone,

    Maybe its just a stupid question but, hey, I rather ask experienced guys like you than to risk getting a nasty shock!
    I just bought my first power supply(ebay cheapo), and just wondering ... Is it safe to use it if its not grounded(it has only 2 prongs), or should I change it to a 3 wire cable with 3 prongs,with the ground wire connected to the metal case of the power supply?

    And also the funny thing is that it has 3 output connections , + , ground, and -.
    Why would it have the 3rd output connection(ground) when it's not even grounded that damn thing!!!

    Thanks!
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Can you give us more details?

    You say it has a metal case?

    But you say the mains supply lead is only two core or has the earth core been truncated?

    You say the supply terminals are marked + earth -

    Is this correct or are they marked + 0 -?

    If it has a metal case it would be illegal in the UK not to ground the that case for safety reasons which would still apply wherever you are.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    197
    Can you post some clear photos of the unit?
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    What country are you in and what psu model number is it?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    In general, yes you should use a three-wire power cable to ground the chassis.

    The ground output pin connects to the chassis. Without a connection to the power line ground, it probably doesn't do much.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    513
    Hey guys we don't know enough about this psu.

    The actual power supply may be floating and the ground terminal (if it is one) may be to ground the case, perhaps in an equipment rack to the rack ground. It may not be to supply ground to anything!

    Bear in mind this is an unknown, possibly second hand unit off Ebay.
     
  7. Stuntman

    Active Member

    Mar 28, 2011
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    47
    Few thoughts.

    A.) Yes, that is silly that it has a ground terminal yet no ground to your outlet.

    B.) A simple continuity tester will tell you if the ground terminal is at least tethered to the chassis. Find a bare piece of metal on the chassis to probe between it and the ground terminal.

    C.) In a few minutes, a hacked computer cable could be rewired into the PSU to fix it. Often, the line power wires hook on with solder lugs or 1/4" quick disconnects.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    3,371
    Just to reiterate what others have said.

    1) Tell us what country you are in so we know what are the AC mains in that country, or show a photo of the AC mains outlet.

    2) Tell us the make and model of the PSU and take pictures of the front, back and power cord.

    3) Take and ohmmeter and measure the resistance between the GROUND terminal and the metal chassis.

    4) Disconnect the 2-conductor power cord and install a 3-conductor power cord with the proper grounded power plug.
     
  9. casulica

    Thread Starter New Member

    Apr 17, 2013
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    Hi guys, I have attached a few photos of the power supply, it clearly shows that it has only 2 wires which connects to the outlet.
    A brown (hot) with a fuse in series and blue(neutral), and there is no ground wire.
    Also thats why I was saying how funny this thing is because it has an output ground which is connected to the case(it can be seen in the picture) but the power supply itself is not grounded!!!

    And btw, I live in Canada.
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,452
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    That PSU would not meet CSA approval.

    The power switch is wired incorrectly.
    Rip out the 2-conductor cable and install a 3-conductor cable:
    BLACK - LIVE
    WHITE - NEUTRAL
    GREEN - GROUND

    BLACK - FUSE - SWITCH - TRANSFORMER
    WHITE - TRANSFORMER
    GREEN - CHASSIS

    Put some heat shrink tubing over the fuse connections.
     
    ErnieM and casulica like this.
  11. casulica

    Thread Starter New Member

    Apr 17, 2013
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    ok, so should I change it to a power cord with 3 wires and connect the ground to metal case?
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Yes. But just as important, you have to rewire the power switch.
    The switch goes between the fuse and the transformer.

    Also, put some heat shrink tubing over the switch connections.
     
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  13. casulica

    Thread Starter New Member

    Apr 17, 2013
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    thank you!!!
     
  14. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Yes, Mr Chips is correct this unit dangerous, and his advice sound.

    The above is an often missed point that it is part of the wiring regulations that any conductor at live potential (eg the end of the mains fuse as shown) must be covered so that it cannot be touched.
    Don't only insulate the fuse terminal, make sure there are no other mains voltage bare conductors accessible inside the box.

    From the photos there would appear to be adequate spacing between conductors for the potentials involved.

    Yes

    From what I can see there is a cable retainer at the entrance to the case. This must be maintained.
    Also the earth wire must be the longest of the three so that if the cable is yanked then the earth will be the last wire to separate.

    go well and safely
     
    Last edited: Jun 27, 2013
  15. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
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    I almost buy this power supply too, guess I need to look for another one now
     
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