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Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

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    Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

    Hello!

    I recently successfully repaired two Apple PowerMac G4 MDD power supplies by swapping out bad caps, per excellent info found in various threads on this forum - yay!

    I was doing testing on one of them, the ACBel API1PC36, with it's case open and had it setup outside of the PowerMac resting on the table with it's power connected to the system (power was also connected to a couple SCSI HDD drives, for a bit extra load). It was working fine, I shut down the system but did not unplug the PS from the power strip (and power strip was left on) so power was going into the PS.

    I touched the edge of the PS case (as noted with the red arrow in the photo) while I was going to unplug it, and got quite a decent shock.

    I certainly understand the danger of working with a PS with the case open, but of course am not expecting to get a shock from the grounded case. I'd been working with this PS for a bit like this, being extra careful to switch off the power strip it was plugged into before even touching the PS, unplugging it, etc. So now I'm not sure if this was a one off or if it was somehow doing this all along…

    I need to figure out how I can tell whether there's live current running through the case/ground somehow. The PS still works completely normal.

    It's of course possible that I may have simply accidentally touched something inside the case, especially since the side where I'd touched it is the AC side.

    How can I properly tell with a multimeter whether there's some sort of current running through the case?

    I measured voltage of the PS case and it's showing 2V when it's plugged in, though of course I had to connect the multimeter ground lead to something else (I connected it to either another PowerMac G4's case or a metal drive enclosure. I also get 2V when doing the same on another PS (a PowerMac G4 Samsung one) that I'm not getting shocked off of so not sure if that's expected.

    Just in case, I also confirmed the electrical socket (and power strip) it was plugged into is properly grounded (and correct polarity if that matters).

    I'm somewhat of a novice at power supply stuff, so any advice you may have, please be explicit.

    Thanks!!

    #2
    Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

    ... here's the photo
    Attached Files

    Comment


      #3
      Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

      The case of the unit is tied to the earth safety ground via green/yellow wire, so you need to check the resistance between the chassis and the ground prong of the AC plug to see if it shows low resistance of less than an Ohm (or the same as probes wires resistance).
      You also need to test the outlet strip ground resistance between the Outlet strip ground and the ground prong of the outlet strip AC plug, the plug the power supply into the strip then check the resistance between chassis of the power supply and the outlet strip AC plug ground prong.
      How did you verify that the AC outlet is grounded properly and not having high resistance back to the breaker panel?
      BTW, when you got shocked, one hand was on the power supply chassis, what other part of your body or the other had were touching?
      What AC V do you get when measuring between the chassis of the power supply and the AC outlet GROUND of the second wall socket where the strip is plugged into? You do have Duplex outlet on the wall, correct?
      Last edited by budm; 07-30-2020, 02:40 PM.
      Never stop learning
      Basic LCD TV and Monitor troubleshooting guides.
      http://www.badcaps.net/forum/showthr...956#post305956

      Voltage Regulator (LDO) testing:
      http://www.badcaps.net/forum/showthr...999#post300999

      Inverter testing using old CFL:
      http://www.badcaps.net/forum/showthr...er+testing+cfl

      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
      http://s807.photobucket.com/user/budm/library/

      TV Factory reset codes listing:
      http://www.badcaps.net/forum/showthread.php?t=24809

      Comment


        #4
        Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

        I confirmed there is at least continuity between the ground pin on the PS power socket and the case (as well as a few other ground points, like the PS screws, etc)

        For the AC outlet, I checked it was grounded by using a multimeter to confirm I was getting a voltage (120V AC) between the hot (short) socket slot and the ground slot.

        Comment


          #5
          Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

          How many Ohms of reading do you get? is that testing done when the power supply connected to the outlet strip?
          What Voltage do you get between Neutral and Ground?
          You do have Duplex outlet on the wall, correct? So what Voltage do you get between chassis and ground of the second outlet of the duplex? The test is done without anything connected to the power supply, correct?
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment


            #6
            Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

            The resistance of the power strip's ground pin to the PS case is good, less an an Ohm (with the PS power cable plugged into the power strip).

            I get the same, less than an Ohm measuring between the PS power connector ground pin and the PS case.

            When I was shocked, I don't believe I was holding or touching anything else but I really can't say for sure.

            "What AC V do you get when measuring between the chassis of the power supply and the AC outlet GROUND of the second wall socket where the strip is plugged into? You do have Duplex outlet on the wall, correct?"

            Correct, it's a dual socket outlet. I get about 1.2V AC measuring between the PS case and the ground pin on the 2nd socket, with the PS connected to the power strip, power strip on (thus power going into PS, but it's off). Nothing is connected to the PS in this case.

            I get no voltage testing between neutral (longer slot on AC socket) and ground.

            Comment


              #7
              Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

              BTW here's a zoomed out photo showing the exact setup it was in when I was shocked.
              Attached Files

              Comment


                #8
                Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

                Primary side heatsinks are often live, are you sure you simply did not touch one of the heatsinks?

                It would be impossible to be shocked by the chassis as long as you have a working PE (protective earth) conductor with low impedance to the breaker panel.
                If the PE was open however you could get a small tingle* when touching the chassis from the Y caps connected from Neutral and Live to PE.
                This would however not be a shock carrying high energy*, it would just be unpleasant...

                * = If the PSU where not a good brand like Apple but china crap with non-safety rated Y caps (the blue caps near the IEC receptacle in your unit) you could have a live chassis with high energy in this case.
                Last edited by Per Hansson; 07-31-2020, 08:48 AM.
                "The one who says it cannot be done should never interrupt the one who is doing it."

                Comment


                  #9
                  Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

                  Exactly, looking at all the data, I agree it's possible I simply touched something live inside the case (ideally don't touch one with the PS live and case open...). I see there's a copper colored heatsink relatively close to the edge of the case.

                  My concern is there's no way to be sure unless I touch it again with it in the same/similar setup to see if I get another shock - or better yet if there was some way to measure it.

                  Would it be possible for the PS to still work ok if the case was live?

                  Thanks

                  Comment


                    #10
                    Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

                    By this: "It would be impossible to be shocked by the chassis as long as you have a working PE (protective earth) conductor with low impedance to the breaker panel."
                    What I meant is that in such a case the circuit breaker in your panel would trip, this is the purpose of the PE: to provide a low impedance path that is good enough that the breaker trips.
                    If you do not have a working PE then yes, the chassis can be live and the unit still working.

                    To know if you would be shocked simply use a meter like you did: if you can not measure any voltage from the chassis to "ground" then it is safe.
                    I think you understand this subconsciously when you ask how to tell if it is safe:
                    There must exist a completed circuit for the voltage to go anywhere, and to be shocked you must be part of this circuit.
                    "The one who says it cannot be done should never interrupt the one who is doing it."

                    Comment


                      #11
                      Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

                      OK thanks all!

                      Comment


                        #12
                        Re: Shock when touching power supply case (ACBel API1PC36 from PowerMac G4 MDD)

                        " I get about 1.2V AC measuring between the PS case and the ground pin on the 2nd socket"
                        best check your house wiring really . because that should be same as a piece of wire in free air . 0v

                        Comment

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