Fail Of The Week: How I Killed The HiPot Tester

Have you ever wired up a piece of test equipment to a circuit or piece of equipment on your bench, only to have the dreaded magic smoke emerge when you apply power? [Steaky] did, and unfortunately for him the smoke was coming not from his circuit being tested but from a £2300 Clare H101 HiPot tester. His write-up details his search for the culprit, then looks at how the manufacturer might have protected the instrument.

[Steaky]’s employer uses the HiPot tester to check that adjacent circuits are adequately isolated from each other. A high voltage is put between the two circuits, and the leakage current between them is measured. A variety of tests are conducted on the same piece of equipment, and the task in hand was to produce a new version of a switch box with software control to swap between the different tests.

This particular instrument has a guard circuit, a pair of contacts that have to be closed before it will proceed. So the switch box incorporated a relay to close them, and wiring was made to connect to the guard socket. At first it was thought that the circuit might run at mains voltage, but when it was discovered to be only 5V the decision was made to use a 3.5mm jack on the switch box. Inadvertently this was left with its sleeve earthed, which had the effect of shorting out a DC to DC converter in the HiPot tester and releasing the smoke. Fortunately then the converter could be replaced and the machine brought back to life, but it left questions about the design of the internal circuit. What was in effect a logic level sense line was in fact connected to a low current power supply, and even the most rudimentary of protection circuitry could have saved the day.

We all stand warned to be vigilant for this kind of problem, and kudos to [Steaky] for both owning up to this particular fail and writing such a good analysis of it.

Our Fail Of The Week series has plenty to entertain the reader who is not of a nervous disposition. This isn’t the first fail to feature a suspect bit of connector wiring, not an unexpected short or even some magic smoke.

18 thoughts on “Fail Of The Week: How I Killed The HiPot Tester

  1. TRS connectors are guaranteed to short some of the pins together on insertion/removal. You have to be *absolutely* sure the hardware connected to it can handle that before choosing it as a connector… even then, you should probably find something else :D

    1. Hmm? How about using the right thing for the right task? IMO that’s the main things an engineer is trained to do…

      I’ll also note that designing hot-plug interfaces for non-trivial tasks can be very hard.

    1. pelrun is right because if a trs connector is an earphone connector they can short the contacts upon connections case in point it is possible to get mono briefly on stereo connection until the plug is in all the way.

      1. And the inverse is also true. When I was a kid I used to listen to tapes on a mono tape player but I only had stereo earphones. Carefully half-inserting the plug would get me sound on both ears. :)

        1. I’ve been in a certification lab that has power switching to 3 test bays. The input is switched with large jumpers that are like 0/0 gauge banana plugs. They can provide up to 480V at 40A with this!

    1. Apple should release a limited eddition iPhone with 4-wire speakon connectors, ya kno, for portability of course.

      Back to topic:
      Could of been worse, i.e. some critical function IC or so…
      At least it didn’t cost an arm and a leg to repair, that’s what we hacker/maker/engineer/repairer people are about.
      Acheive the most with the least.

  2. I thought of using the same technique of an 1/8″ connector for a laptop extension on the DC side, I believe I put the positive on the sleeve and left ring open and the negative on the tip so it wouldn’t touch anything accidentally.
    so 19 V DC with about a 3A supply. it worked great and it even solved my need for a quick breakaway that wouldn’t yank the laptop off the table.
    I realized it was actually melting the socket and connector on every insertion and removal.
    what would happen is even though there was a separation, the sleeve collar on the Jack would short between the tip and the sleeve of the 1/8 connector.
    the PCB socket didn’t fare so well, the connector fared a little bit better since it was an expensive metal jacket right angle one. i ended up replacing everything with “M” connectors.
    3-5x revisions later I finally have what I wanted of an modular extension system with standard connectors.
    however I am now left with a set of 3.5 MM connector and socket that are melted that I don’t dare use for anything else. it’s hard to see in the photos that the insulating rings are dis-formed.
    I would not recommend 1/8″ connections for anything above 6V and/or 0.5A.
    and now the photos… I wish I had more of the damage however it has been few a years since now. ( a note to past time traveling self’s, take more photos ).

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