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Miter saw electrical fault (when class-II isn't class-II)

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  • 08-08-2023 2:38pm
    #1
    10-10-20
    Registered Users Posts: 6,661 ✭✭✭


    More of a whinge and word of warning than anything else, but I had my mitre saw out yesterday to complete skirting board work at home. It was setup on a workbench on the patio and I powered it off a fully unwound extension lead with an RCD fitted. The power was coming from a sub-board in the shed which has an 20A/30mA RCBO and that all comes off a 20A MCB in the house (double-RCD'd, yep, I know). I was using it on and off for about two hours, no faults or problems were noted by me.

    Anyhow I was inside putting on the dinner and my son was outside messing with the dog while I was inside. Curious about all of the dust he went and rubbed a spot of saw-dust off the base of the saw and felt shock and a slight jolt up his arm, so hollered into me. I initially thought it was probably static from the hairy dog, but went out and isolated the saw off the extension lead and had dinner.

    Afterwards I went out to test the saw as static shouldn't have given such a jolt, so I plugged it back in and measured the potential between the earth pin on the extension lead and the base of the saw. Initially it read 16v, jumped to 60v, then down to 0v for a few seconds then back to 16v and then up to 233v. Tried it again and observed the same type of voltage signature; very odd. So I did a basic test of the extension lead - L to E was showing 233v, L to N was showing 233v, N to E was showing 0v; a plug-in tester also found no problems. The saw is marked as double-insulated on the sticker.

    So then I had a theory that maybe the voltage on the saw was induced current from the wiring, so decided that I should probably measure it, or at least see if it's something to be concerned about. I decided that I'd run a wire from the base of the saw to the earth pin in the extension lead - see if it trips the RCD.

    image.png

    Anyhow, I did so and plugged the saw in... bang! The RCD on the extension lead dropped, the RCBO in the shed dropped and the 20A MCB in the house dropped.

    Within a few minutes I had the unit opened up and found the cause. A screw-damaged wire:

    image.png

    This is in spite of the device being "double-insulated". I pulled off the cover to see if it was earthed (which it should not be as it's supposedly a class-II device), and sure enough, no earth.

    image.png

    Shoddy wire nuts though! There is an EMF/filter capacitor on the circuit just below the wire-nuts, that's what I think was causing the variable voltage readings on my DMM as it initially discharged and then charged up while I was measuring it.

    To keep a long story short:, I had been using the saw with a pair of work-boots with rubber soles while my son was only wearing socks, hence he was a less resistive path to earth on the patio.

    I'll rewire the saw so that it has an earth and tap it into the saw's head-stock and base. I'll also replace the wire nuts and that damaged stranded cable. But yeah, RCD's are potentially a life-saver.



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