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  1. #46
    Join Date
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    Without commenting on the rights or wrongs of T&T requirements, I threw out a heap of old Australian made laboratory power supplies today for a variety of reasons. Some failed the visual inspection miserably. Others failed the earth conductivity test. On many of them the aluminium front and rear panels had >20 ohms of resistance to earth (exposed metal has to be <1 ohm). It could have been more- the machine tops out at 20 ohms. I also failed a few IEC leads due to poor earth continuity. So there is some benefit to the T&T process.
    Chris

  2. #47
    Join Date
    Aug 2019
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    Revesby - Sydney Australia
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    Quote Originally Posted by racingtadpole View Post
    Cutting the plug off creates a bigger hazard as it creates a potential touch point at the back of the plug should someone put the plug into an outlet

    True. Remove the active pin/blade as well, just in case?

  3. #48
    Join Date
    Mar 2011
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    Adelaide
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    Nope. Put an Out of Service tag on it closest the plug end of the cord and give it back to the owner is the safest option.
    At the site I work on electricians get sacked if they cut cords off for failing appliance testing.

  4. #49
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nigelpearson View Post
    True. Remove the active pin/blade as well, just in case?
    Reminds me off the time when I went round to my sisters place to find accountant BIL play electronics with his 4 sons at the dining table.
    I'm not sure why. but they needed an earth so BIL had taken an earth pin out of a standard plying and had it naked poking out of a mains GPO with a wire attached running up to the table top.
    Apparenty they had been doing this fopr a few months and could not understand why I was speechless.

  5. #50
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    Jul 2010
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    Melbourne
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    Quote Originally Posted by jack620 View Post
    I threw out a heap of old Australian made laboratory power supplies today for a variety of reasons. Others failed the earth conductivity test. On many of them the aluminium front and rear panels had >20 ohms of resistance to earth (exposed metal has to be <1 ohm).

    Just how is this test done?
    I'd hate to think it would fail a front panel to front panel resistance test.

  6. #51
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    Nov 2007
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    melbourne australia
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    For Class 1 appliances the test checks the resistance between the earth pin on the plug and a metal part of the appliance. You plug the appliance into the machine and touch a probe onto the metal part.
    Chris

  7. #52
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Hi Guys,

    I believe that they stuff a known current into the earth connection and measure the voltage across it, about 1 amp I think. The only problem with this is it doesn't actually tell you where the high resistance is ! In the days of hand wired plugs it very often was the earth wire that was loose in the plug.

    A multimeter will give you an idea of the earth wire resistance, but it doesn't put enough current through the circuit to ensure that the reading is valid.
    Best Regards:
    Baron J.

  8. #53
    Join Date
    May 2012
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    Dandenong
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    43

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    Quote Originally Posted by BobL View Post
    Good point and I have thought of this and doubt they are under much stress.
    The resistors are rated at 1W.
    The tester has 250, 500, 750 and 1000V test settings.
    At least 2 x 1MΩ resistors in series are always in use
    The highest test V setting used is 750V so using V^2/R so power would be 0.28W.
    Even at 1000V, power would be 0.5W, but I don't use this to check the meter.

    Like I said its not meant to be a definitive but rather an indicative test. If they were breaking down due to exposure to the tester voltages I would expect their resistance to change over time but they're still reading the same as when I first set them up. I don't use my tester all that often, maybe 6 times a year but it sure is handy to have one at home.
    If you were concerned at possible variance, you could grab 4 of each value and create 2 parallel pairs connected in series. This will make any variance a much smaller ratio and give you closer to perfect values.

  9. #54
    Join Date
    Oct 2015
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    melbourne
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    Possibly not..
    Parallel series pair will average out the variation between them, but if they were all from the same batch, they are likely to be all very close anyway, even if on the edge of their tolerance.
    If you could get 4 completely random components you could end up with something closer.

    (unless manufacturing has changed a lot in the last 30 years

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