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  1. #1
    Join Date
    Dec 2015
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    Default earth leakage trips in wet weather

    Refers to an old Scripta engravers pantograph currently used for miling aluminium and brass. Every time I switch it on during wet/humid weather it trips the circuit breaker. Leave it in the sun for a while on a sunny day and it's back in operation. Any suggestions about cause/repair/way around?

  2. #2
    BobL is online now Member: Blue and white apron brigade
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    I suspect the machine has a "near short" somewhere and it becomes "shorter" with increasing humidity.
    A Megger tester used in sequence will identify the problem. I'd start with the motor coils - how dusty are they - maybe blow these out with a compressed air line?

  3. #3
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    Default

    I agree with Bob about there being contamination in the electrics somewhere, but it doesn't need to have a particularly low resistance for this to occur, and I would suspect anywhere where cables connect to something else like a switch or connector block. If the machine is the only thing that triggers the RCD in the entire power system, the triggering current path needs to be 8K ohm or less, which would not generally be considered a "near short".

    However it is quite possible that there are other very minor leakage paths around the house as well, that normally do not divert enough power to trip the RCD regularly as the combined leakage current is below threshold level, but a minor leakage when you use the machine could combine with those and exceed the trigger threshold. Example of things that can generate minor leakage currents are mains input filters at the front end of switch mode power supplies in computers, refrigerators, TVs etc. These are intended to minimise RF interference backfeeding into the power cable and being radiated creating interference for other devices, but to do so, they also channel a very small current from both active and neutral power connections to earth.

    Things that I have tracked down and resolved include sawdust, ant nests, spider webs, dust etc within switch housings, junction boxs, and even mains plugs and sockets. I would be isolating the panto from the mains and opening every connection point in the chain from the power point to the motor and verifying that there is absolutely nothing within that that can harbour damp and provide a leakage path.

    I also have one of these testers which can generate small amounts of leakage to check RCD's and fixed wiring, as well as verifying wiring configuration at power points and in extension cords and power boards. To use the device in your situation, I would plug the tester into a power point with the control knob at the 10mA position, turn on the power point, and press the red button. Nothing should happen.

    Advance the control knob to the next position and press the red button again, you should be able to repeat this until you get to the 25mA position, when pressing the button might trip the RCD. If the RCD trips at settings lower than 25mA, this suggests that other items in the electrical system have leakage as well. This would be worth investigating, but the leakage is not overly dangerous, it hasn't started a fire or electrocuted anyone yet, and most likely never will, it just increases the RCD's sensitivity to leakage currents created by other devices when they are connected and activated.

    The RCD however should trip once you reach the 30mA setting and press the button, or the RCD has lower than claimed sensitivity and may not adequately protect your family.

    Bob's suggestion of having the complete unit tested with a Megger is also very valid. This will show up leakage that might not show with a multimeter because the Megger uses high voltage (up to 1000V)to test for insulation breakdown, whereas a multimeter is using 1-9V to test for continuity and this is often way to low to initiate leakage. DO NOT use a Megger to test electronic items, the high voltages that it creates will destroy virtually every semiconductor in existence and render the item being tested scrap.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  4. #4
    Join Date
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    Does anything else running and plugged into that outlet trip the RCD - or conversely, will that engraver trip the RCD when running from a diferent outlet?
    The reason I ask is: many years ago I chased my tail for a while tracking an earth leakage fault, only to eventually find a roofing srew had grazs a cable going to just one powerpoint in our old house.... on a rainy day, the tin roof would be live and leak to earth via the rainwater - on a dry day the fault could not be found...
    I eventually found it with a megger and disconnected circuit on a rainy day.....
    Cheers, Joe
    again completely retired - more time to contemplate projects and spend more shed time....

  5. #5
    Join Date
    Dec 2015
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    south coast, nsw
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    Well, the sun's out and I'm back at work. Thanks for all the suggestions. I tried all the easy ones without success, the more complicated testing is out of my areas of competence, it looks like getting a professional the next time it rains.
    Thanks again!

  6. #6
    BobL is online now Member: Blue and white apron brigade
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    I assume other things will run on the same GPO?

    Here is another trick that I have used occasionally. When the breaker is tripping try heating the motor or switch gently with a hair dryer or hot air gun and see if it starts.

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