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Thread: Boiler element tripping safety switch - can this be intermittent - if so why?

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    Boiler element tripping safety switch - can this be intermittent - if so why?

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Recently I cleaned the boiler and replaced the boiler O-Ring in my Quaha Junior. I ended up opening the boiler twice during the cleaning and fitting. On both occasions it initially tripped the earth leakage safety switch with only one side of the heater element connected. But after a few minutes worked normally again and continues to do so. I put it down to some moisture in the wrong places that eventually dried up, but don't really know the cause.

    Anyway, today I picked up a Quaha Napoletana (cheapbay, supposedly working, couldn't resist - great for work or maybe even the holiday machine), but when I got it home it does exactly the same thing. With one particular side of the heater element connected and the other disconnected it trips the earth leakage switch. I've left it for an hour or so but it continues to trip the switch. The resistance of the element is a healthy 50 ohms.

    Can someone explain to me what is going on. If the element has a crack wouldn't this affect the resistance? Or at least wouldn't it trip the switch no matter which end of the element was connected? And if it is due to a damaged element why did my Quaha Junior exhibit the same problem twice but not any more?

    thanks
    Taan

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    I take it by the resistance test of 50 ohms you did was just the element. A crack in the outside insulation most likely won't effect the resistance you are measuring. You need to measure the insulation resistance to earth which you'll need a megger for. From memory it needs to be greater than 0.01Megohm. Heating elements are notoriously finicky with earth leakage devices
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    Try measuring the resistance from the heating element to the chassis

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    Quote Originally Posted by CaffeinatedAndrew View Post
    You need to measure the insulation resistance to earth which you'll need a megger for. From memory it needs to be greater than 0.01Megohm. Heating elements are notoriously finicky with earth leakage devices
    Quote Originally Posted by kaanage View Post
    Try measuring the resistance from the heating element to the chassis
    From either terminal to chassis It is a few hundred KOhms. Certainly more than 0.01 MOhm.

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    I am just wondering if this is something that I can resolve or whether to complain to the seller who sold it as used but working.

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    If the seller didn't have a RCD on the circuit (old switchboard) then they may well not have been affected

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    Quote Originally Posted by kaanage View Post
    If the seller didn't have a RCD on the circuit (old switchboard) then they may well not have been affected
    too true.... buyer beware. not the bargain I'd hoped for but still repairable.

    next question: I'll open the boiler and remove the element - can this be patched up / re-sealed, or am I simply looking at a new element?

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    Whilst I am not familiar with the workings of this machine, it may pay to fully isolate the element and make some resistance readings to earth on the rest of the circuit to make sure there is not some other source of leakage which is contributing to the earth current.
    If you don't have access to a megger, a digital multimeter on ohms range will give a rough indication. Also, if the machine has any control electronics it could be damaged by the high voltage from a megger.
    If all of the leakage seems to be confined to the heating element you could try thoroughly cleaning and drying the insulation around the terminals and drying the whole assembly for a couple of hours in a warm oven.
    I believe that the minimum allowable resistance to earth is 1 megohm and the maximum earth leakage current is 5mA.
    At the worst, the leakage could be due a hole corroded through the element sheath allowing water from the boiler to enter, in which case the element would have to be replaced.
    Trev
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    Quote Originally Posted by Xanthine View Post
    If you don't have access to a megger, a digital multimeter on ohms range will give a rough indication
    Nope, this test absolutely needs to be done with a megger on the 500v scale. Using a standard multimeter to check insulation resistance is absolutely pointless, may as well pick a number out of a hat.


    Quote Originally Posted by Xanthine View Post
    I believe that the minimum allowable resistance to earth is 1 megohm and the maximum earth leakage current is 5mA.
    Nope, Insulation resistance to earth has to be greater than 0.01Megohm for enclosed heating elements (everything else is 1Megohm, heating elements are the exception). There is no maximum earth leakage current rating for appliance's, it's all tested by insulation resistance and a number of other tests to meet compliance .

    The standard household rcd/elcb's must trip either before or at 30mA/300ms (the before is where there's good and bad points). Good - Trips earlier. Bad - those households with appliance that have sometimes finicky toasters, ovens, kettles will get annoying trips. Rcd/elcb's can be tested to get exact trip measurements and sometimes those households who have them on the more sensitive side of the scale swap them out for one with more tolerance. Or get there appliance repair/replaced.
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    Senior Member Xanthine's Avatar
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    Andrew

    Thanks for the info on insulation testing.
    You seem to know what you are talking about and I would not want to pick an argument with your comments.
    I was aware that devices with heating elements had a lower allowable insulation resistance but failed to fully comprehend when reading your reply.
    I must admit that I am a little confused by your statement that there is no maximum earth leakage current for appliances as I was under the impression that earth leakage current testing was an alternative to insulation resistance testing in certain types of appliances such as those with MOVs or non-mechanical switches.

    My suggestion for taking some measurements with a DVM was simply for getting some comparative readings which could indicate alternative leakage paths in Taan's machine on the assumption that he probably did not have a megger.

    Perhaps we were talking at cross purposes here between fault finding and tripping RCDs.

    Trev

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    KFT
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    With respect to Megger testing of elements most modern meggers have a 250 volt setting so as not to damage electronics. The minimum insulation resistance though on any setting should be 0.1 Mohms or 100,000 ohms. any less and the item is failed.

    If the issue persists it may be worthwhile getting the RCD tested to see if it is tripping at less than 30mA. Some do tend to do this after being in service for a while but then some of the cheaper units are like that from new.

    frank
    Last edited by KFT; 11th September 2015 at 07:22 PM. Reason: added info



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