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Is this how a CU Fire might start?

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  • 27-02-2024 9:34am
    #1
    John.G
    Registered Users Posts: 3,510 ✭✭✭


    Stayed in daughters house at weekend, had a shower on Saturday, checked trip switch on the RCBO a few hours later, RCBO tripped & reset . Got a call on Sunday morning, no power on shower, found no continuity between the RCBO live contacts, renewed RCBO. Checked shower power at 8.8kw (9.0kw shower) using the smart meter flashing LED. Broke open the failed RCBO (Hager 40A/30ma) and found signs of overheating, the contact (shown below) closes downward, there is a layer of what looks like melted plastic as shown by the arrow which was attached underneat the contact as well which probably prevented the contact closing or it closed with this plastic insulation preventing continuity, I get OC on the M.Meter when testing from the blob of plastic to the contact.

    I wonder what would have happened if I hadn't tested the RCBO?, I do test it ~ once/annum, this was the original RCBO installed when the house was built in 2006.


    RCBO failed contacts..jpg




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    #2
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    Interesting. Did she blame you for futhering with it? 😁

    Was the plastic housing around the contact area damaged or brittle in any way or were there signs of heat damage to the area surrounding the contact chamber? Was there an arc chamber within the device and how did it look?

    In theory you might have triggered the RCBO when the load voltage was on or near it's highest point on the sine wave (rather than at the zero crossing point) and this may have created an arc which wasn't quenched properly or generated excessive heat, etc, or maybe the UV off the arc damaged the plastics over time (which the plastics should be resistant to). But at the end of the day these devices are normally rated for thousands of operations and testing should not cause a failure. But we all know that stuff wears out.



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    #3
    John.G
    Registered Users Posts: 3,510 ✭✭✭John.G


    No, not really, no signs of big heat damage, don't know about the arc chamber as I had to really break it up.

    I never tested this RCBO with the shower on so it never broke a 40A load.

    I test my own own every 6 months, no load. but I did test it on load right at the shower terminals with a resistance between L & E way back when I installed it in 2007.

    Post edited by John.G on


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    #4
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    https://www.boards.ie/discussion/comment/121822826#Comment_121822826

    Although switching while under load is detrimental alright (causes pitting of the contact surface), it's not the amp or watt load which I'm referring to, it's the voltage point on the sine-wave which can also cause an arc. Some of the more advanced switches use zero-crossing detection so that the contact is only opened when the AC waveform voltage is crossing the 0v threshold. But I think that's here-nor-there, in fairness.



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    #5
    John.G
    Registered Users Posts: 3,510 ✭✭✭John.G


    To my simple way of thinking, it looks like this RCBO was failing under a normal load even if approaching its nominal rating of 40A, I have occasionally checked mine for voltage drop across the poles under load but never pick up anything but maybe a 0 to 200V range is too high or there simply isn't any.



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