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Failed Niglon B20 M.C.B.

Zoomup

A Niglon B20 M.C.B. has failed after about ten years use. It protected a summer house and small potting shed. The summer house has a 2.5kW space heater in it and the potting shed a small frost heater of about 750 Watts. The breaker became unreliable and stopping passing current but had not tripped off.

What would cause this to happen. Surely if overloaded the breaker would just trip off and not become damaged internally? Or would it? Could the contacts become damaged by over currents just below the tripping threshold, but above the current rating of the contacts?

Z.

  • 0

    MCBs are reasonably complicated and delicate mechanical devices, subject to heat stress and well as potentially damage from arcs and mechanical wear. They're dependent on springs and calibration of gaps for proper operation. Add in the potential for manufacturing errors and the natural variation in the materials they're made from and if anything I'm surprised they're as reliable as they appear to be. I've come across a few now that have failed (usually fail to reset). Thankfully they all seem to 'fail safe' (i.e. refuse to close rather than refusing to open on a fault/overload) ... but then perhaps if they did fail dangerously we'd be less likely to notice...

       - Andy.

  • 0

    Where is this? The main CU? If so, perhaps as Mike says an odd failure.

    If it's in an outbuilding, and not in an IP-rated consumer unit, external influences could be an issue. IP55 would give a longer life in outbuildings, but even then condensation occasionally might be an issue that causes early failure dependent on ventilation and actual location.

  • 0 in reply to gkenyon

    The M.C.B. was located in a new garage unit in a potting shed outdoors. The enclosure does show some signs of external  rusting but the innards are dry and are not affected.

    Z.

  • 0 in reply to Zoomup

    Just because the innards look dry doesn't mean mildew etc. haven't formed inside the device, and that there is no dust in there.

    It's difficult without being able to forensically examine it.

    For example, you say the outside has rust, but not the inside (e.g painted surfaces) ... but what about springs etc. in side the mcb?

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