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Faults With RCBO's in Static Caravan in External Power Supply

Postby Southsidebobby » Fri Apr 21, 2017 12:46 am

I have a static caravan with an external power supply box protected by a class B RCBO'SI, 16 amp rated. The installation is about 7 years old, and has never given any problems - but now trips constantly when any appliance with a motor, ie fridge, freezer, microwave starts. I assume the problem is surge current on start up. Is this likely, and is it likely the RCBO has developed a fault? Do they fail this way? The freezer is less than 12 months old and all tests seem to be perfect - I have an insulation tester, and am a retired appliance repair engineer, but don't have much experience with the reliability of RCBOs. I was thinking of replacing the RCBO, or fitting a type c one which I believe is less liable to surge problems. Any suggestions? Thanks in anticipation.
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Postby kbrownie » Sat Apr 22, 2017 2:56 pm

it would be wise to have your rcbo, ramp tested. If it established that the rcbo is tripping at the normal amount (about 26mA). Then it would be a factor of measuring the earth leakage of the equipment, it could be that one or more are leaking too much.
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Postby ericmark » Sat Apr 22, 2017 7:47 pm

A guess of course, but most likely an earth neutral fault. I will try to explain. Most supplies have the neutral tied to earth, so with no current draw, even with a direct connection between neutral and earth the RCD will not trip as no current will flow.

However as the load increases neutral voltage starts to climb towards line voltage so now a connection between neutral and earth will mean current flows and the RCD will trip. Since motors have a high start load they are likely to make it trip.

The problem is we tend to fit MCB's and fuses into the line only, often the switch is also in the line only, so a toaster with a bit of wet bread causing a leak neutral to earth is still causing that leak when plugged in even if not being used. So you plug in the kettle and the RCD trips but not kettle at fault or RCD it's the toaster.

So unplugging any item not used, or testing between line and neutral clearly with caravan unplugged is the start point before looking at a faulty RCD.
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