RCD Tripping - Is it an earthing problem?


Postby havenbridge » Tue Jul 24, 2007 11:54 am

I live in a rural area and had a heat pump installed 3 years ago which has been working perfectly. Since the recent rain, the heat pump started tripping the house RCD (30mA). The heat pump has its own separate 300mA RCD (which has never tripped) but shares a common earth with the house. There is an earth rod in the ground which has been there about 30 years. The 30mA RCD is new (I thought this was the problem - obviously not!)

If I switch the fuse box off in the house and remove all the fuses, the heat pump still trips the house RCD? The problem therfore cannot be with an appliance in the house. If I switch the heat pump off (isolate it) then switch the cooker on with all plates on maximum, it too trips the RCD. The problem cannot therefore be with the heat pump.

Is it an earthing problem? Has the recent rain washed away the soil surrounding the earth rod thus reducing its effectiveness. Should I install a new earth rod? Help!!!
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Postby Stephen the sparks » Tue Jul 24, 2007 8:07 pm

The heat pump, being in the garden, is outside the equipotential zones. You are right when you say they share a common earth and that is the problem.

The solution is to disconnect the earth at the heat pump end and then stake this down on its own earth stake. This should cure the tripping issue

Your existing installation should have a split load board with a 100mA RCD main switch, this protects the installation from fire etc, and a 30mA RCD on the split side where the socket outlet circuits are, the 30 mA unit provides supplementary protection for portable equipment, or electricity in bathrooms etc. The use of two RCD's is for discrimination purposes.

Electric cooker elements, particularly when old, can cause RCD trip issues since they can be prone to condensation within the element. For this reason best to be on the unprotected side, ie, only protected with the 100mA RCD in this case. Cooker control units, with sockets, have to be on the protected side.

Best of luck!
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Postby sparx » Wed Jul 25, 2007 7:19 pm

Hi, one further point to the valid comments from Stephen;
the main RCD needs to be a time-delayed type or it will still trip first most of the time.
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