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R.C.D. tripping and earthing concerns.

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Evnin' All,

                  I have had a hard day and my brain is in meltdown so please excuse nonsense and silliness.


I installed a new socket in a house today. TT earthing presumably. New A.B.C. overhead supply. Nearest D.N.O. P.M.E. pole earth rod about 15 metres away from the house. When I tested the R.C.D. it worked on the test button, but the switch seemed a bit graunchy. It would not trip off on 30mA test, but did trip off on the 5X R.C.D. test at a socket. I replaced the R.C.D. The same thing happened. No trip on 30mA test setting, but it did trip off on the 5X setting.


An earth fault loop test at a socket showed a reading of 185 Ohms.


A  length of 6.0mm2 green and yellow cable was found in the front yard but the electrode is not inspectable as it is covered by block paving is old. It may have rotted away. With the main switch off the resistance from N. to the earth bar in the consumer unit is about 180 Ohms. 


The consumer unit is a 10 way unit with just one 30mA R.C.D.


I converted the earthing to P.M.E. and then nuisance tripping was seen. I intend to install an all R.C.B.O. board tomorrow.


Sometimes a fridge when turned on will trip the R.C.D., other times it is a freezer, then the kettle. But the appliances test o.k. with an insulation resistance test. Once or twice the tungsten lighting circuits tripped off the R.C.D.


I assume that the addition of individual leakage current is responsible for the R.C.D. tripping, but there is no logic to which individual appliances/circuits seem to cause it. Two suspected rings test o.k. for insulation resistance.


The house owner reported that the gas fitter received a shock when doing something under the AGA. The main gas bond was loose at its clamp. I have replaced the earth clamp.


Any thoughts please?


EDIT. P.S. I have a Voltage tester that will trip off a 30mA R.C.D. if connected to the outgoing R.C.D. L. and E. at the same time. When this was connected to the original R.C.D's LIVE INCOMING terminal and the E. bar the R.C.D. tripped off. Did this raise the Voltage on the C.P.C.s sufficiently to trip off the RC.D?


Z.


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  • 0
    Current injected into the CPC, that does not come from the load side of the RCD (neutral or live) will not trip the RCD.

    There may be a partial path from CPC to the load side neutral of the RCD, in that case pulling up the neutral earth would destabilise the RCD.


    Not tripping at X1 is more of a concern, presuming the tester is  pushing current from outbound live to CPC - though again a partial  NE fault might put some of that 30mA not into terra-firma but back into the load-side neutral of the RCD.


    Be aware on TT, with a significant electrode resistance, an NE short is not the immediate trip on any light loads that it would be on a TNS or TNC-s supply , as if the rod is 180 ohms, even if the NE fault is dead short, load current shares in the ratio of 180 ohms to Rn, and that may mean you need a very large load current before false tripping occurs.  (Rn may be tenth of a ohm, so 30mA * 180/0.1  ~ 60A before tripping such a current may never be seen on the defective circuit.)


    I'd be wondering if drilling and driving a second rod was on the cards,  if the original is in doubt, and also looking for an N-E problem in an appliance or one cct.

    250V or 500V test L+N to E will help shake that out.

    It may not be N to E the CPC of the cct, it could be N to E as in some pipe or fence post in the ground  or something, so test not just to CPC but to electrode too.

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  • 0
    Current injected into the CPC, that does not come from the load side of the RCD (neutral or live) will not trip the RCD.

    There may be a partial path from CPC to the load side neutral of the RCD, in that case pulling up the neutral earth would destabilise the RCD.


    Not tripping at X1 is more of a concern, presuming the tester is  pushing current from outbound live to CPC - though again a partial  NE fault might put some of that 30mA not into terra-firma but back into the load-side neutral of the RCD.


    Be aware on TT, with a significant electrode resistance, an NE short is not the immediate trip on any light loads that it would be on a TNS or TNC-s supply , as if the rod is 180 ohms, even if the NE fault is dead short, load current shares in the ratio of 180 ohms to Rn, and that may mean you need a very large load current before false tripping occurs.  (Rn may be tenth of a ohm, so 30mA * 180/0.1  ~ 60A before tripping such a current may never be seen on the defective circuit.)


    I'd be wondering if drilling and driving a second rod was on the cards,  if the original is in doubt, and also looking for an N-E problem in an appliance or one cct.

    250V or 500V test L+N to E will help shake that out.

    It may not be N to E the CPC of the cct, it could be N to E as in some pipe or fence post in the ground  or something, so test not just to CPC but to electrode too.

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