by ericmark »
Wed Jun 03, 2020 7:46 am
I don't think you understand how a RCD works, it does not actually detect current flowing to earth, it detects an imbalance line to neutral and it assumes if not the same, then some current must be going to earth, so you can't have a RCD which is solely for the SWA cable.
Personally I would not fit an earth rod to a PME supply, I have seen the result when the supply authority lost their neutral/earth, with a rod at 60 ohm not really a problem, but with a good earth, and with the case in question there were 4 earth rods connected with bare earth tape designed as the earth for a radio transmitter, when the supply earth/neutral was lost the whole street tried to use his earth, so likely swinging up to 300 amp, and the 4 mm earth cable melted.
It is OK when every house has water and gas earthed as the load is shared between all the houses, but if only one house is earthed that house takes it all, so let the DNO do any multiple earthing, either use PME (TN-C-S) or TT don't mix them.
The problem with TT is you need to test, there are two methods, one you use a loop impedance tester and compare your earth with the DNO earth, and second is you fit temporary test probes at 15 meters +/- 2 meters and 30 meters and test the earth using a proper earth test meter, both require expensive equipment to test, and the second needs access to the ground to fit probes. So for the DIY man it is an expensive exercise to use TT because of the testing involved.
Of course all installations should be tested, including the RCD, we all know you can't measure 40 mS with a stop watch so again should use a special tester, but in the main if the RCD trips at 30 mA then we just cross our fingers that it does it fast enough.
