It said back in 17th Edition
Every installation shall be divided into circuits, as necessary, to:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced
by equipment in normal operation.
the fact it says "RCD" clearly RCD's form a circuit as far as BS7671 goes, however back before the 30 mA we would have one 100 mA RCD feeding all the home without any problem with nuisance tripping, so would have complied with the above, caravans have two 30 mA RCD's in series and seem to have no problems, so again even a single RCD can be compliant.
We should test to see the leakage, I will hold up hands and say I have never tested, simply as I have never had a clamp on meter that will measure 0.001 amp. So knowing once it exceeds 9 mA we should split is all very good, but we don't know if it will exceed 9 mA.
Also we have no idea what will be plugged in, I remember doing an office in the days of desk top computers, one ring would have powered the 20 odd PC's, but at that time PC's were well known for leakage so we ran multi-circuits three I seem to remember each with own RCD as we knew with one circuit it was likely to trip.
My PAT tester did show the leakage current and I have had new equipment which has been returned as leakage over 3.5 mA. Actually scales and the filter was removed and sent back to us. Filters like this
are common in equipment and we should plan so the small current to earth will not trip a RCD in normal operation. I am sure a small flat can like a caravan work with a single RCD, maybe with an emergency light, remembering most caravans have a battery for lights, except for German Hobby.
But my home three floors, two kitchens, two electric showers, five bedrooms, and to reset a RCD if it trips one has to leave main house and go down outside steps to reset it, there is a very real danger if you trip a RCD with a socket fault and it puts out the lights, and sockets in main house wired side to side and lights wired up/down, so only way to have two RCD's so sockets do not trip lights is all lights on one RCD and all sockets on the other, which in turn means little water in outside light and no lights for whole house, which is clearly a danger. If lights fail due to general power cut, one does not need to do anything, if they fail due to local trip, does not matter how long you wait they will not come back on until you reset them. And general power cuts are rare.
But if the home is small then having two RCD's is possibly compliant, it is clearly compliant with a caravan with one. So the question is how big does the home need to be before we would say non compliant?
On this forum electricians have said how their house rarely trips a RCD, and two were plenty, but I have lived with two 30 mA RCD's feeding two Wylex fuse boxes (fuses swapped for MCB's) for around 25 years before moving here, and I would have bouts of tripping, maybe 4 times in a week, did all tests, no faults found, then 2 years trip free. I did note more likely in a thunder storm, so seems likely caused by spikes on the supply, the RCD's clearly old, so likely no electronics in them, they were 4 module wide each, so maybe the modern RCD is not affected with spikes in the same way, or the SPD removes the spikes?
Never tripped in mothers house 2 RCD's and 4 RCBO's so maybe in may case just age.
But the number of threads on here saying how RCD's have tripped is seems there is a problem, I think the leakage should be measured and entered on the installation certificate.
