Twin RCDs

P

Puzzler

New Member
Scenario - two consumer units each with their own individual RCD are wired back to a splitter coming from the electric meter. Is the tripping fault from one CU likely to trip the RCD in the other CU ?
 
KC is half correct

The RCD's on each CU would be protecting only those circuits on the individual CU. RCD's will detect out of balance phase/earth and neutral/earth fault on the circuit.

RSS
 
Surely its any inbalance between the phase and the neutral it detects.

The fact that it is often a leak to earth (and an indirect one via the person to the physical ground) is by the by.

It could be via a neutral on a non RCD section - in that case there would be no earth current but it would trip because the RCD neutral would be carrying less current.

The question was if a fault occured on one RCD C/U would the other RCD trip. If the fault doesn't unbalance the phase/neutral of the second RCD then no the second won't trip.
 
Hi KC

The original post stated that both CU's were connected to the meter via a splitter. Please explain how the RCD's in one CU can be affected by an earth fault in the other?

RSS
 
That's just the key, RCDs don't trip because of an earth fault - they trip because there is an inbalance of current between the phase and neutral that exceeds their rated trip current.

Now I am not saying that this would happen in normal circumstances but say, for whatever reason, a neutral on one circuit on the first RCD came in contact with a neutral from another circuit on the other RCD. If those circuits where drawing a load, there would be an inbalance between them and their corresponding phase lines, hence the RCDs would trip but no earth current would exist at any time.

This is, as I state, unlikely, but it demonstrates that the RCDs dont care about the earth connection, simply a state of inbalance of flow and return current between phase and neutral. As you quite correctly imply, it is often the case that this inbalance is achieved by effecting an earth current, but as far as the RCD is concerned it has popped because there is more current flowing through its phase connection than its neutral.
 
Hi KC

RCD's detect out of balance current in phase and neutral, accepted. However, the out of balance must be caused by an earth leakage fault from either the neutral or the phase. RCD's will not detect phase to neutral shorts or circuit overloads, these are picked up by the MCB's and other overload devices.

RSS
 
or a cross leakage fault between a curcuit not on the rcd and one on the rcd (for example a borrowed neutral)

i would expect such a fault to show up on initial rcd installation though
 
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