I think the confusion comes from your rather blunt "No, it doesnt." in response to the Harry's comment "A neutral to earth problem is just as likely to be tripping it, so also needs to be checked."
This is fair enough for many appliances, but as most responses were highlighting, it does not help find a N to E fault in the foxed wiring, that can cause trips even without the involvement of the appliances.
I hope that the foxed wiring was done by Reynard the Fox:-)
Adam
Well.. Take a PME system and then connect a RCD to the incomer where the earth and neutral will be connected together in the incomer supply cable.
Now with the neutral conductor side of the output of the RCD connected to Earth assuming the same earth as the incomer, then how does that trip as there will be no current flowing or if that is any current due to Capactive leakage effects then it will be a damm sight lower than the current needed to trip the RCD!.
However now apply a "load" to the output then see it trip. As soon as the "imbalance" current is sufficient ..
In article , Harry Bloomfield scribeth thus
Can you go into that in a bit more detail Harry?..
>
I can confirm that the same happens here. As recently as last Friday the electrician wiring in a new circuit for an aircon cut the cable and tripped the main RCD by momentarily joining neutral to earth. The MCB was not even fitted, so no possibility of any current flowing from the live on the new circuit.
Richard.
Yup, as in "what the fox making that RCD trip?"
L 240V 235V
----------------------------------------------------- | | | ----- ----- ----- | | | | | | TX | H1 | | H2 | | H3 | | | | | | | ----- ----- ----- 0V | | | 5V
----------------------------------------------------- N | 0V -------------------------------------------------- E
Imagine you live in house H2, and there are a number of other properties that are also fed from the same phase as you. The earth is a TN-S arrangement and tied to the neutral at the transformer. The effect of the load drawn by each property will be to drop a small voltage over the length of the supply wires. In effect it will pull the 240V leaving the transformer "down", and pull the neutral "up" away from ground potential when measured away from the transformer (how much depending on the total load, and the impedance of the supply wiring).
So even if you are not currently drawing any current, you may still see a potential difference between your local neutral connection and your earth since the neutral has a substantial current flowing through it due to other properties, but the earth does not.
If you now short N to E in your house wiring, a current can flow between them, that will not also be seen in the L. Hence the RCD trips.
Same can happen with TT due to the above, and also due to the local earth rod potential being different to that at the substation.
TN-C-S makes it less likely, although N E shorts can still cause trips due the the same effect happening in your house due to the effect of other circuits pulling the N potential up in the wire runs after the PME bond.
I finally took action as the RCD has started tripping up to three times a day (is it a coincidence that it has started snowing?) I girded my loins and undid the connections in the consumer unit for the ground floor ring. With the multimeter connected to N and E we plugged in each of the appliances one by one. The kettle, microwave, TV and washing machine all read high momentarily then settled back down to infinity but the boiler which is on a plug read permanently high (or should that be low?) The plug was grotty and some builders dust fell out as we opened it but was otherwise well wired, we replaced it anyway and the meter reading returned to normal.
Could that have been it, a bit of dust in a plug? I find it hard to believe but if you don't hear any more about this then it's because it worked.
Thanks to everyone for their contributions.
yup, good job
NT
I'm afraid not. Two weeks later the problem returned. I tried the multimeter on the L & E again and registered nothing for any of the devices. Completely perplexed I went for my original idea of splitting the ring into two radial arms with an RCD on each. While doing this I found that the ring was already broken and I have isolated the offending length of cable.
So at the moment I have two radial arms, each with an RCD in line, I'll give it a few weeks to see what happens.
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