Odd Fault Tripping Rcd

dave2 · Nov 5, 2015

Today I was working on my last job before retirement, still plenty to do though.

Anyway, next door asks me if I can have a look at his house as when he resets his rcd his neighbours trips.

Don't want to get involved but a quick look found the following.

Originally the property was one house and converted to two.

Both TT but no sign of any rods.

Incoming supply has been split to separate meters.

House A has a 30mA rcd at the front end.

House B has 100mA at the front end.

House B rcd tripped, first time he can remember in years.

When he reset the rcd House A rcd tripped. House A resets no problem.

I tried this several times with the same result.

If I pull the 5A lighting rewirable fuse from House A then reset house B there is no tripping.

Oddly enough house B has one light which is supplied from House A.

Definitely not getting involved with this but your thoughts please.

sparkey999999 · Nov 5, 2015

dave2 said:
Today I was working on my last job before retirement, still plenty to do though.

Anyway, next door asks me if I can have a look at his house as when he resets his rcd his neighbours trips.

Don't want to get involved but a quick look found the following.

Originally the property was one house and converted to two.

Both TT but no sign of any rods.

Incoming supply has been split to separate meters.

House A has a 30mA rcd at the front end.

House B has 100mA at the front end.

House B rcd tripped, first time he can remember in years.

When he reset the rcd House A rcd tripped. House A resets no problem.

I tried this several times with the same result.

If I pull the 5A lighting rewirable fuse from House A then reset house B there is no tripping.

Oddly enough house B has one light which is supplied from House A.

Definitely not getting involved with this but your thoughts please.

Sounds like a neutral crossed between the two houses

Murdoch · Nov 5, 2015

sparkey999999 said:
Sounds like a neutral crossed between the two houses

If that is the case why had this only stayed now? Surely it would have happened since day 1. What changes have been made very recently?

steptoe · Nov 6, 2015

If I lived in house B I'd have a BFO extension lead and about 20 D/skts running off that light,

Rob_the_rich · Nov 6, 2015

could it be due to the front end 100mA RCD being a double pole switch? If the neutral connection is made first, and if there is a small n to e leakage, there could be an imbalance on the neutral which the 30mA RCD detects. Exacerbated by the possibility that earthing and bonding to both properties are likely shared.

ProDave · Nov 6, 2015

As above, the key to understanding these "trip at switch on" or "trip at switch off" faults is that one pole of the DP trip will open or close slightly ahead of the other.

This can exacerbate an imbalance due to an earth fault, when ordinarily the earth leakage due to the fault is too low to trip the rcd.

The key to fixing the fault is earth leakage testing on both properties. But in this case you have identified one particular lighting circuit as the culprit so I will bet when you test it you will find poor IR on that circuit.

Sidewinder · Nov 6, 2015

The convention for internal design of a switch would, for such an application be late break, early make on the neutral.

There is no reason to believe that an RCD should be any different.

However, the product standard may require simultaneous switching, one would have to check.

Lurch · Nov 6, 2015

Sidewinder said:
The convention for internal design of a switch would, for such an application be late break, early make on the neutral.

Are all RCD's polarity concious?

dave2 · Nov 6, 2015

Thanks guys for your input.

The owner of house B is the landlord of house A. His getting his kitchen lighting for nowt!

Good point from Rich Rob about neutral disconnecting at a different time.

Bet that the earth for this house(s) is via a water pipe and both boards have been cross bonded.

Not getting involved as I'm counting my days down.

Rob_the_rich · Nov 6, 2015

Best of luck for the future

Odd Fault Tripping Rcd

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