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Dumela

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Got a friend who called me saying his electrics keep tripping a few times per day, popped over and on the TN-S supply found a 80A 100mA RCD fitted by the supply authority after the meter and before the DP isolator.
It was sealed like the meter etc hence my confidence it was SA fitted.

It was this 100mA RCD that was tripping a lot, my suspicion was this 100mA trip was faulty as it was random and could be any time of the day such as when nothing was being used.

The consumer unit it feeds is a split board with all but 3 circuits protected a 63A 30mA RCD.
I added 30mA RCBO's to the board on the remaining 3 circuits (2 x Lights & 1 x Immersion heater so they all have 30mA protection and removed the suspect 100mA from the system.

Since then the tripping issue has happened in any of the 30mA trips so I can only assume it was a faulty 80mA RCB which has been removed.

The system is getting a full EICR next week as some of the Zs readings I picked up were crap which is probably not helped by a very dubious sized bit of sub main wiring in place.

What I could not work out was why this 100mA RCD was there in the first place. I found the Ze is 0.4 Ohms (0.8 is max on TNS so not an issue).

If anyone has any ideas why it may have had this 80A 100mA RCD was fitted by the supply folks I would be interested to know.
1651067670203.jpg
 

Dumela

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Could you not have tested this RCD?
I am more interested in why it was there in the first place to be honest, I will be replacing the consumer unit and bringing it up the spec soon with an RCBO board. I did test it on an auto test but due to the poor Zs reading my meter threw the towel in even on the 30mA x 1 test as it was raising the test voltage too high. I did not go any further with testing it to be fair.
 

steptoe

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I am more interested in why it was there in the first place to be honest, I will be replacing the consumer unit and bringing it up the spec soon with an RCBO board. I did test it on an auto test but due to the poor Zs reading my meter threw the towel in even on the 30mA x 1 test as it was raising the test voltage too high. I did not go any further with testing it to be fair.
Why would you be testing a 100mA RCD on 30mA setting ?
 
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Test the RCD and get your clamp meter on the tails too.

And I doubt the supply company fitted that RCD. I suspect it was done by a DIYer / builder / spark so the installation has RCD protection.
 

Dumela

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Thanks for the replies but most of you seem to have missed what I was asking, why is there a 100mA RCD fitted, seemingly by the supplier, on a TN-s system with a Ze of 0.40 Ohms ?
 

Andy™

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Thanks for the replies but most of you seem to have missed what I was asking, why is there a 100mA RCD fitted, seemingly by the supplier, on a TN-s system with a Ze of 0.40 Ohms ?

supplier / DNO will not have fitted it

its also possible that it was TT and its been changed to TNS at some point
 

Dumela

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I doubt it was fitted by the supplier.

Who removed it

supplier / DNO will not have fitted it

its also possible that it was TT and its been changed to TNS at some point

I am pretty sure the TN-S is original it is centrally located in the middle of Llandudno amongst lots of other homes.

The RCD had a supplier seal on it which led me to believe it was their kit although with hindsight it is possibly an over zealous meter fitter who sealed it.
I removed it as there is no logical need for it, the rest of the system has full 30mA RCD protection now and it was nuisance tripping 4 times a day.



1651067670169.jpg
 
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RCDs aren’t bullet proof and they do become unreliable and can be the source of nuisance tripping.

simply changing things or removing them isnt really a thorough way of fault finding
 

binky

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I am pretty sure the TN-S is original it is centrally located in the middle of Llandudno amongst lots of other homes.

The RCD had a supplier seal on it which led me to believe it was their kit although with hindsight it is possibly an over zealous meter fitter who sealed it.
I removed it as there is no logical need for it, the rest of the system has full 30mA RCD protection now and it was nuisance tripping 4 times a day.



View attachment 13456
That's an old beasty!
 
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