Discuss 100mA Type S RCD on a TN-S supply in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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I

izools

Just wondered...

Found 100mA time delayed RCD on a TN-S supply between meter and CU.

Not mandatory, but there's nothing to say this should not be installed is there?
 
split load mem, 30ma rcd for one side, standard fare, bit old but done properly.

Might have been to compensate (improperly) for a bad earth at some point?
 
Quite. It's new-ish (i'd say under 10 years old). Yes, it's time delayed for discrimination and what I'd expect to see on TT but the earth is DNO supplied (lead sheath; but not original clamps) and Ze acceptable; so not sure why it's there? Precaution?

Maybe the Ze was rubbish when it was installed and new crimps have rectified things?

Main thing is it doesn't need removing does it? :)
 
Well you don't state the purpose of your visit to the board, but if it's lead sheath it's been round the block I reckon longer than a decade, 10 years in, well if the RCD holds on half times trip, and no nuisance trips, I don't see problem, is the enclosure metal ? they may have added the 100mA hoping to protect the tails like a TT, if you were binning the board for a CU change I would bin it and 30mA all the circuits..Jobs a good 'un

J
 
To be honest I'm not actually an electrician, I've just bought a house and the set up is

Lead sheathed DNO cable in to 80A service cut out. Earth attached to lead sheath. 100ma s type RCD leading to plastic MEM CU, split loads, some circuits protected by 30ma RCD others not.

A friend of mine is an electrician and he noted that the earth clamps to the lead sheath of the DNO cable look relatively new and he said the Ze reading is within TN-S limits; he can't see anything wrong with the setup we're just both a bit weirded out by the apparently unnecessary presence of this 100ma rcd.

- - - Updated - - -

His thought was that the RCD was put there because of a poor Ze reading rather than trying to get the DNO out or staking the property converting to TT, but as the Ze is now OK we suppose the clamps on the lead sheath were replaced negating the need for the 100ma rcd.
 
izools, sometimes you'll get someone who's had a bad day. Nothing wrong with your posts, although giving the whole background in your original post might have helped. Don't let it put you off the forum.
 
Thank you.

My sparkie friend has confirmed that the Ze readings are good for a TN-S system but he doesn't know whether having a 100ma type-s RCD protecting the entirity of a non-TT system where it wouldn't be required was compliant.

All that we needed to know is whether it would be mandatory to replace the main incomer RCD with a 100A plain old switch or not. If it is, we'll get the DNO to pull the service fuse and we'll replace it. If not, we won't.

I hope someone is able to clarify the situation.

Thanks in advance and apologies for not making this clear in my OP.

If it helps I do understand the frustration a lot of sparks have with their having had to pay an inordinate amount of money to get qualified and what might appear to be a jumped up know-it-all DIYer "get away with doing it without paying for quals". I'd be frustrated, too.

But hey, I work in an IT Support environment. I have to support people all day and all night on a 24/7 shift rotation who haven't got a freakin' clue what their on about and the number of people I'd like to say "ok well while you figure out how to click OK I'll just go slit my wirsts" to... well, I've lost count. But we've got to sit there and be patient and kind. Can you imagine what it would be like talking auntie Moe through a CU change? Pulling teeth? Yeah. Same as having to walk a SME through installing an SDSL router over the phone.

Anyway. Enough of the animosity, any help that can be offered is greatly appreciated :)
 
Come on guys this is a public forum there is no justification for all this handbagging! :grouphug:

There is no need to remove the up-front time-delayed RCD from this set-up, providing it is within its BS specification when tested. As already stated the most likely reasons for its fitting are: a) previously the installation was a TT system, until such time as the DNO provided TN-S, or more likely b) it was provided because the TN-S had an out of limits Ze, intended as a temporary measure until such time as the supply earthing was improved and removing it later was considered unjustified, this is increasingly typical if installed by the DNO as the cost of removal outweighs the material cost of the RCD.
 
As Markie says^^^
The original reason for the upfront RCD will probably remain a mystery, but as said it doesn't affect compliance with BS7671. There appears to be no need to remove it, so don't. :)
 
Agreed, almost certainly the RCD was installed due to extremely low Ze reading. However if the Ze is now within limits then remove It by all means or leave insitu, no probs.
hope this helps and please don't let this experience put you off coming back, you are welcome here. Good luck
 
Agreed, almost certainly the RCD was installed due to extremely low Ze reading. However if the Ze is now within limits then remove It by all means or leave insitu, no probs.
hope this helps and please don't let this experience put you off coming back, you are welcome here. Good luck

Dagrat

High Ze, I'm sure you meant high?
 

Reply to 100mA Type S RCD on a TN-S supply in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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