TN C system. RCD ???

Gamma · Jun 3, 2009

Hello all.
According to BS 7671, (411.4.4 Note 1), on TN C systems it is prohibited to use RCD protection.Could someone please explain why???
Background info is: Small industrial unit supplied by a TN C system. DB then splits to supply a set of offices and toilets via a 6 way domestic CU. Client wants to upgrade to a 17th edition RCD protected unit. Would this present a problem ?

Mr Mark Sparks · Jun 3, 2009

hello mate i dont want to teach you to suck eggs
are you sure its tn-c and not tn-c-s
tn-c is very rare

Earthing system - Wikipedia, the free encyclopedia

baldsparkies · Jun 3, 2009

With TN-C the earth is combined with the neutral, This means that an RCD will have no way of seeing the inbalance between earth/ Neutral should a fault develope.

TN-C is a rare combination these days so check it really is that kind of earthing arrangement.

scotsparky · Jun 3, 2009

Gamm to tell the difference you have to find where the neutral and earth split. Is it before or after the meter?

If its before its a TN-C-S if its after at the Main D.B ther will be a Link betwenn Phase and Neutral. TN-C

lister · Jun 3, 2009

Damm good point though, if it was tnc and rcd is a must what would you do?

scotsparky · Jun 3, 2009

Lister TNC systyems have to be "Under supervision of a competent person" so you dont have to use RCDs . They are also thankfully very rare.

Have a look at 411.4.1 its pritty clearly worded that no rcds on a tnc system.

In that system its down to the sparks to ensure the ZS is low enouph for disconnection times by either supplimentary earthing, use of overcurrent devices. for example HRC fuses instead of MCBs you have to be VERY CAREFULL.

I would try and change the earthing system to TN-C-S myself if possible.

Gamma · Jun 4, 2009

Thank all that have responded to my dilemma, I havnt checked the system myself, but client produced an 11 year old EIC which clearly stated system type : TN C. I will drop by today and look for myself and post result tonight.

Gamma · Jun 6, 2009

Update as promised: The TN C system turned out to be TN C S so no probs with the RCDs. Thanks again to all who responded.

mikeymoo · Jun 6, 2009

To add to this.. I think i may have come across a TN C system. Would like some advice is possible. After taking a peek in the meter cabinet i noticed a strange layout that i cant say ive seen b4. This had a yellow sticker on saying "WARNING This installation is connected to a network with protective multiple earthing Please consult YEB regarding earthing?. Have some pictures but will need to upload from my mobile when i get the chance..

scotsparky · Jun 7, 2009

muikeymoo PME is also known as TN-C-S lets see the photos to prove it though

Guest123 · Jun 7, 2009

Agreed, sounds like it's a TN-C-S (PME).

It is very rare to come across a TN-C system particularly in a domestic situation for the simple reason that you cannot use RCD's.

mikeymoo · Jun 7, 2009

scotsparky said:
muikeymoo PME is also known as TN-C-S lets see the photos to prove it though

Isnt TN C also known as PME? Will upload pics.
The installation is approx 28 years old if thats a clue. When would you normaly find a TN C system? Was it used just to reduce the risk of broken neutrals?

Mac · Jun 7, 2009

mikeymoo said:
Isnt TN C also known as PME? Will upload pics.
The installation is approx 28 years old if thats a clue. When would you normaly find a TN C system? Was it used just to reduce the risk of broken neutrals?

Take a look at this link mate
Untitled

mikeymoo · Jun 8, 2009

Thanks for the link mate.. From info, if i were to take alook inside the cu and the earth and neutal are seperate then its TN-C-S? because in TN-C system its combined throughout??

Just phoned YEDL tec helpline and can you believe it they dont hold info regarding the earthing arrangments!!. And wanna relieve me of £150 to come fit an earthing block. Also to add insult to injury will take 6-8 weeks!!. Whats the crack with me adding the earthing block myself?? Will get around to unloading those pics today so if someone could take a look i would be grateful.

ian.settle1 · Jun 8, 2009

If you look at page 32 of the big red book you are NOT allowed to use a TN-C as a consumer under ESQCR Regulations 2002.

The DNO can supply you via this way but then it must split at the point of intake giving the cosumer a seperate earth and neutral.