rcd question?

[laughingjester]

i am an self employed electrician and have a quick question im stumped on...i've just changed an old re-wireable fuse board for a new wylex split load duel rcd consumer unit, and whilst working on the new installation found that after i isolated the circuit wich im working on, when i cut through a piece of 2.5 mm twin wich was spurred off the ring main, the rcd DIDNT trip, i know that an rcd usually trips wether there is a load on the circuit or not when a nuetral to earth fault occurs. so i bridged the cable nuetral to earth (still isolated of course) and it still didnt trip. any one shed some light on this please i think that maybe because its an tnc-s system where the earth and nuetral are combined by PME and that there was no load on said circuit that that is why it didnt trip ??? and is this the case for any other earthing system ie tn-s tt etc

 

[holmslaw]

..

 

[Spark123]

Yes, is owing to it being TN-CS. If there is no potential between N and E then no current can flow via the N-E short.
TT and TN-S are a little bit more complicated, I'd be suprised if an RCD didn't trip with TN-S as any voltages on the distribution network neutral and earth conductors will cause a potential difference.
TT depends on the potential of the neutral conductor above true earth and the resistance of the fault path back to the transormer - if it doesn't trip the RCD it can play havoc when conducting an RCD test as what you would have is a pretend TN-CS link downstream of the RCD.

 

[ColJack]

of course the RCD should trip in a PME ( TN-C-S ) system..

the neutral runs through the rcd, the earth doesn't..
any current going out of the RCD must ballance with the current coming back..
any neutral - earth fault bypasses the rcd on the way back so it's got more current going out than coming back so it will trip...

if it's an RCD for one circuit only and there is no load, then there is no current so can be no imballance...

if it's an RCD for a split load board, and the other circuits have a load, then there is current traveling in the neutral..
given that the neutrals are all connected together, then a neutral / earth fault anywhere will cause current to flow to earth via the neutral evenif the live of that particular circuit is isolated..

how did you isolate the circuit?
if it's a DP switch then the neutral is also disconnected..

 

[Spark123]

The neutral runs through the RCD and the earth doesn't yes, however as N&E are tied together in the head next to the meter, given he is doing a CU change so all circuits will be isolated, current using equipment unplugged etc, where is the potential difference required to make the current flow via the CPC created from?

 

[ColJack]

i isolated the circuit wich im working on

doesn't say he has the whole installation isolated..

since "the RCD didn't trip", it's safe to assume that he has some circuits on.. other wise it wouldn't be switched on anyway...

 

[Spark123]

Where is the potential difference to create the current which didn't trip the RCD created from?

 

[ColJack]

the potential difference comes from the resistance of the wires..

in a split load board, there are several circuits on the RCD side..
all the live currents and all the neutral currents run through the RCD..
all the neutrals are connected together..

when a neutral / earth fault occurs, the neutral current will be split between the 2 paths back to the TN-C point where it splits to become TN-C-S..

for arguments sake, lets assume these paths each have the same resistance then the current running in each path is half of the current running through the live...

so the RCD only "sees" half the current coming back that is going out..

 

[Spark123]

So, as in the original post - how do you explain that with a TN-CS when a N-E fault is introduced on a circuit in which no current is flowing, that the RCD does not trip?
Given the resistance of the meter tails etc will likely be much lower than that of the circuit live and protective conductors?

 

[ColJack]

what's that got to do with the price of fish????

you're having a hard time grasping this concept, or I'm doing a poor job of explaining it..

you have 25mm live and neutrals going to the main switch, then 16mm live and neutral going to the rcd, then 2.5mm live and neutral going to the sockets..

you'll have a 16mm main earth to the earth bar, then 1.5mm earth wire going to the sockets..

your live and neutral have equal resistance throughout, so equal current in both conductors..

your earth will be a higher resistance than the neutral and live conductors so the current will not be split equally, I only used that as a rough example..
it will be roughly in the ratio of 5-3.. with 30mA going down the earth for every 50mA down the neutral..


there are 3 posibilities why the RCD didn't trip in the OP's problem..

1. the only circuits he has on are non RCD protected or on the other RCD in a dual RCD board, such as lights.. ( in which case why bother to have the RCD switched on at all....)?
2. the circuit he had isolated to work on has a double pole isolation device on it somewhere.. ( such as a double pole switch or a switched fused spur which normally have double pole switches in them nowadays )
3. the RCD itself is faulty or incorrectly wired......

 

[Spark123]

4. There is no load on any of the circuits, as holmslaw pointed out in the 2nd post.

 

[ColJack]

I think you and I have been trying to beat each other with two ends of the same stick...

You've been trying to get me to realise that in a no load situation on a TN-C-S system, there will be no current flow between Neutral and earth since they are ( or should be ) at the same potential..

I've been trying to get you to realise that in a load situation on a TN-C-S system, that the returning current in the neutral will be divided between the neutral and earth if there is a neutral / earth fault anywhere after the RCD, enev if that particular circuits live is isolated..

both of which we both already knew...

I think I misinterpreted you original reply since you didn't mention there being no load at all on the board.. and I had come to the conclusion that the OP had some circuits on...

 

[RF Lighting]

Just because circuits are switched on, it doesn't necessarily mean a current will be flowing through the RCD.

Think of a lighting circuit with all the plate switches switched off, or a socket circuit with nothing plugged in.

 

[laughingjester]

to coljack and spark 123.....sorry i caused this argument lads to clarify all circuits were on to begin with 10 circuits in all, 1-5 A circuits protected by one rcd and 1-5 B circuits also protected by rcd. customer wanted extra socket installed on circuit 2 A witch was dowstairs ring main. i isolated that particular circuit (knock off lock off procedure) left the rest on as well as rcd. i cut into one leg off the ring "thats when i was expexting rcd to trip" but alass no tripping.? i fully understand now thanks to you two that if there was'nt a load on that cicuit given that its a tnc-s system there would be no tripping, but the other 4 circuits were on and drawing a load so why didnt it trip the rcd? all test results were bang on the money even rcd auto tests were spot on am i looking for a problem that does'nt exist thanks again to all input from everyone.

 

[laughingjester]

also there is a double pole main switch wich feeds both rcd's..