How come some rcd's......

[hoover]

........ trip on a neutral to earth fault but then some dont

 

[lj.l]

I take you mean some circuits dont trip, check that the neutrals are in the correct neutral bar for the rcd that is feeding the live on that circuit. the rcd will detect any inbalance in current that is flowing through it.
Hope this helps,could with more info

 

[hoover]

I take you mean some circuits dont trip, check that the neutrals are in the correct neutral bar for the rcd that is feeding the live on that circuit. the rcd will detect any inbalance in current that is flowing through it.
Hope this helps,could with more info

Sorry mate, just to expand. I do most of my work in houses which are fed by tt system. I've noticed if your working on a existing lighting circuit for example, that when you've pulled the fuse or knocked off the mcb, and then if you touch n to e together the rcd protecting the db will trip however in many cases it will not. I understand about the imbalance but surely if the circuit is isolated and therefore there will be no current in the live of that circuit, why trip in a n to e short of that circuit?

 

[sarge]

Sorry mate, just to expand. I do most of my work in houses which are fed by tt system. I've noticed if your working on a existing lighting circuit for example, that when you've pulled the fuse or knocked off the mcb, and then if you touch n to e together the rcd protecting the db will trip however in many cases it will not. I understand about the imbalance but surely if the circuit is isolated and therefore there will be no current in the live of that circuit, why trip in a n to e short of that circuit?

If the rcd is covering more than one circuit, it could be that there may already be some small earth leakages from other circuits, and by touching n +cpc its just enough to tip the ballance of 30ma or whatever its rated at.

 

[Deleted member 9648]

I'ts a common phenonemon....way I understand it is there is sometimes just enough potential difference between N-E for sufficient current to flow to operate the RCD,

 

[ghost]

It will all depend on how good the earth is to each house, current always takes the easiest path back to the source, therefore if the neutral connection is an easier path than a high impedance earth then the neutral it is, therfore no tracking down to earth when you short between N and E.

 

[hoover]

It will all depend on how good the earth is to each house, current always takes the easiest path back to the source, therefore if the neutral connection is an easier path than a high impedance earth then the neutral it is, therfore no tracking down to earth when you short between N and E.

Aye that makes sense i suppose. Thanks mate

 

[GT1]

hmmmm
I have a job coming up in a rather secluded 200 odd year old house in the country, where owner tells me he can still get a shock off stuff even with the mains switched off..?
was rather sceptical at first and was going to be carefuly checking with the meter first before i did anything..
could this be similar situation..?
(he had a new CU about 2 yrs ago apparently)

 

[ghost]

that sounds a bit bizzare, approach with caution!

 

[GT1]

yeah... it rang loads of alarm bells when he told me...!

 

[jjjinx]

some older houses have been converted from say 3 small houses to a single property but may still retain the other c/u's.

Beware of multiple supplies & back fed supplies for who knows what reason.

It may be that he doesn't mean that the main switch has been opened but just one purticular circuit or that the main switch is failing to open the circuit.

 

[Jason Smith]

if there is no load at all on the board, all circits off, then you can touch the Earths and Neutrals together and nothing will happen. However if there is some load on say the ring main, there will be some leakage in the neutral bar. A tiny amount. However, even if you switch off an MCB( to a different circuit ) and isolate that circuit, , if you touch the n and e together, evn though the circuit is isolated, it will trip. The school of hard learning taught me to cut each conductor independantly when decommisioning circuits in a loft illuminated by another circuit !! Was working on a lighting circit once and the ring main which fed my work light kept tripping off, and it was the circit I had just rewired, spent ages trying to find the fault, untill Oldsweat put me right. You cannot beat experience. Gimme some !!

 

[ghost]

Think of current like this. It's the end of gig at a big arena. The crowd are pushing their way out of the main doors, it's a slow process but people are getting out. Somebody opens an emergency exit and there's a rush of people to get out of the emergency exit, it's an easier route. Bouncer comes aong and the door is shut.
Think of the main doors as the neural bar, the emergency exit is the short you create between N and E, the bouncer is the RCD. In most cases, usually due to parallel earth paths, the earth path is a much easier path than the neutral conductor, thus current from the neutral bar tries to take the path via the earth, thus an imbalance is created and the RCD trips.
An old lecturer at Gateshead tech used to tell that story and 20 years down the line it still sticks in my mind!