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LOL. You obviously prefer separates.
I am the only one at my company who doesn't use an mft. People seem to waste hours fiddling with the leads and my boss thinks I am mad but my 20 year old rcd tester still does the job. Get the usual comments every time at our yearly assessment.
 
I have megger separates BMM2000 and LRCD 2000/2 and liked them but did buy a Megger 1552 - that is when the BMM came into its own. It makes a brilliant fault finder with continuity voltage and IR all in a handy little package (just like me) so I do agree with you @westward10 - for once!
 
OK people, I'm going to throw another curved ball in here.

Notwithstanding discrimination of two RCD's, the other discrimination factor is the over-current devices. This is a length of 6mm going out to the outbuilding. At the house there is an MCB of what?.... lets say its 40A.
What is in the outbuilding? MCBs of 32A perhaps?

There's no discrimination betweeen the MCBs, so the design is flawed. You'd need a FUSE ate the source end.

Suggest a Henley Block off the supply and a switchfuse of a suitable size that will discriminate with the (whatever they are) MCBs in the outbuilding.

That also solves all the bleating in the above three pages about RCDs in series.

Simples!
 
Absolutely. It's surprising how many people are unaware of this - that circuit breakers to BS EN 60898 of any rating in series will not discriminate.

Of course I'm not as knowledgeable as you chaps, but having done a bit of research I'm led to believe, that such a sweeping statement is not quite true. In truth, from what I've read, it is quite a complicated subject and true discrimination is problematic to achieve. However, in this suggestion of an up stream mcb of 40a, would probably provide no discrimination of a downstream 32a mcb, I don't believe or should I say think that is the case across all mcb ratings?

I link to one piece I've read on the subject;

Guide to discrimination | edmundson-electrical.co.uk - http://news.voltilink.co.uk/articles/guide-discrimination
 
It would discriminate between overcurrent but not necessarily fault current. What about a type B, 2A protected by a type D, 63A would the sweeping statement cover this too.
 
You tell me Westward, only too willing to learn.
I don't know I was responding to #48 but I would put money on discrimination under fault conditions and the 2A would go. It was a rather sweeping statement.
 
There will be no discrimination with mcbs in series except maybe in in westwards case :)but i think in the situation were it just a supply to a garage with a light and a socket in it i would not be to worried, what the worst that going to happen , you have a fault current on the socket circuit and so it trips the local mcb in the garage and the mcb in the house , not the end of the world , i have seen loads of commersial sites with 3 or 4 mcbs in series and 30ma rcds in series ,and yes a right old bodge in my opinion, but when you speak to the customer they say they have never had any problems so why change it . Even when i have explained the disruption it could course, they would rather put up with that than pay for an mccb board and rewiring
 
There will be no discrimination with mcbs in series except maybe in in westwards case :)but i think in the situation were it just a supply to a garage with a light and a socket in it i would not be to worried, what the worst that going to happen , you have a fault current on the socket circuit and so it trips the local mcb in the garage and the mcb in the house , not the end of the world , i have seen loads of commersial sites with 3 or 4 mcbs in series and 30ma rcds in series ,and yes a right old bodge in my opinion, but when you speak to the customer they say they have never had any problems so why change it . Even when i have explained the disruption it could course, they would rather put up with that than pay for an mccb board and rewiring
So if I had a 20A breaker supplying a radial socket circuit from the garage board, and there was a fault on that circuit - are you saying the 20A wouldn't trip independently of the 40A at the house end?
 
So if I had a 20A breaker supplying a radial socket circuit from the garage board, and there was a fault on that circuit - are you saying the 20A wouldn't trip independently of the 40A at the house end?
No i am saying thay would both trip at the same time if there was a fault current , but if you just overloaded the 20 amp mcb by less that 40 amps then only the 20amp mcb would trip
 
were it just a supply to a garage with a light and a socket in it i would not be to worried,
He has quite an unecessarily complicated installation.
For some reason he has this
The garage board is a dual board with an rcd main switch. At present I have it connected with rcds on both sides,

While this is an occassional use outbuilding, it has the potential for something much more substantial in the future.
 
He has quite an unecessarily complicated installation.
For some reason he has this


While this is an occassional use outbuilding, it has the potential for something much more substantial in the future.
Sorry by dual I didn't mean a split board - just two way, i.e. two breakers - purpose garage board.
 
2 RCDs in the same circuit is only a problem if tripping of the origin one will be a
nuisance. So it depends what other circuits are on the CU RCD.
Doing away with the RCD at the garage does nothing to help.
You can acheive discrimination if the CU RCD is higher opertaing current and/ or faster.
When I had a similar problem I tested a variety of 30mA RCDs to find a make that would discriminate on speed. I found Hager would always react faster than the existing RCD which was Merlin if my memory serves.
 

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