Garden Shed problem

Walker34

Hi,
a mate of mine has decided to wire his garden shed after reading a discussion on one of these groups. He used 6 squa swa cable and ran it the 30 meters from his house fuse board to the shed. He took the connection from a 20A socket mcb and ran the cable to the shed and a small sub board. This in effect means there are two rccd/elcbs on this cable and when even a light load is put on the circuit, the shed rccd trips. The cable checked out with a meter as not having any shorts or leaks to earth. With no load the rccd stays set. Is there a problem with having 2 rccds (one at either end) on the same circuit. What is the most likely cause of the tripping. The connections have all been double checked, and are sound.

bertie1

is there any chance he has the neutrals at the shed fuse board mixed up

davelerave

you're creating unnecessary problems to begin with by tying the shed supply into the household rcd .connect it direct to a 32amp mcb or switchfuse at house and provide earth leakage protection at shed.thats assuming supply cable has been properly meggered.two rcds will function in series but is not recommended,you may have a neutral crossed or earthed even if it trips under load conditions.

Walker34

Hi, thanks for the reply Bertie1 btw,

That was one of the first checks he done, and it is defiantly right.

The cable going from the house dp to the shed is in two sections, the first from the board goes under the floor, to a junction box on the inside of an outside wall. The second comes from that junction box to the shed along a garden wall.

When a test lamp is connected to the section at the junction box, all is good and the first rccd in the main DP stays set. When the lamp is then taken to the shed sub-board and is connected to an mcb and neutral, the rccd in the sub-board instantly trips. the conclusion is that the fault lies in the second section going from the junction box to the shed sub-board.

There are a number of other possible scenarios which could cause a leak, but he wants to eliminate the presence of 2 rccd`s as a possibility. I think from memory in a previous life that two elcb`s did not interfere with each other on the same circuit, at least not a socket circuit. I think some lighting fittings, like fluorescents were troublesome due to the starters arching inside.
I worked in a place which had rotating assembly benches with both sockets and fluorescent lights on them.

davelerave

get some professional help if possible for this

Walker34

davelerave wrote: »

you're creating unnecessary problems to begin with by tying the shed supply into the household rcd .connect it direct to a 32amp mcb or switchfuse at house and provide earth leakage protection at shed.thats assuming supply cable has been properly meggered.two rcds will function in series but is not recommended,you may have a neutral crossed or earthed even if it trips under load conditions.

Hi Dave,
I think he felt he was taking the safest option by doing it as he did. By doing it as you suggest the section of cable going under the floor to the shed is not protected by an rccd, just a fuse/mcb. Its probably overkill, but its nice to have a friendly rccd close by. He has earthed the amour as was the practice in the past. Not sure if that's still done. The load on the shed is negligible btw, one 200w lamp and possibly a small drill occasionally, no lawn mowers....no lawn:-).

What is the argument against dispensing with the rccd in the shed and relying on the one on the main dp. It does not answer the question.......why is the shed rccd tripping ?, and I suppose the right approach is to answer that question first.

Cheeble

Walker34 wrote: »

..... the conclusion is that the fault lies in the second section going from the junction box to the shed sub-board.....

It sounds to me more like a fault in the wiring of the shed sub-board itself. Are you sure that both L and N are correctly wired through the RCCB in the shed?

The two RCCBs won't interfere with each other, but the problem with using them in series is that (assuming they're both 30mA) there's no way to predict which one will trip first. This causes two problems: you don't know which board to go to to reset it; and you can't say with certainty which sections of the circuit are dead. In a large scale or industrial installation, this can be a real problem, I'd have thought it was something you could live with in your friend's case.

Cheeble-eers

Walker34

Hi Cheeble,
that has been checked multiple times and it is right(wish it wasnt....and it would be easy to fix). He is now changing the rccd in the hope its faulty....long shot, but any port etc.. Is certainly an unusual one. He could I suppose rely on the main dp rccd, but that really lacks the comfort factor of knowing exactly what the problem is.

This guy is very cautious, and I think he is of a mind to call a spark at this stage. I think all the obvious ones have been eliminated. Thanks for the wall to bounce thoughts off folks.

davelerave

Walker34 wrote: »

Hi Dave,
I think he felt he was taking the safest option by doing it as he did. By doing it as you suggest the section of cable going under the floor to the shed is not protected by an rccd, just a fuse/mcb. Its probably overkill, but its nice to have a friendly rccd close by. He has earthed the amour as was the practice in the past. Not sure if that's still done. The load on the shed is negligible btw, one 200w lamp and possibly a small drill occasionally, no lawn mowers....no lawn:-).

What is the argument against dispensing with the rccd in the shed and relying on the one on the main dp. It does not answer the question.......why is the shed rccd tripping ?, and I suppose the right approach is to answer that question first.

supply cable doesn't need rcd protection.get a pro to give it the once-over .there's plenty of sparkies available with the right tools now.you cannot be coached safely through these jobs

2011

supply cable doesn't need rcd protection.get a pro to give it the once-over .there's plenty of sparkies available with the right tools now.you cannot be coached safely through these jobs

+1

It is either:

is there any chance he has the neutrals at the shed fuse board mixed up

Or an earth to neutral fault.

Stoner

Walker34 wrote: »

Hi,
a mate of mine has decided to wire his garden shed after reading a discussion on one of these groups.

Sorry for adding little to the thread , +1 to the advice given above, but I'd like to point out that in a sticky on the main page (circuit diagrams) we identify a main consumer unit (thanks to fishdog) and and typcial shed consumer unit. We also clearly indicate that the feed to the shed should be taken from a supply wired like the Cooker supply, i.e. no RCD protection
http://www.boards.ie/vbulletin/showthread.php?t=2055400462

Just wanted to clear that up, I hope the problem is cheared up soon, by the sounds of it your friend was trying to do the right job in the first place, many people skip the shed consumer unit and complete a quicky job, spur of the nearest house socket so hats off to him/her for the effort, it wont cost much to get an electrician to check the cabling into the shed.

Walker34

Well wouldn't you just know it, got up this am at 9.00 for a spark to call at 11.00, and took one last look at the board and of course, as many had suggested, the neutrals were reversed. It was so obvious this morning, but last week?, nah its never that simple! Gonna get him to check it out anyway.....bah! and humbug! there`s another one that starts with a "b" but I wont use it here, and yer right , its "blast"!:-)

2011

Fair play to you Walker34! Now make sure all of the connections are good and tight.

The great thing about learning lessons the hard way is that you only have to learn them once!!!

davelerave

yes.welcome to the world of electrical work and hard lessons and knocks(speaking from painful experience and sleepness nights) .you will now remember to give your connections the once-over visually:D

Cheeble

Walker34 wrote: »

Well wouldn't you just know it.........

Yes, been there, done that, used the same words, and some more besides

Cheeble-eers