Earth rod needed or not for outbuilding supply?

[shazchaudary]

Hi

I have an outbuilding that is some 25m (cable run length) from the CU of the main house. The two are connected in 10mm T&E in waterproof ducts already existing on the site. The cable has been replaced for a new one of the same size.

The main house has a new MCB installed on the RCD side feeding the outbuilding. The outbuilding is installed with another RCD to protect the sockets etc in the outbuilding. The problem I am having is that when all is connected between the two buildings, all seems fine. The minute I put any load on the outbuilding, the RCD in the outbuilding and the main house CU trip. I have disconnected all circuits from the outbuilding and just wired in a single socket to a MCD hanging off the RCD side and I get the same. If I move this single socket to the non-RCD side of the outbuilding I get the same problem.

I think this could be down to a poor earth in the outbuilding. (The 10mm T&E has a 6mm earth). So the question is really do I need an earth rod for the outbuilding? Does it sound like it will be causing the issue? With an earth rod for the outbuilding, Do I still connect the 6mm earth from the mainbuilding to the outbuilding?

Thanks for any suggestions...

Shaz

 

[Gaz the Sparky]

Are you sure that you have connected the 10mm t+e correctly in the main house consumer unit. There are two neutral bars in the split load board if you connect the neutral to wrong one then the RCD senses the imbalance and disconnects the supply

so just check which bar you have connected the cable to.
G.T.S

 

[Steve]

And you dont need 2 RCDs. Mainly for the reason you give here. You have to trail back to the house to reset every time it trips. Its also pointless having 2 RCDs in a row. They do the same job.

Best to just have local protection, the cable doesn't need it. (Unless, as I suspect your "waterproof ducting" is plastic and therefore offers no electrical protection via an earthed outer sheath.)

 

[shazchaudary]

I have checked the neutral bars and i have connected to the RCD one. And yes the 'waterproof' duct is plastic hence the desire for 2 RCD units...

 

[Steve]

You still only need one! At the house in the case. Sell the other on ebay!

 

[davy_owen_88]

I think this could be down to a poor earth in the outbuilding. (The 10mm T&E has a 6mm earth). So the question is really do I need an earth rod for the outbuilding? Does it sound like it will be causing the issue? With an earth rod for the outbuilding, Do I still connect the 6mm earth from the mainbuilding to the outbuilding?

What earthing arrangement do you have? TT, TN-C-S, TN-S??

Regardless of this though, a poor earth does not cause an RCD to trip. It is more than likely that you have just made a mistake with the neutral bars. What testing was done on the circuit? What were the insulation resistance readings like?

 

[RF Lighting]

You still shouldn't have buried T&E in a plastic duct regardless of wether it is RCD protected or not.

 

[shazchaudary]

Ok understood. If I was to run armoured round the garden a fresh, what size should I use and? I shall check the neutral connections again!

 

[davy_owen_88]

What testing did you do on the circuits? It may be a damaged cable that is causing the tripping.

What is your earthing arrangement? This will determine whether you need an earth rod or not.

To determine what size cable you need you will need to provide us with some clues as to what you need. What is the outbuilding for? What is the maximum demand and what kind of loads will it be supplying?

 

[scott6471]

Hi Shazchaudary.

RCD's inside and out is not that bad an idea as has been suggested. If done correctly then you get the benefit of RCD protection but without the fault taking out the entire electrical system as oppossed to just the shed. In order to do this though you'd need "time delay" RCD's which can be adjusted so as not to have both trip at the same time.

That aside, what about the Neutral connections in the external CU, have you checked them? Does the problem occur when plugging in different bits of equipment or just the one? (could be the appliance). What is the appliance you're using to test the circuit? (fridges/freezers can nuisance trip RCD's if they are slightly faulty).

Scott
(Man with hole in pocket feel cocky all day)

 

[Steve]

RCD's inside and out is not that bad an idea as has been suggested. If done correctly then you get the benefit of RCD protection but without the fault taking out the entire electrical system as oppossed to just the shed. In order to do this though you'd need "time delay" RCD's which can be adjusted so as not to have both trip at the same time.

This wasnt the point. Most installations have no need for a TD RCD, and they aren't widely available at the sheds. My point really, I guess was that so many DIYers waste money on unessecary RCDs, simply because they see them on the shelf at B&Q. So their shed might have RCD protection from the house, RCD in the shed CU, and an RCD spur supplying an RCD double socket! FOUR RCDs! All because they saw it in B&Q and thought it was a neat idea and nessecary (and didnt know what they were doing!) And they dont even know what an RCD does

However, he shouldn't just change his house RCD for a TD one, as it is probably providing protection to the house circuits too.