Use RCD for 130m cable run to outbuilding?

[Ewok]

Hi all. I have a couple of outbuildings which I laid cables to from the house. Both are about 130m cable runs away individually.

Just thinking about the time it takes for a RCD to "trip" - do I need to put RCD's in the distribution units? at the buildings? Also I presume it would be wise to also fit a good earth rod too.

Any advice would be greatly received.

 

[Ewok]

So I see most of the "garage" consumer units have RCD's in so I guess multiple RCD's on a circuit is OK. Our earthing in the house is TN-C-S - but we are out in the sticks and with overhead wires - which we have lost once before in a gale since we have been here - so an earth rod I guess would be prudent.

 

[Resu]

Why multiple RCDs? What type of cable have you run?

 

[telectrix]

i'm confused. why do you mention multiple RCDs? each outbuilding should have it's own RCD, independent of the other. what cables have you installed (type, no. of cores, size).

 

[Ewok]

10mm 3 core SWA. Just required for lights and a plug socket.

 

[telectrix]

one cable or two? estimated load for each outbuilding. c'mon mate more info. this is like pulling teeth.

 

[Deleted member 26818]

SWA is a cable used as an alternative to RCD protection and for situations where RCD protection is not acceptable.

 

[Taylortwocities]

There is an excellent guide to power for outbuildings here
http://electrical.------.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf
This was written under the regulations of the 16th, but still valid.

Normally you do not have an RCD at the supply end but you do at the remote end. You don't want a 260metre trek back to the house every time the RCD trips!

So, RCD at remote end, none at the source end and yes a separate earth rod for each outbuilding with the usual precautions if you have livestock out there!

 

[Spoon]

You don't want a 260metre trek back to the house every time the RCD trips!

I've been seeing this type of comment a lot. Just out of interest, how many times has your RCD tripped in the past two years?

Someone should set up a pole to see how often RCD's trip for the people on this forum..

 

[Ewok]

Thanks everyone. Info - sorry - two separate cable runs on different directions - total 130m each not 260m total. Load is for a few lights and a plug. Cant see any more than about 10A required peak max - mainly lights at say 300w.

One last question - no RCD at the source end - what is the proper way to bypass the RCD in my house distribution board then - job for my sparkly this one - and to run his eye over what I have done.

And one more last question... Can I terminate the SWA cable in a Wiska box next the distribution board and then cable from that. You can't really fit glands to a domestic distribution board. Also the earth. Earth rod to distribution board - then can I piggy back from that to earth the SWA armour or should it have it's own? - sorry if that is in the guide - I have not read that as yet.

 

[Andy78]

If you are not an electrician but are getting an electrician to oversee this installation, then all questions regarding the installation would be best directed at them.

 

[Taylortwocities]

If you are not an electrician but are getting an electrician to oversee this installation, then all questions regarding the installation would be best directed at them.

I absolutely agree. You need an electrician on site to advise and design. There are so many gotchas, especially these days with AMD3 requirements and TT supplies being near the top of the list. In any case, this is NOTIFIABLE work!

Re the 260metre comment. That was my take on how far you have to walk if you in the outbuilding and trip the house RCD. Its then 130 metres to the house and another 130 back again..

As an overview here what i would do.

House end. Henley block supply and provide a mini DB with 100mA Time Delay RCD. two MCBs one for each cable to the outbuildings. Armour earthed from this end ONLY. This gives protection for the cables.

At the outbuildings a mini DB with 30mA rcd. Separate earth spikes making the supply types
in the outbuildings TT. MCBs for the lights&sockets in each outbuilding.

 

[Taylortwocities]

I've been seeing this type of comment a lot. Just out of interest, how many times has your RCD tripped in the past two years?

Not many. But then I like to think I have an appreciation of things, electricity-wise.

I'm looking at the worse case, we don't know what he's going to plug in. A dodgy power tile cutter, a crappy outside light, failing pond pump, faulty milking machine, who knows...

 

[bigspark17]

I absolutely agree. You need an electrician on site to advise and design. There are so many gotchas, especially these days with AMD3 requirements and TT supplies being near the top of the list. In any case, this is NOTIFIABLE work!

Re the 260metre comment. That was my take on how far you have to walk if you in the outbuilding and trip the house RCD. Its then 130 metres to the house and another 130 back again..

As an overview here what i would do.

House end. Henley block supply and provide a mini DB with 100mA Time Delay RCD. two MCBs one for each cable to the outbuildings. Armour earthed from this end ONLY. This gives protection for the cables.

At the outbuildings a mini DB with 30mA rcd. Separate earth spikes making the supply types
in the outbuildings TT. MCBs for the lights&sockets in each outbuilding.

I have to agree with this install but question what are your thoughts behind the 100mA rcd at house side what are you achieving with this that a main switch doesnt?

 

[Taylortwocities]

I have to agree with this install but question what are your thoughts behind the 100mA rcd at house side what are you achieving with this that a main switch doesnt?

The cable to the outbuilding would be protected by an MCB, lets say its 32A. If there is an earth fault on the cable the MCB should trip, but the earth fault current will be restricted by the impedance of the earth/ROD at the house and, and that is never going to be enough to trip a 40A MCB.
So an RCD is needed to provide this protection. Its not for personal protection, so 100mA fits the bill, but it needs to be time-delayed or an earth fault in the outbuilding could trip both the remote 30mA RCD, and the 100mA RCD at the source end.

 

[Deleted member 26818]

No rod at the house, house is TN-C-S.

 

[bigspark17]

Tncs at house!

 

[Risteard]

two MCBs one for each cable to the outbuildings.

I would far prefer to see a HRC fuse instead of MCBs for the distribution circuits. Otherwise you may still end up with your 260m walk!

 

[Risteard]

No rod at the house, house is TN-C-S.

Although it should be mentioned that there is nothing inherently wrong with having an earth electrode installed with a TN-C-S supply. (In fact much of the world requires this!)

 

[Taylortwocities]

No rod at the house, house is TN-C-S.

Oh poo. How embarrassing.
I was responding to another topic elsewhere and got mixed.

Agreed, no need for RCD at house end.