RCB protection for outbuilding

[bananaman]

Hi,

I have a question regarding RCBs and would appreciate some help.
I have a new consumer unit fitted (split load) and one half includes the circuit for my garage, which in turn supplies appliances in the garage and also goes off to supply a pond pump. I know the existing garage RCD device that protect this circuit need to be removed, to prevent nuisance tripping at the consumer unit. But, specifically, how do I protect the pond circuit, such that the rest of the garage does not go down, if the pond circuit develops a fault?

 

[Taylortwocities]

How long ago was the consumer unit fitted?
How many RCDs are there in it?

 

[ericmark]

If you have a "New" consumer unit with split load how many splits? Sometimes two splits both with RCD protection and sometimes three way with one way with no protection.

The "Old" split consumer units would only have one RCD.

Although there is mention of discrimination in the 17th Edition it also says you must fit a 30ma on supply and in a caravan so plainly there is nothing to stop you fitting two RCD's supplying each other.

However having said that 10ma RCD's are available and I would use a 10ma RCD for a pond supply. However you will need to go to an electrical supplier I failed to find any in screwfix.

There are also RCBO's but I don't know if available in 10ma version.

 

[Taylortwocities]

But a 10mA RCD in series with a 30mA RCD will not provide discrimination. If the pond pump fails with an earth leakage of greater than 30mA then it could be the 10, or the 30mA RCD that trips - it will just depend on the trip time.

Best would be a 30MA RCBO in an unprotected way to serve just the pond pump, but lets see what sort of consumer unit he has....

could we ask bananaman to post a picture?

 

[bananaman]

Thanks for your replies. Here is a more complete description:

The consumer unit is brand new: that's why I am addressing this issue now, as I know that I may get nuisance tripping otherwise.

The new consumer unit is split two ways, with a 30mA trip RCCB on each side of the unit.

Basically, the layout from the consumer unit goes in order, like this:

Consumer unit
Cable to garage (reinforced beyond the house)
Garage 20A fusebox
(Fused spur to garage lighting)
Cable to RCD 30mA trip twin socket (used to protect power tools etc with old consumer unit). Proposed to replace with plain sockets, as no longer required?
Cable to RCD 30mA trip twin socket (used to protect power tools etc with old consumer unit). Proposed to replace with plain sockets, as no longer required?
Cable to twin socket (NO RCD), used to power appliances, fridge, freezer.
Cable to RCD 30mA trip junction box supplying pond
Cable to pond isolating switch and swap to armoured cable to run out of garage.
Armoured cable to pond pump and filter

I understand that I should replace the RCD sockets, as the consumer unit protects these sockets now. But I want to ensure that any failure of the pond equipment does not trip the consumer unit and take down the fridge and freezer in the garage. Is there a way of ensuring that a device that protects the pond circuit will not trip the main unit?
Hope this is clearer and thanks for the help

 

[bananaman]

PS

The RCD devices at the consumer unit are Hager CD 263U devices and there are two of them.

 

[ban-all-sheds]

Simple - you need to get the pond equipment on a different RCD group to the fridge and freezer.

Ways to do this would be to not have the garage CU on an RCD supply, and to have that one split or use RCBOs, or to take the pond supply from the other side of the house CU, not from the garage one.

BTW - having two (or more) RCDs in series on the same circuit will not cause nuisance tripping, and discrimination is only a regulatory requirement where the lack of it would be dangerous.

 

[bananaman]

Thanks for your excellent replies. I have (hopefully) attached a picture of the layout at the box, but that will possibly soon change as explained below.

So, If I understand things correctly, my options seem to be:

1. Get the pond on a separate circuit protected by a RCBO
2. Get the pond and remaining garage equipment on separate halves of the CU in the house, so that they trip separately in the event of fault.

I have tried to avoid considering this, as it involves major re-wiring outside, digging cables in the garden etc. The other reason for avoiding this has been the shortage of spare fuse slots, as explained below:

When the new CU was fitted, there were space restrictions. So, I was led to understand that I could have a minimum of 8 fused slots within the space available on a split load dual RCCB arrangement. I currently have 5 circuits, with a requirement for a total of 6 in the near future. That leaves 2 spare for future expansion. Well, come the day, with the wires hanging off the wall and the old fusebox in bits, the electrician discovers he's got the wrong CU!. Panic. Run off to the supplier to get the right one. When he arrives back, he announces that they didn't have the one he originally requested and offers a solution of 6 fused slots. With the gun to my head, I reluctantly agreed. (Well, what would you do?).

In the picture, you will see that 5 slots are used and there is a spare 6th slot (50 amp for new cooker expected). This leaves me nothing spare.
Having just spoken to the electrician again, he is proposing to free up some space by removing one of the RCCBs and fitting an RCBO on one of the existing circuits. This will create two spare slots and still be within regulations apparently. So, I have a couple of follow up questions:

1. Can you point me to a list of acceptable (wihin regs) combinations of MCB, RCCD and RCBO within the CU? I don't mind about the additional cost of putting RCBOs on individual circuits. I just want a set-up that is safe, within the regs and flexible.

2. Concerning the pond, if I replaced the existing garage 1-way wire fuse box with a 2 way RCCB or RCBO arrangement (one for the appliances and one for the pond), would that guarantee that the pond doesn't trip the appliances?

Once again, thanks for your help. Just realised I don't know how to attach a picture, so I will post that separately.

 

[Taylortwocities]

You need to retain RCD protection for all of the existing circuits, including the garage.

IMO, the best way would be to re-organise the consumer unit and install an RCBO to serve the pond only. The RCBO would be an unprotected way after the main switch and before the RCD protected ways.

If there is no unprotected ways available, the board will need re-working, for Hager this will mean installing an additional mini busbar. This one: http://www.tlc-direct.co.uk/Products/HGVAT05.html

PS, notifiable work.

 

[bananaman]

Untitled

• bananaman
• 7 Jul 2009
• 1

 

[Taylortwocities]

Well, you have no space because of your sparky's error.

In France it is regulation that you must leave a spare two ways in dist board at the time of installation. i would suggest this should apply here too.

If he is going to swop out the RCD & MCBs for RCBOs then I hope he is doing it at his cost. It will be around £30 for each RCBO and he will have to retest and certify each circuit, again.

 

[ColJack]

one with the cover off?

if the bussbars can be re-configured to suit then..
ground floor sockets could be on a rcbo, lose the first rcd giving you 2 free spaces, no need for the garage to be on the RCD if it's got it's own CU in there with rcd / rcbo protection...
that leaves you with the b50 not on an rcd, which if it's intended use was for a shower is a necessity.
so that's another RCBO you'll have to buy..

is that a 16A to the garage? an you've a fridge AND freezer on that?

 

[ban-all-sheds]

When he arrives back, he announces that they didn't have the one he originally requested and offers a solution of 6 fused slots. With the gun to my head, I reluctantly agreed. (Well, what would you do?).

I would have told him to re-fit the old one, or temporarily fit the wrong new one, and then at a later date to return and fit the one you had agreed on. I would also point out that until the agreed CU had been fitted and tested etc that the job would not be satisfactorily completed, and that until the job was satisfactorily completed there would be no payment made.

1. Can you point me to a list of acceptable (wihin regs) combinations of MCB, RCCD and RCBO within the CU? I don't mind about the additional cost of putting RCBOs on individual circuits. I just want a set-up that is safe, within the regs and flexible.

As long as all final circuits end up RCD protected, and not all on one RCD, then you can't go wrong.

2. Concerning the pond, if I replaced the existing garage 1-way wire fuse box with a 2 way RCCB or RCBO arrangement (one for the appliances and one for the pond), would that guarantee that the pond doesn't trip the appliances?

Yup, as long as that CU isn't fed by an RCD device on the main board.

If you remove the RH RCD you'll then have 5 ways available (assuming Hager RCBOs are single-width):

• 2 RCBOs for sockets & cooker
• 1 MCB for garage
• 2 spares

50A is one heck of a lot for a cooker - what is it? If you have space in the kitchen for a small CU you might like to consider having one, supplied by an MCB, so that you can break the circuits down into hob, oven, hood etc...

 

[ban-all-sheds]

that leaves you with the b50 not on an rcd, which if it's intended use was for a shower is a necessity.
so that's another RCBO you'll have to buy..

Or redeploy the RCD removed from the CU in a new small enclosure next to it....

 

[ColJack]

he didn't have room for a bigger CU, what makes you think he's got room for an extra box next to it now?

ah I see that it's for the cooker.. must read the whole thread more carefully in future..