Hi, new guy here & some help needed with RCDs

[Sooby2]

Hi,

Just qualified City & Guilds 4555, 2393, 2382 and (EAL) 2394 with Able Skills in January and now a full time handyman - I have no intention of ever rewiring a house, just doing the smaller jobs.

I'm looking to practice at home first and my garage needs a rewire. I'm happy with the requirements, but I'm having some difficulty with the differentiation in the main (upstream) consumer unit. I have a Wylex CCU (10 way) with two 30ma RCDs and one spare space on each.

I have bought a 5 way Wylex unit for the garage that also has a 30ma RCD on it.

If I'm correct I should use a time-delayed form of protection in the main consumer unit, but I don't have room to fit another RCD, so I looked into RCBOs and came up with a 100ma RCBO Square-D-KQE132C10 on eBay (I can't get the link to work properly)

Will this do the job correctly or will the 30ma RCD it's under trip first anyway?

Many thanks for the help. Stu

 

[Leesparkykent]

Hi, what earthing arrangement do you have and what installation method is the distribution circuit?

 

[davesparks]

Why does your submain need RCD protection? And why would about want a square D Reno if you have a wylex CU?

 

[Sooby2]

I have a TNS system and it will be 4mm 3 core SWA clipped to a fence. In the house it will be under floorboards, with plenty of space around it (two feet between floor and the ground).

I want RCD in the garage as I want to fit an external socket for use in the garden and secondly, if there is a problem with something, I don't want to trek all the way back into the house (25 metres away) every time to reset the RCD there.

 

[davesparks]

I want RCD in the garage as I want to fit an external socket for use in the garden and secondly, if there is a problem with something, I don't want to trek all the way back into the house (25 metres away) every time to reset the RCD there.

Which doesn't answer the question of why your submain needs RCD protection?

 

[Leesparkykent]

is the board in your house high integrity?

 

[Sooby2]

Which doesn't answer the question of why your submain needs RCD protection?

OK, please explain why it doesn't - which would make life much easier I guess? If I don't need it, I can take it out. I'm going by what was demonstrated on my recent course, which was as I have described and I assumed it was correct.

And the board is high integrity.

 

[ChrisElectrical88]

If you install the armoured correctly it will have a earthed mechanical protection, therefore will not need an RCD.

 

[davesparks]

OK, please explain why it doesn't - which would make life much easier I guess? If I don't need it, I can take it out. I'm going by what was demonstrated on my recent course, which was as I have described and I assumed it was correct.

And the board is high integrity.

I don't know whether it does or doesn't as I have not visitited the property and looked at the job, which is why I asked why it needs it.
You wouldn't normally expect a submain on a TNS supply to need RCD protection.

 

[Leesparkykent]

OK, please explain why it doesn't - which would make life much easier I guess? If I don't need it, I can take it out. I'm going by what was demonstrated on my recent course, which was as I have described and I assumed it was correct.

And the board is high integrity.

The earthing arrangement and installation method doesn't require 30mA RCD protection for the distribution circuit. The final circuits will be covered by the RCD in the garage board. Having a high integrity board means you can feed the distribution circuit from a non RCD protected way. Doing it this way means you wont have RCD's in series with lack of discrimination. The only discrimination issue you would have would be having to circuit breakers in series of a similar current rating but you'll have to decide if that's going to be an issue and if it is look at a way of providing over current protection for the distribution circuit with a fuse or similar. Are there any extraneous conductive parts within the garage that will require bonding?

 

[Sooby2]

Leesparky, thanks for the advice. I'll look into it and see what I can do with the main board. I've already bonded the steel frame and metal doors of the garage several years ago, but I have a very old (as in ancient) fuse box in there that needs replacing.

 

[Leesparkykent]

Leesparky, thanks for the advice. I'll look into it and see what I can do with the main board. I've already bonded the steel frame and metal doors of the garage several years ago, but I have a very old (as in ancient) fuse box in there that needs replacing.

Are they actually extraneous conductive parts tho? Where have the bonds been connected to? If they've been connected to the earth marshalling terminal in the garage board that is using the TNS earthing arrangement then the CPC of the 4mm armoured cable is not of an adequate CSA to form part of a bonding conductor.

 

[Octopus]

And this is notifiable under part p

 

[westward10]

See you had to spoil it

 

[Octopus]

See you had to spoil it

Not spoiling - just educating....

 

[westward10]

Not spoiling - just educating....

Yeah I know was in jest, took 13 posts for it to raise its ugly head though

 

[mickfred]

If values permit fit a c type MCB in the first board , therefore a overload should trip the b type in garage unit fist

The earthing arrangement and installation method doesn't require 30mA RCD protection for the distribution circuit. The final circuits will be covered by the RCD in the garage board. Having a high integrity board means you can feed the distribution circuit from a non RCD protected way. Doing it this way means you wont have RCD's in series with lack of discrimination. The only discrimination issue you would have would be having to circuit breakers in series of a similar current rating but you'll have to decide if that's going to be an issue and if it is look at a way of providing over current protection for the distribution circuit with a fuse or similar. Are there any extraneous conductive parts within the garage that will require bonding?

 

[westward10]

A circuit breaker is not going provide good discrimination but a type C, assuming fault protection is maintained is the best option and they are not critical circuits should the wrong one trip.

 

[Leesparkykent]

If values permit fit a c type MCB in the first board , therefore a overload should trip the b type in garage unit fist

That will not provide short circuit discrimination.

 

[mickfred]

No it won't but it will solve overload problems and if plugging in transformer in the garage only the b type will have to be reset not walking back to the house every time

That will not provide short circuit discrimination.