Need some advice

[Essex]

Just had a look at a job and need some advice.

Partners relative has brought a garage CU and wants it fitted, it's a MK board with a 30mA RCD and two MCB's.

The property is very old, CU is BS3036 rewirable type with MCB's and protected by a 100mA RCCB.

Supply is Type IIA 60A, tails and earthing is 16mm.

I'm gonna run 4.0mm from CU up into the loft then 4.0mm SWA on catenary into garage into CU.

Do I need to upgrade the 100mA RCCB to a 30ma RCD?

I am NIC/EIC registered but don't do a lot of domestic.

Thanks

 

[Manator]

I would check the earthing arrangements to make sure of the type, with the inclusion of a 100mA RCCB it sounds like it could be TT.

you should not upgrade the 100mA, but any final circuits you put in will require 30mA RCD protection.

 

[fletchflash]

depends on where the 4mm run into the loft is installed, if its within 50mm of a surface of a wall then it will require 30mA RCD protection, but as stated, not a good idea to change the 100mA. Is the 100mA a time delayed RCD? If not you will not acheive discrimination between the garage RCD and this one.

If the 4mm cable does need RCD protection then you could run this through a seperate housing unit at the origin (next to DB) and not fit the RCD into the garage Board

 

[Doc Hudson]

Do I need to upgrade the 100mA RCCB to a 30ma RCD?

Why do you consider this an "upgrade"? Having one 30ma RCD protecting the whole property would be bad practice IMHO.

Doc H.

 

[Essex]

Thanks for the response guys

The cable would be run in the loft clipped direct to the beams and on show, I was gonna run T&E from CU to joint box in loft and then SWA outside into garage CU, or I could run SWA all the way, either way I should be able to utilise the RCD on the garage board.

I'm not too sure if the RCCB is a time delayed RCD, where would be the best place to feed the garage from?

Thanks

 

[fletchflash]

what i mean is how you gonna get the cable into the loft, clipped to the wall/trunking or plastered in? if plastered then the <50mm reg applies.

 

[Doc Hudson]

I'm not too sure if the RCCB is a time delayed RCD,

If the initial 100ma RCD is not time delayed there with be little use in putting another non-time delayed RCD downstream. You will have problems knowing which will trip first.

Doc H.

 

[Essex]

Yes, I see what you mean

The existing CU is in a cupboard above a doorway and the place is a bungalow, there would be 8"-12" of cable burried from the top of the cupboard to the ceiling, so a RCD would be required at the main CU end.

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Would it be best to take the tails off the RCCB an put it into a henly block then one set of tails back into the RCCB and another set of tails into a RCD then off to the garage CU and replace the existing RCD for a main isolator switch?

 

[jameseye12]

Or you could replace the RCCB with a 100ma time delayed RCD and the out of the this henley block the tails after they come of the RCCB. This can then feed your existing CU and a new 30ma RCD in a separate enclosure feeding the garage.

I don't think you could split before the RCCB if this a TT install as I think you need to provide RCD protection for the tails which is provided here by the RCCB. I'm not 100% on this so I may have got this wrong.

 

[NozSpark]

Don't forget...

Your sub main will also need over current protection

 

[Essex]

Yes, has MCB's already.

 

[PC Electrics]

If you surface mount the t&e just above the fuseboard (in mini trunking, say) then you won't need to RCD protect the distribution circuit.

You won't be able to achieve RCD discrimination, but your sockets in the garage will need 30mA RCD. The existing installation won't be any worse off than it was before but your new section will comply.

A better (electrically) solution (but less aesthetically pleasing if it's all on show) would be to henley the tails and fit a garage board next to the existing. Then run a 20A circuit out to the garage sockets and just use a 3A FCU for the lights. This way you could use 4 or 6mm H07RN-F across your catenary instead of swa (easier to terminate and handle).

 

[batty]

How much cable is being run in loft surely by the time you have brought a box terminated it it would be easier to run swa all the way.

 

[Essex]

Why do you consider this an "upgrade"? Having one 30ma RCD protecting the whole property would be bad practice IMHO.Doc H.

Doc H, Correct me if i'm wrong but a 30mA shock would probably give you cramps, a 50mA-80mA shock would probably kill you but you can be revived, anything greater and your chances of revival are very slim.

On a fault condition of greater than 100mA, both RCD's would trip, on a fault condition of 50mA, only the 30mA RCD would trip (Do I really need a time delay RCD?)

By upgrading the 100mA RCCD to a 30mA RCD, would that not make the installation safer?

If i put the meter tails onto a henley block and split the circuits, as the garage CU has 30mA RCD, would that not give me discrimination?

 

[Doc Hudson]

Doc H, Correct me if i'm wrong but a 30mA shock would probably give you cramps, a 50mA-80mA shock would probably kill you but you can be revived, anything greater and your chances of revival are very slim.On a fault condition of greater than 100mA, both RCD's would trip, on a fault condition of 50mA, only the 30mA RCD would trip (Do I really need a time delay RCD?)

By upgrading the 100mA RCCD to a 30mA RCD, would that not make the installation safer?

If i put the meter tails onto a henley block and split the circuits, as the garage CU has 30mA RCD, would that not give me discrimination?

Hello Essex, greater sensitivity isn't always the best thing. Most real world faults will exceed 100ma and it would be pot-luck which RCD trips if you have two non-time delayed RCDs wired in series. Non-time delayed would have to be wired in parallel to achieve discrimination. Increasing the sensitivity of an RCD can also increase the number of nuisance trips due to natural leakage currents. If the whole installation is protected by one RCD and goes dark when it trips, safety can become a physical injury issue not an electric shock issue. I am not clear if you have said what the earthing type is? I am assuming TT by the fact you have this 100ma RCD already installed?

Doc H.

 

[Essex]

Hi Doc H

Thank you for your response, yes, the system is TT.