Distribution circuit protection

[pencilpusher]

Hi all

I am planning a CU change for assessment. My detached garage is currently fed from a 16a MCB in my CU. The garage CU has RCD protection for sockets and lights. My question: Can I run the SWA from an unprotected way in the house CU (as it is now) so that I don't have to consider discrimination between two RCD's (One in house CU and the second in the garage CU). The original installation is to the 16th edition so I am OK with bonding etc but this circuit is puzzling me.

The db T&E cable goes immediately out of the house wall to a box where the swa is terminated and runs 2 meters underground to the garage. The only reg I am thinking of here is 522.6.6 I do not know if the cable is less than 50mm from the surface of the kitchen wall that the cupboards sit on.. The CU is on an outside wall and goes down it and then through it to the box. The CU is in a kitchen cupboard. Just wanted to give all the facts so you know what i have here!

I don't want this causing problems during assessment.

Thanks in anticipation. :smiley2:

PP

 

[not clever]

First thing what is the over current device rating for the sockets in the garage?

 

[pencilpusher]

They are protected by a 16a MCB.

 

[not clever]

Well you'll be needing to up the mcb at the house end then if the submain cable permits (20amp)

SWA from an unprotected way in the house CU (as it is now) so that I don't have to consider discrimination between two RCD's (One in house CU and the second in the garage CU).

You can.

The only reg I am thinking of here is 522.6.6 I do not know if the cable is less than 50mm from the surface of the kitchen wall that the cupboards sit on..

So leave submain circuit in place & covered by rcd at the house & change garage rcd to BS EN 60947-3
Or
Remove t/e part of circuit & rewire with swa & move to non-rcd side of cu.
Or
Look in the BGB to see how you can negate the use of an rcd for the submain....it's all in there:wink5:

 

[Richard]

What is your earthing arrangment?

 

[pencilpusher]

Its TNCS

 

[pencilpusher]

Having had a good look at the detatched garage CU, it has a separate TT system running to an earth rod that was not marked and I only found it by digging! So I am on TNCS with the garage having a TT sytem. This now prevents me from changing the garage RCD to a main switch. Rewiring the short T&E run with swa will be a mare as its behind the kitchen cupboards. Am I able to fit a time delayed RCD or RCBO?? in the house CU to achieve discrimination? (That will mean 3 RCD's in CU though?, if Time delayed RCBO not available)

Thanks

PP

 

[KFH]

Oh what fun we have.

I would suggest that unless you can prove the T&E run to the SWA is more than 50mm deep then you have to have RCD protection on it. I would think that it is unlikely to be so deep as it was not a 16th reuirement. Also some say that where it runs through the wall it would also need RCD protection if not in metal conduit.

I think your options are:

Put a separate S type RCD in an enclosure next to the CU with surface wiring or trunking to the CU.
Fit SWA all the way, as we cannot see the job it is difficult to know if this is pratical.
Try and fit a third S type RCD in the CU to protect the circuit. I don't think you can get time delayed RCBOs.
Run new T&E in trunking until it goes through the wall.
FInd a different job for your assessment. :icon7:

 

[pencilpusher]

Oh what fun we have.

I would suggest that unless you can prove the T&E run to the SWA is more than 50mm deep then you have to have RCD protection on it. I would think that it is unlikely to be so deep as it was not a 16th reuirement. Also some say that where it runs through the wall it would also need RCD protection if not in metal conduit.

I think your options are:

Put a separate S type RCD in an enclosure next to the CU with surface wiring or trunking to the CU.
Fit SWA all the way, as we cannot see the job it is difficult to know if this is pratical.
Try and fit a third S type RCD in the CU to protect the circuit. I don't think you can get time delayed RCBOs.
Run new T&E in trunking until it goes through the wall.
FInd a different job for your assessment. :icon7:

Thanks KFH. Some excellent points there. I am not sure the garage needs a TT system. There is no earthed metal work in there and I have a TNCS supply to the house. I would have thought the earthing could have been exported to the garage CU via the CPC in the swa. If that was possible I could then protect as outlined above and change the garage RCD to a main switch. What a pain in the ar*e! Any views?

 

[KFH]

I would have no problem exporting the earth especially if there are no services in the garage, I should have put that as an option but it is very easy to look at the detail and miss the big picture. You need to check the size of the armour for CPC sizing but with an RCD it should not be a problem but the assessor may ask. However I did an outbuilding once with the same assumption and tested a bit of miscellaneous metal work and got a Ze of 70 Ohm had to convert to TT and bond lots of metal work I had assumed was not extraneous, I could have bolted the MET to the wall for an earth. Good luck.