Swa & cpc

[chloebear]

I have a 3 phase sub board to run in SWA - I am proposing to use 16mm 4 core with the armour as the main Cpc for the cable. I was also concidering installing a seperate 16mm as a seperate earth.

I am having a battle with myself as to the necesity of the additional 16mm earth - I can see the logic of using one of the cores of the cable as a CPC (in this case I would need to run 5 Core!!!!) as this would be an integral part of the cable protection should the armour fail for some reason.

But what additional protection would a separtate 16mm cable afford for the cable & sub board????????

 

[Chr!s]

Please note that the term 'copper equivelance' used in the chart, does not refer to equivelance of conductance as refered to in the note for Table 54.8 of BS7671.

I disagree there spin

 

[pushrod]

Would the sink not fall into supplementary bonding which we dont have to do anymore?

It depends on if it is a special location and whether or not there is 30mA rcd protection.
If, however, the sink has a metal waste pipe it will definitely require a main protective bond.

 

[chloebear]

I love these discussions - The sub main will only be feeding 3x cooker isolators which will be surface cabled and mounted. The isolators are 3 meters away from the metal sink unit & the nearest cooker outlet point would be about 1.5 metres from the sink unit. Probably not going to provide RCD protection on the cooker circuits.

If the pipes feeding the kitchen sinks have been bonded already, should there be any requirement to bond them to the sub main board???

The area that the sub main will be in is an internal room and no adverse conditions apply, the same applies to the board that it will be fed from.

Myself, I can't see any problems with using the armour of the swa and leaving the bonding as it is - but always happy to be proved wrong.

 

[pushrod]

Think you need to test resistance between simultaneously accessible exposed and extraneous conductive parts (if there are any)
R needs to be equal or less than 50V/Ia for the PD concerned if no rcd protection.

 

[Deleted member 26818]

I disagree there spin

Your choice Chris, I don't suppose you'd like to state your reasons, for doing so?

 

[Chr!s]

Spin

Your choice Chris, I don't suppose you'd like to state your reasons, for doing so?

Well i was referring to calculations for a bonding conductor SWA, so copper equivalent is used. Though having re read the thread i assume you are referring to a SWA/CPC.

And i agree if used as a cpc we dont use direct copper equivalent, as we need to include K values table 54.7.

So i do apologizes, should of read the whole thread

 

[malcolmsanford]

I love these discussions - The sub main will only be feeding 3x cooker isolators which will be surface cabled and mounted. The isolators are 3 meters away from the metal sink unit & the nearest cooker outlet point would be about 1.5 metres from the sink unit. Probably not going to provide RCD protection on the cooker circuits.

If the pipes feeding the kitchen sinks have been bonded already, should there be any requirement to bond them to the sub main board???

The area that the sub main will be in is an internal room and no adverse conditions apply, the same applies to the board that it will be fed from.

Myself, I can't see any problems with using the armour of the swa and leaving the bonding as it is - but always happy to be proved wrong.

As a kitchen is not classed as a special location in the BS 7671-2008 then is there a need to have that sink bonded at all?

If your not geting a low enough Zs for disconnection times on an existing circuits protection device then yes a way to achieve a safe enviroment by limiting the touch voltage is by the use of supplementary equipotential bonding in the kitchen and the metal work, that is proved to be an extraneous conductive part, within it. Though as your installing new circuits there are better ways to achieve disconnection times, by installing conductors of a large enough CSA to obtain the low enough Zs or the use of an RCD, my prefered option is cable CSA. IMO supplementary equipotential bonding should be something that is only used in a special location, which a kitchen in not classed as.

Also you can check the effectivness of the bonding by making a test between the pipe work, extraneous conductive part, if it is an extraneous part, and an exposed conductive part, case work of cookers, check out section 415 and in particular reg 415.2.2 for this.

You may find that the original installation was done to the 16th edition or perhaps by a sparks use to the 15th where the attitude was metal pipe ..bond it .........must be safe!!

If it does need bonding, which to be honest I doubt, unless it was fitted to achieve safety on an existing circuit, then as it would be supplementary equipotential bonding, then it should be done exactly the same as you would in a location containing a bath or shower, and that is connecting the bonding conductor to the CPC within the location.

 

[Chr!s]

The other issue is if you bond the sink and its not extraneous, it will be placed at system potential under fault conditions or even normal if PME, this may increase the risk of electric shock, especially if the floor is tiled and a bit earthy for example.

 

[Ladders]

Hi. I need to run a 40m length of SWA in domestic premises for TNS. It will be a RCD protected radial circuit and connected to own 32a MCB in house CU. The run will go to detached garage with another non RCD CU with main switch, running x1 20a and x1 6a MCB. Is 3 core 2.5mm SWA OK? Cheers.

 

[pushrod]

Hi. I need to run a 40m length of SWA in domestic premises for TNS. It will be a RCD protected radial circuit and connected to own 32a MCB in house CU. The run will go to detached garage with another non RCD CU with main switch, running x1 20a and x1 6a MCB. Is 3 core 2.5mm SWA OK? Cheers.

If clipped direct 2.5 mm will be ok for CCC at the garage but needs protecting by a smaller mcb at the main CU
- 32A mcb will not protect it, try 20A. If your load is much above 10A you will fail to meet 3% for volt drop on lighting circuit.

Also the swa does not need rcd protection, better to have the rcd at the garage CU then it is easier to reset, doesn't take out parts of the house when it trips and easier to test when doing the certs
.

 

[Ladders]

Thanks. I forgot about the lighting volt drop and just looked at the other! So will need to go to 4mm SWA, plus some of the run will be buried, about 5 metres. The SWA is being connected to dual RCD in house so not much choice there. Not sure what you are saying about going with 20a MCB in house instead of 32mcb? What then would you use on garage board for sockets? 16a? Cheers.

 

[pushrod]

Thanks. I forgot about the lighting volt drop and just looked at the other! So will need to go to 4mm SWA, plus some of the run will be buried, about 5 metres. The SWA is being connected to dual RCD in house so not much choice there. Not sure what you are saying about going with 20a MCB in house instead of 32mcb? What then would you use on garage board for sockets? 16a? Cheers.

A 2.5mm² swa is only good for 28A clipped direct. So a 32A mcb will not protect a 2.5mm² radial against a fault current of say 30A - the cable would overheat/catchfire before the mcb tripped. 16A would be ok for the sockets.

Re the volt drop you need to decide what loads there are likely to be in the garage

However, if you are going up to 4mm² then that is good up to 38A, so you could put a 32A mcb on that.

You will also be ok on volt drop for the lights, unless you are planning on some heavy loads.

Re RCDs - you do not want 2 30mA ones in series . Sometimes it is possible to re jig a CU to get an unprotected way. No problem though it is just less convenient having the rcd for the garage back at the house