Help with swa armouring as suitability as c.p.c

[JohnS888]

Hello all

Am trying to calculate suitability of using swa armouring as c.p.c. It is a 6mm 2c supplying a log cabin.
From table 54.7 the calculation I need to use to find min csa of armouring is (k1/k2) x S
Taking value from table 43.1 for 90c cable I have k1 = 100 and K2 = 46 (Table 54.4)

So (100/46) x 6 = 13 which is my min CSA size of cpc. My question is where do I find the CSA of 6mm 2C cable and also does the type of supply need to be taken into account at all if either tns or tncs my understanding is that this doesn't. Any help appreciated.

 

[pc1966]

The last page of this data sheet has the various SWA figures:

https://www.whyprysmian.co.uk/BS5467.pdf

Also you need to check the resistance for fault clearing, if it is a fuse-switch feeding it as a sub-main it would be the 5s clearing time for Zs at the end (I think). So see what fuse you are using, look up its curve and get Zs, subtract Ze and check your cable R1+R2 meets that.

There is another thing if it is TN-C-S at the house and there are extraneous conductive parts in the log cabin to be bonded then the requirement is for 10mm earth conductor size (copper equivalent). If that is painful in cable cost then you could put in a rod and TT the cabin supply with an up-front RCD.

 

[Pete999]

Hello all

Am trying to calculate suitability of using swa armouring as c.p.c. It is a 6mm 2c supplying a log cabin.
From table 54.7 the calculation I need to use to find min csa of armouring is (k1/k2) x S
Taking value from table 43.1 for 90c cable I have k1 = 100 and K2 = 46 (Table 54.4)

So (100/46) x 6 = 13 which is my min CSA size of cpc. My question is where do I find the CSA of 6mm 2C cable and also does the type of supply need to be taken into account at all if either tns or tncs my understanding is that this doesn't. Any help appreciated.

I seem to recall that somewhere in BS 7671 there is a table that give SWA cable sizes with the armour csa and the equivalent copper comparison, sorry I can't help anymore than this, I'm retired now and no longer work so no need or means to spend £80 odd on a new set of regs, my old 17th has been sent to a Friend over the pond in a sort of exchange for a set of the US codes. Good luck.

 

[DefyG]

The info from the IET GN1 would indicate that the swa from either PVC or XLPE cables would meet requirements of table 54.7 but as PC1966 indicated the story doesn't end there!

 

[pc1966]

Guessing you have:

• Fused-switch 32A BS88-2 fuse
• Assume the house Ze is no more than 0.8 ohm (TN-S)

Fault limit is 1.4 ohm maz Zs from on-site guide table B3 (ii) so your limit for R1+R2 is 0.6 ohms. From the data sheet the R1+R2 for 6mm 2-core is 10.08 mOhm/m so limit on length would be 59m. If your length is under 20m then going to 40A fuse would help selectivity with and load MCB, etc.

 

[telectrix]

generally the armour of SWA satisfies requirements for a cpc.for the smaller size cables. it's all down to the R1+R2 and hence the Zs. as pointed out by pc1966, it will not serve as a bonding conductor if PME earthing is installled and the house earth equipotential zone is extended to the garage/shed.

 

[pc1966]

That is another thing, most "garage CU" come with 6A and 32A MCBs but you probably don't need 32A out there, so going to 20A (or even 16A in only one or two ever used) for the sockets' MCB will reduce the chance of a hard fault taking out the feed fuse (as will keeping the supply fuse as high as safe for the feed cable / disconnect time).

 

[JohnS888]

generally the armour of SWA satisfies requirements for a cpc.for the smaller size cables. it's all down to the R1+R2 and hence the Zs. as pointed out by pc1966, it will not serve as a bonding conductor if PME earthing is installled and the house earth equipotential zone is extended to the garage/shed.

Guessing you have:

• Fused-switch 32A BS88-2 fuse
• Assume the house Ze is no more than 0.8 ohm (TN-S)

Fault limit is 1.4 ohm maz Zs from on-site guide table B3 (ii) so your limit for R1+R2 is 0.6 ohms. From the data sheet the R1+R2 for 6mm 2-core is 10.08 mOhm/m so limit on length would be 59m. If your length is under 20m then going to 40A fuse would help selectivity with and load MCB, etc.

Thanks guys all super helpful. Especially PC for level of detail in responses. Very much appreciated ?

 

[JohnS888]

The last page of this data sheet has the various SWA figures:

https://www.whyprysmian.co.uk/BS5467.pdf

Also you need to check the resistance for fault clearing, if it is a fuse-switch feeding it as a sub-main it would be the 5s clearing time for Zs at the end (I think). So see what fuse you are using, look up its curve and get Zs, subtract Ze and check your cable R1+R2 meets that.

There is another thing if it is TN-C-S at the house and there are extraneous conductive parts in the log cabin to be bonded then the requirement is for 10mm earth conductor size (copper equivalent). If that is painful in cable cost then you could put in a rod and TT the cabin supply with an up-front RCD.

Just to clarify PC. If had to go down the earth rod route for the 2 way DB would a 30ma main switch be adequate? I believe there is still requirements for S type 100ma in some scenarios. I think this may be agricultural installs

 

[Ian1981]

Taken From gn8

 

[JohnS888]

Taken From gn8

Thanks Ian appreciated

 

[pc1966]

Just to clarify PC. If had to go down the earth rod route for the 2 way DB would a 30ma main switch be adequate? I believe there is still requirements for S type 100ma in some scenarios. I think this may be agricultural installs

You will need 30mA protection anyway for the lights & sockets so it could be used instead as the TT incomer switch. Really the only difference here is if you have the SWA armour used as the earth conductor, or have its armour earthed only at the home end and it is isolated (e.g. plastic gland in to local CU) at the out building and have a separate earth rod.

Where you would need a 100mA S-type is if you want selectivity with downstream 30mA personnel protection RCDs and/or had some fixed load that has high leakage currents that could not be on 30mA protection.

The down-side of the usual "garage CU" with 30mA RCD and a couple of MCB is you lose everything if it trips on an earth fault, and if it is used as a workshop that could lead to a nasty accident with tools, etc. Having an emergency light to avoid complete darkness, and/or using a 100mA S-type as incomer switch and a couple of 30mA RCBO so lights remain on if the sockets trip are both ways around that.
[automerge]1588520976[/automerge]
You are probably thinking of the section 705 for agricultural areas that has a max 300mA RCD for fire protection (along with lower values for other cases).
705.411.1
705.422.7

 

[JohnS888]

You will need 30mA protection anyway for the lights & sockets so it could be used instead as the TT incomer switch. Really the only difference here is if you have the SWA armour used as the earth conductor, or have its armour earthed only at the home end and it is isolated (e.g. plastic gland in to local CU) at the out building and have a separate earth rod.

Where you would need a 100mA S-type is if you want selectivity with downstream 30mA personnel protection RCDs and/or had some fixed load that has high leakage currents that could not be on 30mA protection.

The down-side of the usual "garage CU" with 30mA RCD and a couple of MCB is you lose everything if it trips on an earth fault, and if it is used as a workshop that could lead to a nasty accident with tools, etc. Having an emergency light to avoid complete darkness, and/or using a 100mA S-type as incomer switch and a couple of 30mA RCBO so lights remain on if the sockets trip are both ways around that.
[automerge]1588520976[/automerge]
You are probably thinking of the section 705 for agricultural areas that has a max 300mA RCD for fire protection (along with lower values for other cases).
705.411.1
705.422.7

Thanks for clarification PC ?

 

[pc1966]

Just realised you said a log cabin, so might have TV, etc, there. You might want to look at using a smaller home CU that has space for SPD and a few final circuits (or at least spares for later if they add anything else). But discuss it with the client to see what they have planned.

 

[DefyG]

And an AFDD if a wooden structure!