SWA as cpc to submain-Suitability, & PC's on RCD's.

[Jm1980]

Been looking at a CU change in a small office and would appreciate comments on the following.

Office is supplied from the mains intake in the warehouse via a 2 core 16mm SWA buried under concrete with the SWA used as cpc (60A BS88 is the protection). Mains intake in the warehouse is a 3 phase 300A TNCS supply.

Within the office is 16mm bonding to the incoming water and gas services which runs back to the CU i am looking at changing. My question is about the suitability of the armouring as the CPC for the office supply, especially as it branches out into bonding after. Any thoughts from anyone on the setup?

The 16mm SWA is probably a bit small in all honesty if your looking at belt and braces approach but it has existed like this for years with no issues and loading is fairly light. Replacing the cable would be a bit of a nightmare.

The old board is plug in wylex mcb's and a few 3036's protecting a couple of lighting circuits and computer equipment. Obviously i am required to RCD the socket outlets now. I have 3 ring circuits, each with 4 PC's running from it. In terms of build up of leakage current, what's your guys experience of how many PC's should be the max we put on a circuit to avoid nuisance trips?
I will modify the circuits to high integrity cpc connections.

Also looking at AFFD's for the sockets, new amendment has them as a recommendation for socket outlets but not a must for this type of building. What's everyone doing regarding them as they have left a bit of a grey area with that one!

Cheers

 

[westward10]

The armour is inadequate for PME bonding purposes. Are the gas and water shared services with a main protective bond elsewhere. It is doubtful the main protective bonds you have are of an adequate csa but they could be deemed as supplementary if other bonds are in place.

 

[Jm1980]

The premises has two water supply pipes, one to the warehouse, one to the office. There is no connection between them. Looking back at my photo's it seems the water supply pipe to the office is in plastic tube not metal, the copper pipework is just after the stopcock. Both separately bonded in each location.

Gas service pipe is common between the warehouse and office and runs between them (again, bonded in both)

 

[timhoward]

Office is supplied from the mains intake in the warehouse via a 2 core 16mm SWA buried under concrete with the SWA used as cpc...

My question is about the suitability of the armouring as the CPC for the office supply, especially as it branches out into bonding after. Any thoughts from anyone on the setup?

My cheat sheet says that is 13.5 sq mm copper equivalent. As a CPC it may be ok by calculation, which obviously doesn't solve everything if bonding is a no-go.

It sounds as though the water doesn't need bonding.
I have a feeling that something changed in amendment 2 regarding bonding in this situation but I don't remember what without the book to hand.

 

[westward10]

My cheat sheet says that is 13.5 sq mm copper equivalent. As a CPC it may be ok by calculation, which obviously doesn't solve everything if bonding is a no-go.

It sounds as though the water doesn't need bonding.
I have a feeling that something changed in amendment 2 regarding bonding in this situation but I don't remember what without the book to hand.

I doubt it is anywhere 13.5.

 

[timhoward]

I doubt it is anywhere 13.5.

I think I wrongly assumed it was a brand new cable.

2 core 16 sq mm SWA
I have :
PVC - steel armour is 46 sq mm (BS 6346)
XLPE (unlikely) - steel armour is 42 sq mm (BS 5467 and BS6724)
Doing the calc gives me 20.3 sq mm or 13.5 sq mm respectively.

EDIT - by the way, the AM2 change I was remembering is only for TNS / TT so not relevant.

 

[Jm1980]

Its a BS6346 cable as opose to an XLPE, probably been in service a while. Anyone know if this actually complies with 543.1.1 without me having to remember how to do the calc? I did have a cheat sheet myself but lost it! Zdb at board to be replaced is 0.08 ohms

 

[timhoward]

Its a BS6346 cable as opose to an XLPE, probably been in service a while. Anyone know if this actually complies with 543.1.1 without me having to remember how to do the calc? I did have a cheat sheet myself but lost it! Zdb at board to be replaced is 0.08 ohms

63 amp BS88 fuse and 2875A fault current?
A quick glance suggests we're down the bottom of the graph well before that fault current, so bunging 0.1 disconnection time in gives 18 sq mm steel needed. But I'm not taking responsibility for it!

CPC Sizing Tool

 

[westward10]

I think I wrongly assumed it was a brand new cable.

2 core 16 sq mm SWA
I have :
PVC - steel armour is 46 sq mm (BS 6346)
XLPE (unlikely) - steel armour is 42 sq mm (BS 5467 and BS6724)
Doing the calc gives me 20.3 sq mm or 13.5 sq mm respectively.

EDIT - by the way, the AM2 change I was remembering is only for TNS / TT so not relevant.

You need to divide it by 8 or 8.5 can't remember which.

 

[pc1966]

There are two variations on how you treat steel armour. The adiabatic is kinder as the increase in metal needed to lower resistance also increased the heat storage capability so the conversion is about a factor of 3. Basically although your resistance is still much more than copper equivalent, the extra I2R heating is absorbed by the greater volume of steel to get to the same end temperature in the adiabatic case.

However, for the TN-C-S bonding the problem is the large and continuous current that can flow under open-PEN fault conditions. So the factor is about 8 (resistance of steel/copper ratio) to have the same I2R losses (i.e. heating) as the fault event is far longer than fault-clearing and you no longer assume a finite energy leading to an end temperature when the fuse/MCB/whatever cuts out.

"Steel" is not a single material though, so the resistance varies a little, but I usually simplified to using 8 as the ratio. For A2 stainless it is about 40 times.

 

[pc1966]

63 amp BS88 fuse and 2875A fault current?
A quick glance suggests we're down the bottom of the graph well before that fault current, so bunging 0.1 disconnection time in gives 18 sq mm steel needed. But I'm not taking responsibility for it!

For 0.1s and 63A fuse I see 720A mentioned (Fig 3A3(b) in BBB) so I2t = 720 * 720 * 0.1 = 51,840

Using Table 54.4 and assuming steel armour on 70C PVC cable k=51 so:

s = sqrt(51,840) / 51 = 4.5 mm of steel needed so well within safe margins as my Prysmain sheet lists 16mm 2C as 42mm of steel.

To get 0.08 Zdb at the end of the cable is impressive, it must be quite short (16m max even for Ze close to zero) or already have parallel earth paths!

 

[pc1966]

I have 3 ring circuits, each with 4 PC's running from it. In terms of build up of leakage current, what's your guys experience of how many PC's should be the max we put on a circuit to avoid nuisance trips?

Page 26 of the Schneider RCD guide CA908066E has a handy table:

https://download.schneider-electric.com/files?p_enDocType=Catalog&p_File_Name=CA908066E+(web).pdf&p_Doc_Ref=CA908066E

It lists a desktop PC at 2mA, printer at 1mA, portable computer (laptop I guess) at 0.5mA where it has a functional earth for EMC control. You should aim for no more than 10mA per socket circuit, so {5 * PC} or {4 * PC + 4 * laptop}, etc.

 

[Jm1980]

I did the calc myself too and got it well within the required minimum when compared to the cheat sheets i looked at online. Is the conclusion that the setup is compliant therefore when considering the swa as the cpc and also the bonding setup (16mm's in the office, 35mm's in the warehouse?

Does the fact the supply is TNCS alter it at all? I have always heard conflicting stances when it comes to exporting a TNCS supply, some people rod everything, others don't bother.

I was surprised at the Zdb figure myself when i measured it but on further investigation found the DNO supply transformer is right outside the premises.

 

[westward10]

No the armour cannot support the main protective bonds.

 

[timhoward]

For 0.1s and 63A fuse I see 720A mentioned (Fig 3A3(b) in BBB) so I2t = 720 * 720 * 0.1 = 51,840

I couldn't actually read the graph properly on my phone so didn't reduce the fault current in the formula to be safe!

Is the conclusion that the setup is compliant therefore when considering the swa as the cpc and also the bonding setup (16mm's in the office, 35mm's in the warehouse?

As a CPC I don't see an issue at all.

My reading of 544.1.1 is that we can't use the "size according to distribution circuit CPC size" clause as it's PME.
Water doesn't need bonding as plastic.
The gas bonding is the thing.
I believe it comes down to what the copper equivalent size is and whether it is > 10 sq mm in order to satisfy table 54.8 for a 35 sq mm supply PEN conductor.

I'll bow out here as the table I've always used seems to not be matching what some very clever people are saying!
( https://www.askthetrades.co.uk/hosted_images/Armour CSA.pdf )
That table has higher copper equivalent sizes than are being stated so as I'm confused I'll shut up!

 

[pc1966]

If the armour is the only bond to a large extraneous part it could end badly. It is possible simply to run another 10mm or 16mm copper cable to bond the gas pipe back to the MET?

 

[Jm1980]

If the armour is the only bond to a large extraneous part it could end badly. It is possible simply to run another 10mm or 16mm copper cable to bond the gas pipe back to the MET?

It isnt feasible (without a great deal of hassle digging a drive up) to get another cable from the mains intake in the warehouse over to the separate office block.

The gas pipe in the office block does run back to the warehouse (underground) so will be 'bonded' to the MET indirectly via being fixed to the structural steelwork.

 

[pc1966]

It isnt feasible (without a great deal of hassle digging a drive up) to get another cable from the mains intake in the warehouse over to the separate office block.

The gas pipe in the office block does run back to the warehouse (underground) so will be 'bonded' to the MET indirectly via being fixed to the structural steelwork.

Can you verify that it is the same gas pipe in the main MET location? If so you can make sure it is bonded there (you really can't relay on steel work supports, etc) and the problem is dealt with.

 

[Jm1980]

Can you verify that it is the same gas pipe in the main MET location? If so you can make sure it is bonded there (you really can't relay on steel work supports, etc) and the problem is dealt with.

Yes, from memory, 3 phase supply into warehouse where we have our MET…..35mm bonding to gas, water and steelwork in this building.

What looked to be the same gas pipe (could test to confirm I guess) ran underground from warehouse (where it was metered) into the stand alone office.

In the office, the CU I am quoting to change has its earth bar (earth marshalling terminal?) connected to the gas and water where they come into the office.

 

[pc1966]

Yes, from memory, 3 phase supply into warehouse where we have our MET…..35mm bonding to gas, water and steelwork in this building.

What looked to be the same gas pipe (could test to confirm I guess) ran underground from warehouse (where it was metered) into the stand alone office.

In the office, the CU I am quoting to change has its earth bar (earth marshalling terminal?) connected to the gas and water where they come into the office.

It sounds as if it is fine in that case, but you ought to confirm the pipe is (at least electrically) the same.

When you have the to-be-replaced CU disconnected you ought to be able to check the resistance using a wander lead (I guess, if you can safely get it from MET to pipe) and if that is well under an ohm I would be happy.

If a wander lead is not practical, then once sub-main is isolated and armour simply checking from pipe to sub-main armour at the remote building should give pretty low results as well if the pipework is common. You can link L&N of the 16mm and measure the R1+Rn to get an estimate of that cable length, and from there an estimate of the R2 expected of the armour, and then from armour to pipe the difference should be that more-or-less of the pipe.

The Prysmain sheet I have lists 16mm Cu as 1.15 mOhm/m and the 2C armour as 3.7 mOhm/m