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Topic Title: Does swa need separate protective conductor??
Topic Summary: Help required
Created On: 24 July 2013 09:02 PM
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 24 July 2013 09:02 PM
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eamonn1983

Posts: 88
Joined: 21 October 2010

I need to work out if I will be required to run a separate protective conductor along side the swa cable I'm going to install. The cable is a sub mains feeding a 3 phase db panel in a large workshop. The cable is going to be a 50mm 4core 120metres steel wire armoured installed in a underground duct. The protective device feeding the cable is a merlin gerin NS250N 3 phase mccb and the readings at this end are zs 0.19 and pfc 1.23KA.

I'd be grateful if someone could tell me how I can determine if this needs a separate protective conductor and if so maybe explain the calculation,to help me,in the future.

Thanks in advance

Edited: 25 July 2013 at 08:04 PM by eamonn1983
 24 July 2013 10:19 PM
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UKPN

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your 50sqmm comes within the PME requirement size for which the armouring can be used for a bonding conductor.

the reqd size csa of armouring (mm2) for 10sqmm min bond is 56.37mm2.

actual csa of armouring (mm2) is 122 for 4 core 50sqmm.

nominal csa copper conductor equiv to armouring 10sqmm.

Regards

Edited: 24 July 2013 at 10:37 PM by UKPN
 24 July 2013 10:48 PM
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John Peckham

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Eamon

You do not state the length of run of the cable, the installation method or the load in As and PF. Also what size the NS250N is 160A, 200A or 250A. Also what the settings are on the MCCB for Ir and Im? I think you are going to have a struggle with your final Zs if your Zdb is 0.19 ohms.

PM me your email address and I will send you some useful data on SWA cables.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 24 July 2013 11:08 PM
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Parsley

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I would double check the the proposed cable size, Zdb, pfc and volt drop.

If the submain is protected by a 250 amp mccb the bonding conductor required is going to be bigger than 10mm especially if the supply is pme.

sq root I2 t/k for the cpc csa as per 543.1.3 or table 54.7 by selection.

John, please send me your SWA paper.

Regards
 24 July 2013 11:26 PM
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Legh

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I've come up with something similar.
You are going to have problems with far end sub main earth faults.

Legh

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 25 July 2013 07:16 AM
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eamonn1983

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I have checked picture of protective device and its 200 amp. Im 0 and ir 1. The cable is installed in a duct underground. The installation is TNS.
 25 July 2013 07:19 AM
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eamonn1983

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If I did require parallel protective conductor how do I work out size ? Never had to do this before
 25 July 2013 07:21 AM
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alanblaby

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Originally posted by: Legh
I've come up with something similar.
You are going to have problems with far end sub main earth faults.


What did you do?
Put in an extra protective conductor,or make it TT at the far end?
Thanks
Alan.
 25 July 2013 07:25 AM
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eamonn1983

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Haven't installed cable yet was looking for some good advice first
 25 July 2013 09:18 AM
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Legh

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Originally posted by: alanblaby

Originally posted by: Legh

I've come up with something similar.

You are going to have problems with far end sub main earth faults.



What did you do?

Put in an extra protective conductor,or make it TT at the far end?

Thanks

Alan.


I was actually looking at this problem rather than my own installs.

However, I did once install a 6mm2 4 core SWA run for 90m up a garden to supply some lights, from a PME supply.

The circuit was TT'd.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 25 July 2013 11:05 AM
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John Peckham

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What is the load in terms of Amps per phase and PF? What is the length of run? What is the depth of the duct?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 25 July 2013 12:19 PM
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Parsley

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Surely this cable is going to be undersized?

120m run, Zdb 0.19 ohms and In Ib 200Amps.

Regards
 25 July 2013 01:04 PM
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AJJewsbury

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Put in an extra protective conductor,or make it TT at the far end?

You'd still want adequate Zs for the TN bit though to cover L-PE faults within the SWA itself.
- Andy.
 25 July 2013 01:44 PM
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John Peckham

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The cable is only good for 116A under those conditions, the final Zs with armour only would be 0.41 ohms but the max. Zs for the device is 0.06 ohms and the adiabatic fails with a 10s disconnection time. So a big fail all round!

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John Peckham

http://www.astutetechnicalservices.co.uk/
 25 July 2013 04:31 PM
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AJJewsbury

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The cable is only good for 116A under those conditions

less, if you take into account the 0.9 factor for overload protection for buried cables.

- Andy.
 25 July 2013 05:31 PM
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leckie

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The max Zs for the device is given by John as 0.06 ohms. I know all the other elements need sorting out, but you can never be sure of achieving 0.06 ohms anyway can you. PME can be 0.35ohms, haven't got BGB to hand but there's a bit saying to use, I think it's phrased, "additional bonding" when the max Zs of a device cannot be achieved, say for a MCCB. Basically locally bonding to ensure that the exposed conductive parts within that area, say a plant room, is within the required max Zs.
 25 July 2013 05:44 PM
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AJJewsbury

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PME can be 0.35ohms

Hopefully not for a 250A+ supply - the v.d. in the supplier's lines would be a problem for ESCQR compliance.

I think it's phrased, "additional bonding" when the max Zs of a device cannot be achieved, say for a MCCB. Basically locally bonding to ensure that the exposed conductive parts within that area, say a plant room, is within the required max Zs.

I think you're thinking of supplementary bonding (411.3.2.6) - that should limit the touch voltage within the area to <=50V (making a longer disconnection time acceptable from a shock protection point of view) - but that doesn't necessarily reduce Zs significantly.

- Andy.
 25 July 2013 06:00 PM
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leckie

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Thanks Andy

No I know it doesn't lower Zs, and that it related to touch voltages, but that's the reg i meant. Just looked it up on the NICEIC essential guide once you gave me the number. I'm an idiot, of course it's supplementary bonding. It's not the best method but may be necessary is the Ze is too high to achieve the max Zs for the CPD. The only time I have ever thought it might be required is for CPD's like mccb's where the manufacturers quote very low max Zs values. I only mentioned it as the Op may be able to sort the other issues and still not be able to get the 0.06 ohms value.
 25 July 2013 06:10 PM
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eamonn1983

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What's the solution to this then do I need to downgrade the protective device to a 100 amp Mccb ?
 25 July 2013 06:54 PM
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OMS

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OK - be careful about using bonding to overcome extended disconnection times - everything stops being adiabatic and you really, really need to get the cable thermal withstand under control.

Going back to basics, you have a 200A device that needs a multiplier of about 10 to give you disconnection in five seconds - ie about 2000A or a Zs of about 0.12.

If you are starting with a supply end Zs of about 0.2 then it's simply not going to work - and if the load really is 200A then it will totally kill the supply on voltage drop - the DNO network can't provide what you want.

Realistically, you need to totally re-evaluate the load down this submain and from there pick a reasonable protective device - and also you might want to re appraise that 0.19 Zdb value - what did you measure it with and what is the supply intake to the facility.

If all the numbers are correct, you'll be looking at something akin to a 63A MCCB.

If you really need more than 63A per phase then you need to start looking upstream at the supply chracteristics, or go with 411.3.2.6 and get the bonding in place, determine the touch voltage, determine the actual disconnection time of the CPD and fully evaluate the cable withstand.

Keep in mind if your Zs is low then so will be the short circuit fault current - so keep that in mind for a P-N or P-P fault as well. The 50mm cable may need to get bigger - it's already too small for a 200A service (by some margin).

Regards

OMS

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