IET logo
 
IET
Decrease font size
Increase font size
Topic Title: The risk of undersized Bonding Conductors
Topic Summary: The risk of undersized Bonding Conductors
Created On: 06 August 2012 03:59 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 06 August 2012 03:59 PM
User is offline View Users Profile Print this message



acerday

Posts: 4
Joined: 06 August 2012

I have been asked to test & inspect community centre. It has a PME supply. The incoming cable is a 3-core Aluminium 185mm² with Copper wire waveform concentric layer.
Based upon my observations a 35 mm² to Gas & Water services is required.
The site has several detached brick buildings with buried Gas & Water services to them.
The power is supplied via 16 mm² SWA cables,
I believe that the armour of the SWA is insufficient to be affective as a combined CPC & main bonding conductor.
I am going to recommend that a separate 35 mm² bonding conductor be installed to the MET.

My question is; how large a risk is not installing this? In the old NICEIC codes is it a code 2, requiring improvement before a "Satisfactory" PIR report of a code 4 simply not complying, but not adversely affecting electrical safety.

Any wise words would be greatly appreciated

Acer
 06 August 2012 04:23 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11295
Joined: 13 August 2003

The site has several detached brick buildings with buried Gas & Water services to them.

Are the services plastic or metal coming out of the ground?

If metal, can you clamp the existing bonds and see what (diverted N) current they're carrying? and how does that compare to the current carrying capacity of the existing bonds? (conditions will change over time of course, but it should give you a ball-park idea).

- Andy.
 06 August 2012 04:33 PM
User is offline View Users Profile Print this message



acerday

Posts: 4
Joined: 06 August 2012

Thank you for your swift reply.
The services are all metal.
My concern is under fault conditions and the possibility of diverted neutral current.
The buildings is about 30 years old, the SWA cables insulation resistance test is sound and visually look in very good condition
Acer
 06 August 2012 04:35 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11295
Joined: 13 August 2003

I am going to recommend that a separate 35 mm² bonding conductor be installed to the MET.

BTW - for a EICR you might be best sticking to describing what the problems are, rather than trying to "sell" particular solutions. E.g. IF the bonding does prove to be inadequate, it might be more economic to TT the outbuildings rather than throwing more copper at them.
- Andy.
 06 August 2012 04:41 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11295
Joined: 13 August 2003

The services are all metal.
My concern is under fault conditions and the possibility of diverted neutral current.

OK, clamp readings would be interesting then (to give a very rough idea of diverted N in normal conditions).

Faults internal to the installation are unlikely to be a problem if the bonding meets TN-S standards.

Faults in the distribution system (e.g. broken PEN) are hopefully sufficiently rare (at least with an underground supply) not to be too much of a worry (of they were, then even bonding to PME standards will likely be inadequate - as they're always significantly smaller than even the installation's own N, let alone any N current diverted from other installations too).

- Andy.
 06 August 2012 04:49 PM
User is offline View Users Profile Print this message



acerday

Posts: 4
Joined: 06 August 2012

Thanks Andy once more for wise advice, I work on my own and it is appreciated.
Unfortunately the buildings are all fitted with metal clad DB's and one instance a metal clad SB which I suspect may not be suitable for TT systems and I fear if the RCCB modules are available, won't be cheep. But I shall include both options to the client.

Acer
 06 August 2012 06:59 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1841
Joined: 01 April 2006

So at present, the PME supply is stopped at the service head and TN-C-S to the detached brick buildings, Would you consider yourself that the electrical equipment in the several detached brick buildings is outside the equipotential zone with the supply obtained from within the equipotential zone, therefore disconnection shall occur within 0.4 seconds. The RCD,s for socket outlets in the detached buildings would cater for that but you would need to cater for 0.4 sec for lightning as well, (contraction Fire and detection requirements ?).
Start again: This should have been your system from a previous BS7671 (say 15th)
542-7, 8, Where a protective conductor is installed through more than one installation, each one of which has its own earthing arrangement, then either of the following requirements shall be complied with
1) The protective conductor shall be capable of carrying the maxium fault current likely to flow through it from any of the installation, or
2) the protective conductor shall be earthed within one installation only, and insulated from the earthing arrangements in the other installations, in this case where the protective conductor is part of a cable it shall only be earthed in the installation containing the protective device for the cable circuit.

To bring up to current BS7671, (2) would be costly because of electrode and main S type RCD required at each outlying installation, it would not be prudent to fit RCD at source end, in case of future SPD that may be fitted to the outlying buildings.

Yes; choices are limited, and you might get a high Zs at detached buildings because of long run and the cable size. 16mm SWA strands are very fine check making sure there are glands rings bolted to metal distribution equipment with no corrosion. Yes no 4 now, good luck.

Regards

Edited: 06 August 2012 at 07:06 PM by jcm256
 06 August 2012 08:11 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4437
Joined: 10 December 2004

Does this situation pose an immediate danger? If yes, then code C1.
If no, then could the situation pose a danger, if there were to be another fault? If yes then code C2.
If no, then code C3.
 07 August 2012 06:08 PM
User is offline View Users Profile Print this message


Avatar for davezawadi.
davezawadi

Posts: 2622
Joined: 26 June 2002

I agree with Andy, you are supposed to be inspecting the installation, not deciding on the work you might like to carry out. Lets look at this properly. The main bonding to the services is present at the MET in the community centre. The secondary buildings presumably share the same supplies for gas and water, and therefore the possible neutral current in these buildings is very small as the majority will flow in the community centre bonding. It is not that the services are buried which needs to be assessed, but whether they are separate supplies, meaning that they connect back to the supply mains (and could have large currents flowing) or simply extensions of the community centre supplies, and therefore could not. It would be unusual if they were separate, (have they got their own supplier meters?) in which case the bonding is probably adequate.

You should be aware that main bonding is only required at the point of supply, and not at every distribution board, although supplementary bonding may be present to other supplies at other places in the installation. Also we are now with the big GREEN book and inspection is for an EICR, substantially different to the old PIR. Read 411.3.1.2 and 544.1. You will realise that the definition of premises may mean more than one building, but would normally contain one equipotential bonding zone.

I feel that this job has sufficient pitfalls that you should consider getting assistance from an experienced inspector of larger installations, particularly as public buildings have so many insurance implications.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 08 August 2012 07:42 PM
User is offline View Users Profile Print this message



acerday

Posts: 4
Joined: 06 August 2012

Dear Dave, I have been asked to test & inspect the installation to the clients specification, it's not an EICR. The client is a local authority, the report includes a list of possible solutions to any point I raise. I do not do any installation work for the client, they have an in house team. I am a sole trader with 11 year to go before I retire, I have to do the best I can in the meantime.
The scenario is very similar to the one illustrated in the now out of date Guidance Note 8 figure 5.16 (I haven't received my new copy) and the NICEIC's current guide it is illustrated slimily; An example of the application of Regulation 411.3.1.2 where an installation forming part of a TN-C-S system serves more than one building.
In the section "Main equipotential, metallic pipework common to separate buildings" the NICEIC's current guide tells me "Regulation 411.3.1.2 requires that extraneous-conductive-parts in the area served by each installation are connected to the Main Earthing Terminal (MET) of that installation by main bonding conductors complying with Section 544. In addition, where an electrical installation serves more than one building, the above requirement is to be applied at each building (Regulation 411.3.1.2 refers).
Regulation 544.1.1 tells me that Where PME conditions apply, the bonding conductor csa is to be based on the copper equivalent csa of the neutral conductor of the supply (i.e. the combined protective and neutral (PEN) conductor belonging to the electricity distributor) and Table 54.8 of Regulation 544.1.1.
This is what I am basing my conclusion upon.
My question, unfortunately poorly worded, was aimed at the forum members that have had experience at risk assessment of this situation. However the points raised are always interesting.
Acer

Edited: 08 August 2012 at 07:48 PM by acerday
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.