bonding from met

[just add water sparks]

on a number of pir for commercial premises i have found main bonding i.e gas/water has not going directly back to the MET instead they have taken the bonding cable to a sub distribution board or below it to the external earth terminal block. which is closer than the main intake position where the MET is.I think the principal is that as long as the swa amouring equates to the correct cross-sectional area of the bonding cable which then goes back to the main intake position where the MET is ,it is ok!!I do not think this is acceptable in the regs but cannot see any valid reason why it cannot be installed in this way???This scenario is where the sub board and main intake are in the same building.I presume the MET is classed as being at the main intake position and not at a earth terminal block beneath a sub-distribution board.Could anyone please clarify this and perhaps quotte regs no please.

 

[Des 56]

Being unsure of an answer myself I searched for something simillar
Here is a quote from someone on the Iet forum that may be of interest to you

Whilst the primary function of the metallic armouring of an armoured cable is to provide overall strength, it may be used as a circuit protective conductor (cpc), main protective bonding conductor or, where practicable, an earthing conductor.
Whichever of these three functions is to be performed by the armouring of a cable, all the relevant requirements of BS 7671 need to be satisfied. The armouring of a cable may be used for more than one of the functions if all the appropriate requirements for each function concerned are met.

Like any conductor used as a cpc, armouring will experience thermal stress (heating effects) if it is subjected to earth fault current. To withstand this stress safely, the cross-sectional area (csa) of armouring used as a cpc must be not less than that determined by one of the two methods referred to in Regulation 543.1.1 of BS 7671, namely calculation or selection.

In an installation where Protective Multiple Earthing (PME) conditions apply (that is, where the means of earthing for the installation is a PME earthing facility), main protective bonding conductors have to carry sustained currents under certain conditions, which may result in heating of the bonding conductors. The armouring of a cable should not be used as a main protective bonding conductor where PME conditions apply.

 

[just add water sparks]

Thanks Des 56 I have just found in regs page 46 411.3.1.2 that main bonding conductors shall connect to the main earthing terminal ........................but nothing says it should not be via a sub-main using the armouring of a swa.Again some sparks interpretation of the MET could be at a sub board, my take is the MAIN INTAKE POSITION.BL***Y REGS why is it never definitive i detest the ambiguity in this game sometimes

 

[Des 56]

My own view is that an installation has one main earth terminal and that is sited at the main intake position
An met at a sub main distribution board would be nothing more than a marshalling earth terminal

The protective bonding should go to the main earth terminal but I am happy to accept that it can be via an armouring of a suitable sub main cable

 

[just add water sparks]

just out of interest anyone else have veiws or evidence on this as if client contests the theory of using swa at least i have enough ammunition so to speak and using that sought of idea could potentially save a lot of tome and money any more help would be much appreciated

 

[Sintra]

just out of interest anyone else have veiws or evidence on this as if client contests the theory of using swa at least i have enough ammunition so to speak and using that sought of idea could potentially save a lot of tome and money any more help would be much appreciated

IMO all main protective bonds should go to the MET.

 

[bramboy]

Our area niceic inspector says all main protective conductors need go back to the origin,
incase someone disconnects/ alters sub board.

Also see reg411.3.1.2 which show this.

 

[Guest123]

All equipotential bonding must go back to the MET at the origin.

 

[just add water sparks]

Thanks guys i appreciate feedback sorry if i seem to have self doubt(only due to nature of regs being un-definative and down to individual interpretations) I understand the regs say that the bonding does have to go back to the MET but it is not specific in the manner of this.Just to reiterate what Des 56 read in iet forum and i have heard myself before quote " whilst the primary function of metallic amouring is to provide strength it may be used as a bonding conductor"

 

[slocm3105]

It should definitely be taken right the way back to the MET. It cannot go via the Armouring of the SWA as you will then need to make a join and main bonding conductors must be continuous .

 

[just add water sparks]

Hi Again,
Just had an interesting visit from an n.i.c.e.i.c inspector and he has just informed me that taking multiple bonding cables cables back to the m.e.t is not the forward way of thinking now and is uneconomical in its approach.He suggested i buy the 17th Edition Of The Wiring Regs by Darrell Locke and the ECA Don't know what you think Lenny ,Slocm, Bramboy

 

[just add water sparks]

I have just read ECA book by Darrell Locke MIEE ACBIBSE ITS A GOOD READ page174 shows a picture of ladder rack or tray being used as a part of a bonding conductor and bonding from this to boiler,structure, gas pipe e.t.c So by looking at this it seems to shoot down the THEORY that bonding has to go directly back to the M.E.T So again its down to people's interpretations it does say in regs Bonding must go back to the M.E.T but does not "SPECIFY HOW" A QUOTE from this book page171 there is no need virtually for green & yellow cable at all.Continuity tests on metalwork virtually always indicate a continuity approaching zero figure 63 shows a much better & economical solution where the ladderrack or cable tray being used as a protective conductor figure e61 Running multiple equopotential cables is un-economic

 

[mechelec]

The protective bonding should go to the main earth terminal but I am happy to accept that it can be via an armouring of a suitable sub main cable[/QUOTE]


Just to throw a spanner i n the works, the armour may be used as a bonding conductor but its CSA must comply and if it is being used as the CPC for the sub main then it is unlikely to have sufficient CSA to serve both perposes

 

[just add water sparks]

Thanks Mechelec,
Just looked @ snags & solutions book,part 3,inspection & testing by the n.i.c.e.i.c snag45 page 150 shows an illustration shows a combined cpc and main bonding conductor supplying an outbuilding!!!!!!!!!!!!

re; last post
I don't think the c.s.a has to be for each purpose which effectively means the amouring would have to have a c.s.a that equates to the c.p.c and main bonding conductor.Would be interesting to have further views on this one and perhaps regs no's to go with.????????? Another contentious question

 

[Graeme Harrold]

It should definitely be taken right the way back to the MET. It cannot go via the Armouring of the SWA as you will then need to make a join and main bonding conductors must be continuous .

Got to agree with that and unbroken between services.........