Shared earth conductor

[morqthana]

The grey enclosure is part of a commercially produced item

Nice to see it's not just me who can't get DIN rail terminals to sit straight.

 

[SUNRAY]

Nice to see it's not just me who can't get DIN rail terminals to sit straight.

depends on the terminal make, that particular job I had far too little time to install the bars and wire them so straight terminals was way down on my list of priorities

 

[ebee]

Well I will admit it`s sometimes easy to get the MCBs slightly out from slanted a tadd on some makes. So same thing really to a lesser degree

 

[morqthana]

depends on the terminal make, that particular job I had far too little time to install the bars and wire them so straight terminals was way down on my list of priorities

Oh oops - didn't think about it might be your work.

 

[SUNRAY]

Oh oops - didn't think about it might be your work.

The history of those internally wired stage lighting bars and dimmer: I installed them in 2007 in a school, Approx 80 hours including all of the conduit cutting/threading/bending, metal work in the restricted ceiling void

steel trunking and dimmer etc

then in 2017: working round school use of the hall including working quietly behind the stage curtains during PE & drama classes etc, in a great hurry had less than a day to strip them out and shift it all to the new building.

Then re-install in 3 days

The builders installed and weight certified the Unistrut and ran the SY/earth

Yeah nice install!

leaving me ½ day to hang and focus the PATd lights and fully commission,

with the builders seemingly taking the scaff tower down as I climbed down, or so it felt.

About 40 hours including assistance removing the lights on the first day and on the last to rehang them and commission.
Normally I'd try to make theterminals tidy but chances are they were dislodged when decommissioning and re-installing but the last thing on my mind was: "Are the terminals straight? when the builders are locking up halfway through each day. As least I got a free mug when my delivery of studding, nuts, washers etc arrived; Cheers

 

[ebee]

Provided there are no'downsides', it's never really fair to criticise someone who wants to exceed 'minimum requirements', even though the CPC sizes in T+E we have are generally adiabatically adequate and large enough enough to allow the required 'maximum Zs' (for ADS) to be achieved

However, as for 'downsides', I would have to do some sums to ascertain whether this could ever be an issue, but it's just possible that increasing CPC size could sometimes be somewhat of a 'two-edged sword' - whilst that would decrease Zs (to a level lower than needed for ADS), the consequential increase in PFC might possibly render the CPC adiabatically inadequate. Watch this space whilst I 'investigate'

Kind Regards, John

Well it can "sometimes" be something to consider but most times its merely to consider it is worthy of considering and the answer is usually no I would not consider considering it

 

[JohnW2]

Well it can "sometimes" be something to consider but most times its merely to consider it is worthy of considering and the answer is usually no I would not consider considering it

Maybe

When I find time to look into this ('do some sums') I might well discover that the potential issue I mentioned could never (or, at least, very rarely) arise. However, if it transpires that it IS possible for a CPC to become adiabatically inadequate ' because it is too big', that would presumably be something one should consider considering if one was contemplated a 'larger than usual' CPC?

 

[SUNRAY]

What constitutes a circuit?

I was in the team for the initial installation of this:

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14* 4mm² singles in a 32mm conduit at a height of 6m, that's R,Y,Blu,3Blk,Gn/Y*2 for each tube. Effectively 3 circuits, each fed by individual contactor but only 1 earth, then repeated to make 6 circuits per tube. 2 tubes per row
There was discussion about using a common neutral per 3 circuits to allow us to get 9 circuits per tube, the consultant/QC inspector turned it down but only because it would have made the lighting control clumsey.

 

[JohnW2]

What constitutes a circuit?

BS 7671 has a definition - namely anything/everything protected by a common OPD.

 

[SUNRAY]

What constitutes a circuit?

I was in the team for the initial installation of this:

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14* 4mm² singles in a 32mm conduit at a height of 6m, that's R,Y,Blu,3Blk,Gn/Y*2 for each tube. Effectively 3 circuits, each fed by individual contactor but only 1 earth, then repeated to make 6 circuits per tube. 2 tubes per row
There was discussion about using a common neutral per 3 circuits to allow us to get 9 circuits per tube, the consultant/QC inspector turned it down but only because it would have made the lighting control clumsey.

 

[ebee]

Maybe

When I find time to look into this ('do some sums') I might well discover that the potential issue I mentioned could never (or, at least, very rarely) arise. However, if it transpires that it IS possible for a CPC to become adiabatically inadequate ' because it is too big', that would presumably be something one should consider considering if one was contemplated a 'larger than usual' CPC?

Yes good thinking.
We tend to reckon its safe to use the same as E as a Live conductor for a circuit or reduced size E that have become sort of defacto in standard T & E or 3C & E but we are probably mainly thinking of possible touch voltage of a zero ohms fault , So equal sizes become around 120V and the other sizes become higher at, notably 6.0 T & E upwards ut yet again whicher voltage results we are concentraing on mainly in the band 120 to nearing about 200v .
Probably only worth considering L conductors around 6.0 upwards running enough current to make an R2 approching zero a possible real world problem much of the time.
Bringing touch voltages down in fault conditions and/or causing rapid ADS is what we examine most carefully.
It`s at cutouts and consumer unit level we start to think a bit more about things usually.

 

[JohnW2]

We tend to reckon its safe to use the same as E as a Live conductor for a circuit or reduced size E that have become sort of defacto in standard T & E or 3C & E but we are probably mainly thinking of possible touch voltage of a zero ohms fault , So equal sizes become around 120V and the other sizes become higher at, notably 6.0 T & E upwards ut yet again whicher voltage results we are concentraing on mainly in the band 120 to nearing about 200v .

Indeed. I often find myself having to point that out, particularly in discussions about supplementary bonding. To get 'touch voltages' (relative to 'true earth' - maybe the CPC of a different circuit), in the face of a 'zero ohms fault, down to, say, 50V, would require a CPC with a CSA almost 5 times that of the L conductor - which clearly is not realistic/'practicable'.

Kind Regards, John

 

[ebee]

Indeed. I often find myself having to point that out, particularly in discussions about supplementary bonding. To get 'touch voltages' (relative to 'true earth' - maybe the CPC of a different circuit), in the face of a 'zero ohms fault, down to, say, 50V, would require a CPC with a CSA almost 5 times that of the L conductor - which clearly is not realistic/'practicable'.

Kind Regards, John

Thats just reminded me of a funny one John, a few years back I remember a contractor saying " Regs says Supp bonding is 4.0mm !" .
Being pedantic I asked hime where the reg was and what does it state? nope , OK a will accept a Guidance note explanation then ? Nope.
I then pointed it out that is was worded (not this simply) but any two cpcs the connecting supp bond conductor need be workesd at as the smallest one - at each stage of connecting two or more together, then the overeaching minimum which basically depended upon the fact of wheter the conductor had mechanical protection or not.
So yes you would always get 4.0mm (well nearly always) as the go to size in most domestic stitutions if you did the working out, but it was not exactly how it was worded at that time.

Sometimes add a pinch of a poke to allow for the installtion E conductor not actually sitting near the Supp bond conductor voltage (rarely happend but in theory had to be thought out I was informed (didn`t sound right to me in reality but you supposed to follow your tutor on these courses)

 

[plugwash]

in the face of a 'zero ohms fault, down to, say, 50V, would require a CPC with a CSA almost 5 times that of the L conductor - which clearly is not realistic/'practicable'.

Indeed.

Which is the point of supplemntary bonding, the supplementary bonding conductors are much shorter than the CPCs. So the 'touch voltage' between bonded objects is much lower than it would be with CPCs alone.

 

[JohnW2]

Thats just reminded me of a funny one John, a few years back I remember a contractor saying " Regs says Supp bonding is 4.0mm !" .
Being pedantic I asked hime where the reg was and what does it state? nope , OK a will accept a Guidance note explanation then ? Nope.

Does 543.1.1 not require any standalone 'protective conductor' (which I presume includes a SB conductor) to have a CSA of at least 4mm² unless it is 'mechanically protected'? ....