trivia for discussion

[dingodeano]

We went to a to a big contractors site today to look at some pat testing for a sub contractor. We had a heated disscusion on way back about what we had seen.
While i was there i noticed 5 10kva single phase transformers dotted about the scaffold.
The transformers were all fed in 6mm swa cable tied to scaffold
These transformers were supplied with type c 32a rcbos. Fed from a 240v metal board inside a metal enclosure. This was housed inside a wooden purpose built hut. this was approx 8 lifts up about 20M.
The supply to this sub was a run of 16mm armour straight into site office fed from a 63amp single pole CB where there was a temp tp&n board. Tp&n board was fed by a 50mm swa that dissapeared into a hole in concrete floor where i asssume the incomer was.
Now i have no idea what supply it was but im hoping its not a PME.
I argued the 240v sub main have been rcd protected for additional protection? my mate said not because if transformers are supplying lighting on the scaffold then if it trips then blokes have no light. So we debated if the scaffold should have been bonded where the sub main on scaffold was - i said no because scaffold is not an exposed conductive part.
anyone have any opinion?

 

[ricicle]

The scaffold is not an exposed conductive part. If lighting on the scaffold is a concern then I'd worry about that 63A mcb discriminating with the 32A ones.

 

[ban-all-sheds]

Supplies can fail. If blokes on scaffolding in the dark is not required, then surely emergency lighting is?

 

[dingodeano]

I have no idea if there is provision for emergency lighting. Or indeed if these tranny's supply the work light. They prob all knock off at 2pm. It's all presumption I'm afraid. We were just speculating on how compliant the install was. The thing that I was struck by was 63a sub with no rcd. I'm sure discon times are met but why no rcd? As riccle said 5 x 32 amp loads they can't be that concerned with loosing supply. We thought lack of rcd may be down to nuciance tripping for lighting.
But a.d.s by itself what about some additional safety measures?

 

[JohnW2]

The scaffold is not an exposed conductive part.

That leads me to a question I forgot to ask in one of the recent 'garage electrics' threads. If one attaches a part of the electrical installation (e.g. an accessory or control box) to an otherwise-floating metallic structure (e.g.a metal door frame) by bolting the exposed-conductive-parts of the former to the latter, does the latter then become an extension of the exposed-conductive part?

... and, perhaps more interestingly, what would be the situation if that metal structure was, prior to the 'bolting on', not 'floating' but, rather, an extraneous-conductive-part? If the answer to my first question is 'yes', then that bolting together would clearly represent a degree of 'bonding' - but I presume that there would still be a requirement for an adequate CSA bond back to the MET (so as to be able to cope with possible fault currents, which the circuit CPC would be inadequate for)?

Kind Regards, John

 

[dingodeano]

john you seemed to have got the point across better than me -That was my esteemed colleagues point for arguing the bonding of the scaffold that as soon as the armour was cable tied to the scaffold then it became an extraneous conductive part.
Which i was not fussed about for the trannys fed by rcbos. But i was concerned with the 63a sub main which was not rcd protected.

 

[JohnW2]

john you seemed to have got the point across better than me -That was my esteemed colleagues point for arguing the bonding of the scaffold that as soon as the armour was cable tied to the scaffold then it became an extraneous conductive part.

Not really - whether or not the saffolding is an extraneous-conductive-part depends upon whether it is contact with 'true earth' (i.e. the ground) - which I imagine is pretty likely. My point was that connecting the 'installation earth' (e.g. the SWA armour) to the scaffolding would probably render it an (extension of an) exposed-conductive-part (maybe as well as being and extraneous-c-p), thereby affording some degree of bonding. In fact, with SWA armour, and/or an SWA CPC core, (as opposed to the CPC of domestic wiring) the bonding from scaffold back to MET might be of adequate CSA to satisfy bonding requirements.

Kind Regards, John

 

[dingodeano]

So if the metalic parts of the sub main was connected to (or at least locally to sub main ) the scaffolding with a conductor of equal size to that afforded by the 16mm swa armour you would say it is effectively bonded.
I have no idea if it was or not. But if that scaffold had to be bonded in its entirety then it would look like something wired pre 16th edition.

 

[JohnW2]

So if the metalic parts of the sub main was connected to (or at least locally to sub main ) the scaffolding with a conductor of equal size to that afforded by the 16mm swa armour you would say it is effectively bonded.

I'd have to look up the copper-equivalent CSA to the armour of 16mm SWA to know whether it was adequately bonded per regs, but it certainly ought to be close.

I have no idea if it was or not. But if that scaffold had to be bonded in its entirety then it would look like something wired pre 16th edition.

That's another potential problem, I guess. If one decided (or ascertained) that the mechanical joints in the scaffolding did not provide adequate electrical connection, then it could consist of an 'orrible mixture of exposed- and extraneous-conductive parts - which, as you say, could result in some jobsworth almost using up the world's supply of G/Y cable doing cross-bonding However, I would hope that the (fixed) mechanical joints would afford adequate electrical connections swivel joints etc. could be far more iffy).

Kind Regards, John.

 

[EFLImpudence]

If you are worried about the swa live shorting to the scaffolding then you would need to connect the scaffolding to the cpc with appropriate csa. - earthing an exposed conductive part.

If you are worried about a fault at a light resulting in a potential difference between the light and the earthed (by the ground) scaffolding then you would need to bond it to the light - supplementary bonding an extraneous conductive part.
This would only be necessary at points where the light and scaffolding are simultaneously accessible.
It would be easier if the light and attachments were metal.

It would not matter if anyone was touching the scaffolding elsewhere - bird on a wire.

 

[dingodeano]

I understand what you are saying. Now you have typed it and i have read it what is the likely hood of swa phase to scaffold ? presuming a(nd i have no reason not to) the armour is terminated correctly.
The lighting which i assume is running from one or more of the transformers is festoon. It is obviously reduced low voltage but im pretty sure that it is double insulated yellow arctic flex. So likely hood of phase to scaffold short is more likely.

 

[JohnW2]

If you are worried about the swa live shorting to the scaffolding then you would need to connect the scaffolding to the cpc with appropriate csa. - earthing an exposed conductive part. If you are worried about a fault at a light resulting in a potential difference between the light and the earthed (by the ground) scaffolding then you would need to bond it to the light - supplementary bonding an extraneous conductive part.

Totally agreed.

This would only be necessary at points where the light and scaffolding are simultaneously accessible.

Indeed - but I imagine that there might well be lots of lights, so that quite a lot of the scaffolding could be touched simultaneously with one of them.

It would be easier if the light and attachments were metal.

Indeed so - and that's what I was rather assuming - such that fixing the lights to scaffolding would effect some sort of electrical connection.

It would not matter if anyone was touching the scaffolding elsewhere - bird on a wire.

Probably true, but, in the context of 'within buildings' (which, admittedly, is a bit different), I haven't noticed any waiving of the requirement for bonding of extraneous-c-ps if one can demonstrate that nothing connected to that extraneous-c-p could be touched simultaneously with an exposed-c-p, have you?

The real theoretical problem is presumably the ground itself (particularly if swamp-like) which may well be touchable simultaneously with any exposed-c-ps (of lights etc.) - since it's not usually going to be possible/practical to 'main bond' the ground! I guess the solution to that is to avoid any exposed-c-ps which are touchable from ground level.

Kind Regards, John

 

[dingodeano]

The real theoretical problem is presumably the ground itself (particularly if swamp-like) which may well be touchable simultaneously with any exposed-c-ps (of lights etc.) - since it's not usually going to be possible/practical to 'main bond' the ground! I guess the solution to that is to avoid any exposed-c-ps which are touchable from ground level.

Kind Regards, John

Its a city all Tarmac and concrete paving the 'ground' or earth as in soil you prob need to drive to a park or out of the city. Like i metioned the incomer i reckon is about another one or two floors below street level.

 

[EFLImpudence]

And - are they isolating transformers?

 

[JohnW2]

And - are they isolating transformers?

Good question - I should have asked that! Do we take it that we're talking about 110V secondaries? Mind you, any (two winding) transformer [i.e. any transformer other than a (single winding) 'auto-transformer'] is, per se, 'isolating'. If there is provision for connection of a CPC to the output, then the question clearly arises as to what it's connected to. If there is no such provision, then all the talking that we (at least, I ) have ben doing about exposed-conductive-parts obviously becomes somewhat moot (although it wasn't in the garage thread which stimulated my question here).

Kind Regards, John