Large number of 13A sockets in a steel building: Design considerations.

Engineer54 said:

Who built the steel framed building, you or was this building existing. I'm asking as i want to know if the concrete floor has any reinforcement ??

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Good question: I built it.
No steel in the floor slab, no, so nothing in there which can be bonded.

The concrete is some new-fangled mix with fibre in it, which obviates the need for matting. At least that’s what the contractor said. It is guaranteed for 10-tonne vehicles …

cash-and-dash said:

Good question: I built it.
No steel in the floor slab, no, so nothing in there which can be bonded.

The concrete is some new-fangled mix with fibre in it, which obviates the need for matting. At least that’s what the contractor said. It is guaranteed for 10-tonne vehicles …

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In that case you may have a problem using your PME earth in this building, especially if your kids are going to be playing with hoses bare footed. There could well be a potential difference between true earth and the PME earth in this instance. If you did have a grid in the foundation floor that could be bonded, then that would have been acceptable.

Looks like your going to have to go the TT route and your rods as you suggested, at opposite corners tied on to the structure and the structure to your MET. No need for 25mm cable 10mm will be fine!! Just make sure you go down a good depth with your rods, at least 2 rods deep at each corner, preferably 3 rods if your Ra is decreasing significantly the deeper you go with the second rod....

What I meant Mark was have a sub board in the building itself, in essence 3 boards, Main in the house, main in the out building and then your sub board say in the middle of the proposed ring, and then you could run 2 staggered rings one left one right, or then just 2 radials

malcolmsanford said:

What I meant Mark was have a sub board in the building itself, in essence 3 boards, Main in the house, main in the out building and then your sub board say in the middle of the proposed ring, and then you could run 2 staggered rings one left one right, or then just 2 radials

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Thanks, Malcolm, an interesting idea. But I don’t understand how that might be easier, or any more economic or better designed than running two radials or rings from the single sub-main in the building. Plus it would add complication.

Despite what others have said, 60m of 2.5 for a ring on a 20A breaker is absolutely not too long. Last night I looked up the specs for ‘Standard domestic circuits’ in the ECA 17th Ed guide. Max length of 2.5/1.5 T&E with a 32A breaker on TN-C-S and a 30mA RCD is 106m.

And I’m using SY, which has all the cores the same CSA so a 2.5mm CPC, and clipped direct to cold metal. Therefore 60m is clearly WAY within spec without even bothering to look up the cable specs and calculating volt drop.

The only ‘departure’ (if that’s what it is) is from the maximum floor area guidance. No one’s addressed that issue.

Remember that the main supply is only 25A per phase (no more is needed, so a 10mm incomer could be specified). So multiple 20A radials or even 32A rings on the same phase are pointless, since by definition they could never be fully loaded anyway. I forgot to mention that one reason I was protecting the ring at 20A was to provide discrimination (probably) between that and the 25A main switch.

One can think of it as two 30m radials (regs allow 40m of 2.5/1.5 T&E), both terminated in the same 20A breaker, but with the far ends of the radials then linked (so it then becomes electrically a ring circuit), which I considered good design since if then offers huge redundancy in the cable spec. What is wrong with that??

Enjoy KSA – I was in Dhahran in 1991, and subsequently, and hated the place (As the hookers say, just close your eyes and think of the money!)

micknew said:

Perfect example of a little knowledge being dangerous.

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Perfect example of not reading someone’s posts properly then jumping to conclusions.

Enjoy the exercise!

Engineer54 said:

In that case you may have a problem using your PME earth in this building, especially if your kids are going to be playing with hoses bare footed. There could well be a potential difference between true earth and the PME earth in this instance. If you did have a grid in the foundation floor that could be bonded, then that would have been acceptable.

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The ‘children with bare feet and hoses’ was my over-the-top example of why it is always better to design for worst-case scenarios, that’s all. It’s unlikely to happen in real life! I mean, how could I allow my little children to spray water all over an electrical installation? It’s completely unthinkable.

They might make it rusty!

Looks like you’re going to have to go the TT route and your rods as you suggested, at opposite corners tied on to the structure and the structure to your MET. No need for 25mm cable 10mm will be fine!! Just make sure you go down a good depth with your rods, at least 2 rods deep at each corner, preferably 3 rods if your Ra is decreasing significantly the deeper you go with the second rod....

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OK, and thanks for the advice, and sorry for the delay, I had to sleep on this one.

I still don’t understand why it’s not OK (electrically) to use the ‘exported’ PME, in parallel with a TT system. I’ll come clean: I’ve never really understood this. But I’ve never met an electrician on any site who could explain it to me either, without going round in circles, or using the ‘that’s how it’s done’ response and getting annoyed. (Which means of course he had no clue either.)

In accordance with modern practice, the floor slab is underlain by a 1000-gauge PVC sheet on sand blinding, so is probably electrically isolated from the earth. Rag bolts go from the concrete to the metal stanchions, but these do not pierce the DPC. But then of course the stanchions will be bonded to earth via the new TT system.

In the case of a lost Neutral on the DNO’s side of the N-E link, presumably the problem is that load current will attempt to return to the star point via the earth infrastructure
which includes all the structural metalwork, appliance casings and so on, which someone might be touching, so the current path would pass through the victim and into the general mass of earth. Or is the point of breaking the PME to prevent this happening in the remote building? (Since breaking the PME is really the same as breaking the N-E link at the service head, through which the fault current would need to flow.) BUT if there are additional low-impedance earth paths present on the consumer’s side – ie my stakes, what is the problem? I just can’t see it. Perhaps I’m being thick.
Plus anyone touching exposed metalwork would divert some of the current and trip RCDs.

Job done.

Pic of the shed shows what I was on about – I wanted to run an RFC all around the perimeter.

But OK, I’ve listened, and changed my mind. I installed it as two separate radials, on two 20A breakers, rather than as a single ring on one 20A.

I believe it's electrically not as sound as my ring proposal (the CSA is lower), but I agree more ‘standard’. An example of where strict compliance with regulations is actually worse electrical design than ‘breaking’ the rules.

Also a pic of using SY for the sockets but without the correct SY glands, designed, I believe, primarily for where EM screening is specified.

Unravel the screen, twist, sleeve g/y, then shrink-fit the lot. Outer sheath ends in a normal cable gland. Is this OK? BTW, the holes in the back of the metal boxes will be sealed to keep insects out. Not fitted the accessories yet, and though I’d better consult this wise oracle before proceeding …

Thanks, Mark.

Im assuming this may be for FLT batteries iv seen them done with busbar a few times dependant on the load

Only concern I may have would be the "wosk area" and any welding ect in the presence of battery fumes ect

Massively envious of your 'shed'!

mogga said:

Im assuming this may be for FLT batteries iv seen them done with busbar a few times dependant on the load

Only concern I may have would be the "work area" and any welding ect in the presence of battery fumes ect

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I was asked this before.

Naa, they are only for ordinary batteries: cars, lawnmower, motorbike, children’s quads, that sort of thing. Nothing big. And only little ‘Accumate’ maintenance chargers to stop the batteries self-discharging when left for a long time.

No hydrogen gassing off to speak of.

Archy Styrigg said:

Massively envious of your 'shed'!

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Thanks Archy. I like my shed too. The only way I can afford admittedly ridiculous projects like this is by doing everything myself. Which is one reason I’m on here asking questions …

Oh, and the other thing is to buy nearly everything on EBay, and to throw away your Tv.

cash-and-dash said:

........... is by doing everything myself. Which is one reason I’m on here asking questions …

Absolutely, go for it! (as long as you comply and get by the Notification Police (sarcasm can be taken as read there))

Oh, and the other thing is to buy nearly everything on EBay, and to throw away your Tv.

Ebay is my friend too. Need to get unhooked from trawling Electricians Forum next!

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Hi,
Don't lightly solder where screw terminals are concerned, the screw pressure over time will cause the solder to cold flow. Have a look at farnell part number 1121755 for the correct item to use.

UKMeterman said:

Hi,
Don't lightly solder where screw terminals are concerned, the screw pressure over time will cause the solder to cold flow. Have a look at farnell part number 1121755 for the correct item to use.

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Thanks - Very good advice about the cold flow.

I already ordered some 6mm (yellow) twin-entry ferrules, which two x 2.5mm SY cores fit nicely, but once crimped they won't go into the terminals!

The Farnell site is not clear on the spec of 1121755. Is that TWO x 2.5mm cables or 2.5mm TOTAL CSA?

I don't want to remove strands if it can at all be avoided.

Don't cut any strands!
Try the next size down.

Most likely me Mark, but I see a load of yellow and green cables there but not a brown in sight. I see where you have heat shrank the braid ok, I see a couple of Blues and then 4 what look like CPC coloured cables ..........

malc, there is 2x brown, but it looks like a black in the flash. 2x neutrals, 2 x CPC and 2x braiding