What would you do...

[apprentice87]

Hi all,

Been asked to wire in a shower in an industrial unit. The electrics in this place are a mess. Anyway, that is beside the point...

Picture this, A large shed about 250 feet long and about 80 feet wide. There is a DB right in the middle that powers everything in the shed. Where the supply comes into the shed [in one corner] it is split, and a cable goes off and feeds a portacabin outside.

Now, my problem is, some numpty has run feeds from the portacabin back into the shed, so you now have a shed with multiple points of isolation, all completely unlabelled, and nothing to indicate the supply is split and comes back in again!!

Anyway, to wire this shower, [inside the shed remember] i think they will want me to use a feed from the portacabin outside, as it is about half the cable length.

I want to feed it from the central DB, as this is how anyone with half a brain would do it.

Would anyone be tempted to feed it from outside, as i think they might lean on me a bit to do this, but i would not be happy at all, downright dangerous i would say, and just an irresponsible thing to do.....

If you were doing an EICR and you saw this sort of thing, what code would you give it, a 3 presumably, and what would you do if it actually WAS labelled "multiple points of isolation" or whatever??? Still a 3 i think???

john..

 

[steptoe]

could it be regarded as 'dual supply' ?

is it possible to put a main isolator on at the incomer to isolate everything?

 

[Sharpend]

If I am reading correctly, can you not supply it from where the cable is split in the corner on it's own shower cu??

 

[apprentice87]

Hi Steps,

There is an isolator at the incoming position, but it is one of those places that leave the computers on none stop, so if i tried to isolate the portacabin supply, which CAN be isolated separately, but also feeds back into the shed!! they would have a fit!

I have been working my way through the thing re-arranging circuits to try to sort this daft situation out. Next weekend i am going to remove the "feeds back in to the shed" and run them from the "proper DB" At least this will restore some sanity to the thing!!

Hi Sharpend,

The cable would be about 100 yards long if not more, do not think they would like that..

 

[steptoe]

I think if there is one MAIN isolator for EVERYTHING then it complies,

so, if you supply it from where the supply re-enters what is the actual length from the incomer?

will you get a low enough Zs, and what about volt-drop?

remember, you need to be considering all this from the point of entry, NOT the point of connection.

what cable sizes are already present, and what are the loadings already in use?

 

[apprentice87]

Hi Both,

Here is a picture of where the supplies come in. They did have two supplies in operation at the same time to the one building, the one at the extreme left with the meter, and also the large one at the extreme right, with no knowing what supply fed what board....

It was all small units knocked into one you see.. so any labelling that was there, meant nothing...

I have disconnected the left hand supply, and all the distribution cabling that came from it, and re-connected them to the buildings central DB, fed from the supply on the right. Much better!!

Look closely at the left hand meter...What is the little "black box" gubbins attached to it???? Never seen one before.....

View attachment 4079

john...

 

[Andy™]

I think if there is one MAIN isolator for EVERYTHING then it complies,

there is nothing in 7671 to say you must have 1 isolator for everything

 

[steptoe]

[quote name='Andy

 

[Andy™]

seriously?obviously one of yet another myth even I have fallen victim to,

I always thought there was a need to isolate everything from one point, otherwise it was dual supply.

each installation must have its own switch, but there is nothing to say you cant have 2 installations in the same building. so split tails and DB1 can be its own installation, DB2 can be its own installation

 

[steptoe]

ah, yes, we can have that scenario,

quite correct Andy.

 

[apprentice87]

Hi Steps,

It is impossible to measure the cable lengths, as the place is so tortuous, sooooo, i have not done the cals in their entirety yet, but, the "central" DB is fed in 70mm and there is only about 100 amps total connected load, without any diversity, so current is not a problem. As to volt drop, what i normally do is just measure the supply impedance at the end of the cable in question, and the voltage at that point too, [takes into account what loads there are already] and then work it out from there on in.

Zs at the board i want to connect to is 0.24 and by the time i add on the new run, I allowed 40 metres of 16mm three core, [it feeds a single phase CU] protected by a "B" curve MCB, i will have a Zs of 0.332

The MCB will be a 63A "B" curve one, sooooo, 230/315 = 0.730 x .8 = .584 So, ok up to now..

As to volt drop, i will lose about 6.5 volts in the three core at worst case 60A, so now i need to go back and measure the supply impedance at the board, [to work out volt drop], but if the Zs is anything to go by, i cannot see there being a major problem.

I will have a proper measure in the next few days, and do the calcs properly and go from there, only had a quick look so far!!

john..

---------- Post Auto-Merged at 16:09 ---------- Previous post was made at 16:01 ----------

Hi all,

Yes i see what you are saying about isolation, but this setup has the supply split after the first switch fuse [to protect and isolate the 70mm] and then there is another switch fuse, to protect and isolate the 25mm to to portacabin, soooo, you can shut off the porta cabin, but if you try to shut off the feed to the main buildings DB, you also shut off the porta cabin...

Cannot see that complies, as you need [should??] to be able to isolate individual sections without shutting off others needlessly, [or so i think!!]

[so, what they have, is not strictly, two SEPARATE installations, if you follow me!]

john...

 

[binky]

Whilst the situation is far from ideal, it is also viable to address situation with some simple labelling, in particular the stuff back fed from poratacabin to ensure anyone working on electrics has some idea of what is going on.