Supply for building site cabins - a few questions!

[Short Circuit]

Hi Guys
I have been asked to quote for putting in a supply to a pair of steel site cabins on a construction site.

At the minute there is no electrical supply to the site but there is a proposed position between the two cabins where the supply will be (there is a surface mounted meter box on the wall). The most each run of cable will be is 13 meters. i was plannning on using 4mm 3core armoured for each. Can the armoured come straight out of the main switch / isolator that the electric board supply, or will that need protected by a RCD? I will of course be using the 3rd core as the earth and bond the armourings at the glands

Also Reg 704.411.3.1 says a TNC-S system shall not be used for the supply to a construction site, except for the supply to a fixed building. I appreciate that the buildings are not fixed - but they wont be moved until the site is closing. The same rule in 7671 has a note that ESQR prohibit the use of a TNC-S to a caravan or similar! Does this mean that i need to use an earth rod and make it TT?

I am also trying to understand reg 704.411.3.2 does that mean the supply for the cabins would have to come through a 500mA RCD? If i need to put a 2 way RCD board in, i will struggle for space in the meter cabinet, and i know that the builder will not go for the expensive option of another housing

One more question (sorry!), on the cabins, i need to install a pair of 70w Son lights, i was planning on just using flex tie wrapped where possible - the cabin itself is an upstairs one so will not be used for storing stuff of top and the cable is not likely to be damaged - or should i put it in PVC conduit / or Kopex etc? I know i cant drill into the cabin as the hire company will charge the builder for any damage to it, which is why i thought that i could use tie wraps where possible!

Any suggestions will be greatly received as times are tough and i would like to get the job if possible!

Cheers

 

[High Tower]

What do you mean by a meter cupboard?

You need to split the supply to 2 separate switch fuses either by using a adaptable box, henleys, mcb's hence I ask about the cupboard arrangement.

2x switch fuses supplying 2 x swa cables ( have you done your max demand calculations and relevant cable calcs?)

Each swa needs to be earthed only at supply end for cable protection, you will not be exporting the earths hence reference to tnc-s in regs.

Each swa to a 4 way consumer, main switch only, TT each consumer unit independently and supplementary bond each of the cabins to your met.

4 circuits on the consumer unit, all rcbo's

1, 20 supply for power radial
2, 10a supply for son lights
3, 6a supply for lighting circuit
4, spare/ dedicated 16a for supply to 110 trannys

 

[nick611]

stake the supply dist board down 100 ma time delay to 2 way board in meter box,30 ma rcd in cabins,4mm will give you 32a,or 6mm swa will give you 40a thats plenty for site mobile

 

[TPES]

What do you mean by a meter cupboard?

You need to split the supply to 2 separate switch fuses either by using a adaptable box, henleys, mcb's hence I ask about the cupboard arrangement.

2x switch fuses supplying 2 x swa cables ( have you done your max demand calculations and relevant cable calcs?)

Each swa needs to be earthed only at supply end for cable protection, you will not be exporting the earths hence reference to tnc-s in regs.

Each swa to a 4 way consumer, main switch only, TT each consumer unit independently and supplementary bond each of the cabins to your met.

4 circuits on the consumer unit, all rcbo's

1, 20 supply for power radial
2, 10a supply for son lights
3, 6a supply for lighting circuit
4, spare/ dedicated 16a for supply to 110 trannys

Agree with high tower, Switchfuse each SWA, TT each CU to the met. Bond the cabin frames, and I assume the water feed coming in is plastic pipe so not to worry.

 

[nick611]

why 2 sw fuses? a simple dist board will do.put it on tt then theres no need to fart about at other end,take swa,s direct to cabin dist boards.why over complicate this simple setup?

 

[IQ Electrical]

why 2 sw fuses? a simple dist board will do.put it on tt then theres no need to fart about at other end,take swa,s direct to cabin dist boards.why over complicate this simple setup?

Why overcomplicate? That's what happens when guys offer up designs based on 20% of the information needed to complete a design.

The possibilities are infinite and sometimes fun to read

 

[TPES]

why 2 sw fuses? a simple dist board will do.put it on tt then theres no need to fart about at other end,take swa,s direct to cabin dist boards.why over complicate this simple setup?

I agree nick, the last cabin I did was fed off dis board, but I always TT at the cabin then bond back to this met, so the cabin bond can be connected locally without running back to the met at original intake.

I don't think it's over complicating anything, any eq bonding should be done at the met for the earthing arrangements for the supply, in this case it's the cabin.

I don't think it's easy for any of us to be 100% accurate on how we do a job briefly explained on here without seeing the job our self

 

[High Tower]

Did he not say that there was no dist board and no room for one?

Hence ip rated switch fuses.

I prefer rcbo's over a rcd in this application, because of the environment and type of use.

 

[Short Circuit]

Thanks for that guys, sorry for the late reply - i have been out all day! The meter cupboard is a white surface mounted fibreglass box where on domestic premises the electric board bring their supply and fit the meter and Double Pole switch. There is very little space but i may be able to fit a 2 way board!

Am i right in thinking the RCD or RCBO would have to be 100 mA? The cabins already have their own CU's with 30ma RCD's

I can add the earth stakes no problem, but will i have to connect the earth from that to the one from the supply (TNCS) or simply not connect anything to the electric boards supply - sorry of that seems like a daft question but i want to get it right!

Thanks again

 

[tonys]

You don't fully understand what a TT system is do you?

 

[High Tower]

Thanks for that guys, sorry for the late reply - i have been out all day! The meter cupboard is a white surface mounted fibreglass box where on domestic premises the electric board bring their supply and fit the meter and Double Pole switch. There is very little space but i may be able to fit a 2 way board!

Am i right in thinking the RCD or RCBO would have to be 100 mA? The cabins already have their own CU's with 30ma RCD's

I can add the earth stakes no problem, but will i have to connect the earth from that to the one from the supply (TNCS) or simply not connect anything to the electric boards supply - sorry of that seems like a daft question but i want to get it right!

Thanks again

Ahh more info, thank you.

You do not need to worry about any form of RCD for the cables, as they are already RcD' with 30ma in the cabin, as long as the suppply is SWA then no further protection is required.

Fit a small 2 way CCU with 2x MCB's sized acording to you Ib and cable size
2x SWA glanded and earthed to protect the cable, but no earth banjo on the load side to seperate the earthing arrangements.

TT both of the cabins, the space between the cabins would suggest that 2 seperate earth spikes would be most suitable.

Finally you must bond the framework of the metal cabin!! but check the wiring arrangements as this may already have been done

 

[jaresquire]

Use the incoming TNCS to protect the SWAs, make the cabins TT. Fixed site admin buildings can use TNCS but cabins and the like are forbade from doing so.

edit High Tower beat me to it!

 

[Short Circuit]

You don't fully understand what a TT system is do you?

the supply will be TNCS, and the regs say you cant use that for construction sites - all i was asking was if you link the earth from the rod and that of the TNCS together?

 

[High Tower]

Ok Just incase there is any confusion, for the benifit of the OP.

Incoming supply in TNC-S, Have you verified this?. this stands for Terra Neuteral Combined- seperate. So the Earth uses the neuteral onductor to complete the eart fault loop, if anything was to happen on the site then the fault current could be induced into surrounding installations under certain conditions. as per a pme arrangement.

So, use the incoming TNC-S to protect the SWA in the installations but do not connect the earth from the SWA to the boards in the cabins, drive a eath stake in the ground for each cabin and connect this to the MET for the cabin. beware that the ZE for the TT system needs to be less than 21 ohms so be prepared to drive in more than one per cabin.

I would not suggest linking the TT of both cabins as you would have to install some kind of mechanical protection to protect link cable, it would be far easier and far cheaper to just install dedicated rods.

 

[High Tower]

the supply will be TNCS, and the regs say you cant use that for construction sites - all i was asking was if you link the earth from the rod and that of the TNCS together?

You cannot combine a TT and a tnc-s its either one or the other mate

 

[kmcgold]

i would use a tt system
100ma time delay rcd on the db circuits

 

[High Tower]

i would use a tt system
100ma time delay rcd on the db circuits

Whats the time delayed Rcd for? is it needed on a SWa supply?? i am genuinely interested as im still fairly inexperienced

 

[kmcgold]

tt system must have rcd
time delay otherwise it will trip with the 30ma or instead of the 30ma

 

[IQ Electrical]

Ok Just incase there is any confusion, for the benifit of the OP.

Incoming supply in TNC-S, Have you verified this?. this stands for Terra Neuteral Combined- seperate. So the Earth uses the neuteral onductor to complete the eart fault loop, if anything was to happen on the site then the fault current could be induced into surrounding installations under certain conditions. as per a pme arrangement.

So, use the incoming TNC-S to protect the SWA in the installations but do not connect the earth from the SWA to the boards in the cabins, drive a eath stake in the ground for each cabin and connect this to the MET for the cabin. beware that the ZE for the TT system needs to be less than 21 ohms so be prepared to drive in more than one per cabin.

I would not suggest linking the TT of both cabins as you would have to install some kind of mechanical protection to protect link cable, it would be far easier and far cheaper to just install dedicated rods.

What?

The maximum acceptable value for stability is 200 Ohms or 100 Ohms if you go by BS 7430......

 

[mogga]

IP65 elclosure with RCD swa to cabins 6mm use "Earthing nuts" with fiber washers to maintain the ip65 rating protected by boards if possible to avoid traffic site dumping and people tripping rod each cabin in the ip65 enclosure include an rcbo for probable site tranny your floods flex tywrapped along top and down leg of cabin fix the floods to the bit at enc of container used to fix containers together as for bonding water it probably will be PVS so wont be required

 

[Short Circuit]

Thanks for that chaps, that has certainly helped! It was the customer who told me that the supply would be TNCS, and it will be in by midweek so i will know for sure by then. I was only asked to quote for the job yesterday and had no time to contact the supplier!

Sorry if the questions seemed daft, but i would rather do the job right and agree with the principle - ' if you are not sure - just ask! '

 

[High Tower]

Thanks for that chaps, that has certainly helped! It was the customer who told me that the supply would be TNCS, and it will be in by midweek so i will know for sure by then. I was only asked to quote for the job yesterday and had no time to contact the supplier!

Sorry if the questions seemed daft, but i would rather do the job right and agree with the principle - ' if you are not sure - just ask! '

The only stupid question is the one that you dont ask!

 

[Short Circuit]

exactly, thanks again High Tower

 

[High Tower]

tt system must have rcd
time delay otherwise it will trip with the 30ma or instead of the 30ma

Swa is protected by the TNC-s arrangement, the cabins have 30ma RCD to protect the TT side, so i still dont see why we need the TD- RCD??

 

[mogga]

Im gonny start usin that my usual one is theres no such thing as stupid questions BTW I know its "bad practice !!!" but on sites especialy I put tenby clips round the swa incase the armour get hauled out by nuggats on site moving cables around ect n BTW mind in your quote to include reconendations on testing 3monthly

 

[kmcgold]

Swa is protected by the TNC-s arrangement, the cabins have 30ma RCD to protect the TT side, so i still dont see why we need the TD- RCD??

wouldn't use tnc s
turn to tt system at meter not at cabins.

 

[High Tower]

wouldn't use tnc s
turn to tt system at meter not at cabins.

Ahhh i see what your saying, dont think its going to be possible as he will have to find more space in a already cramped encloseure and and try and cet the cost of a further RCD out of what already sounds like a Skin flint!

But all the same, yes i see another perfectly acceptable way of doing it

 

[malcolmsanford]

wouldn't use tnc s
turn to tt system at meter not at cabins.

Even if you TT at the incoming why would you want to fit a 100mA S type up front RCD to protect the armoured sub mains? The boards in the cabins protect all the final circuits with a main 30mA one.

Plus as you will have the problem of where your MET will be. As your runing 2 sub mains your MET will have to be at the origin of the installation and so at the head. Which means you either have to use a combined CPC/Bonding conductor in the SWA and may need a larger cable or run bonding conductors back to the MET from the cabins.

I would TNC-S the SWA and then local fit rods.

 

[nickblake]

Agree with malcome on this one going to add, depending on the length of cable and ground type soil clay sand , if you TT at the supply you may not be able to get the required Ze <200 ohms , so tncs to each cabin and earth rod each cabin from the CDU in each cabin not combining the two i add Earth TNCS TT.

 

[kmcgold]

Even if you TT at the incoming why would you want to fit a 100mA S type up front RCD to protect the armoured sub mains? The boards in the cabins protect all the final circuits with a main 30mA one.

Plus as you will have the problem of where your MET will be. As your runing 2 sub mains your MET will have to be at the origin of the installation and so at the head. Which means you either have to use a combined CPC/Bonding conductor in the SWA and may need a larger cable or run bonding conductors back to the MET from the cabins.

I would TNC-S the SWA and then local fit rods.

For TT system all circuit must be protected with rcd.
I should have said at the distributor meter position.(or cutout service head ,met,meter)
csa of swa should be big enough for main bonds can check if you want .
The are many ways of doing the job I am just saying
the way I would install it .

 

[malcolmsanford]

For TT system all circuit must be protected with rcd.
I should have said at the distributor meter position.(or cutout service head ,met,meter)
csa of swa should be big enough for main bonds can check if you want .
The are many ways of doing the job I am just saying
the way I would install it .

Regulation please ..............because I would use 411.5.2 that tells me that not all circuits require RCD protection.

We then go to reg 704.411.3.2.1 and that will depend how the termination is done if in a socket and not exceeding 32amps then there is no need for a 500mA RCD protection.

I agree that in some cases you can't adhere to the regs in section 411.3.2..........etc then additional protection by RCD can be achieved, but a blanket RCD is not required, as it can also be achieved by supplementary equipotential bonding.

I'm not sure I would use the SWA as a bonding conductor as that as to go back direct to the MET, I would use a SWA for a CPC though. But why have all this extra calculations and work when TT at the cabins is so much easier.

 

[IQ Electrical]

This thread just gets better, round in circles we go!
This is why I invoice my design and only after a site visit!

kmcgold is now proposing to use the TN-C-S earth and main protective bonding to a steel cabin on a construction site!

 

[nickblake]

Agree with malcome , if you are using the SWA as the cpc then the cable is protected , rod at the cabins and RCD the circuits ,every site cabin ive installed has a board with a main switch RCD ,so the TT is covered the only reason i would install a 100ma rcd would be if the cable is at greater risk of damage due to external influences ,

 

[kmcgold]

no tt system only

tncs?

 

[kmcgold]

This thread just gets better, round in circles we go!
This is why I invoice my design and only after a site visit!

kmcgold is now proposing to use the TN-C-S earth and main protective bonding to a steel cabin on a construction site!

no tt system only

 

[kmcgold]

kmcgold is now proposing to use the TN-C-S earth and main protective bonding to a steel cabin on a construction site!
iq electrical when did i say that!

 

[kmcgold]

I have never come across a ze reading low enough to operate a mcb on a tt system never mind the zs reading
csa sw can be used for bonding and third core for cpc
4mm swa has 21mm csa of steel wire armour

i would use a tt system not a tncs

just the way i would do it
the are many ways of doing it.

 

[nickblake]

I have never come across a ze reading low enough to operate a mcb on a tt system never mind the zs reading
csa sw can be used for bonding and third core for cpc
4mm swa has 21mm csa of steel wire armour

i would use a tt system not a tncs

just the way i would do it
the are many ways of doing it.

thats why you use and RCD the max Ze reading on a TT system is <200 ohms the max Zs reading is 1667 ohms using a 30ma RCD
it can be reached quite easily read reg 411.5 of the regs it gives you all the readings there

 

[kmcgold]

yes nickblake i know i was just replying to a post.
thanks

 

[SimpleSimon]

Never det with these types of cabins before, but if they are coming with their own CU in the cabin with a RCD main switch, wouldnt it be practical to have the bonding already installed in the cabin?
If they are always to be classed as a "temp structure" than wouldn't the same regulations apply?

 

[IQ Electrical]

kmcgold is now proposing to use the TN-C-S earth and main protective bonding to a steel cabin on a construction site!
iq electrical when did i say that!

You were talking about distributing cpc and bonding through the SWA but you didn't say you were talking as a TT system, previous posts proposed using the TN-C-S only for the distribution circuit, isolating the cpc at the cabin.

You need to be clear when you post on this sort of thread, amazingly, rather than utilising BS7671 and relevant guidance notes, some use this type of thread as a free design service.

If I was mistaken by the ambiguity in your post, I apologise

 

[kmcgold]

on previous post i have said i would use tt system
i try to give just short and basic answers
thanks

 

[High Tower]

Why would you
Go to the hassle and expense of TT the supply cables??

There is a far superior TNC-s available to protect the SWA's. Also there are already 30ma RCDs in the cabins so why TT the swa and have the cost of another rcd in a already cramped enclosure???

Only other question I have is that if the cabins come pre wired do they come pre bonded ??

 

[SimpleSimon]

Why would you
Go to the hassle and expense of TT the supply cables??

There is a far superior TNC-s available to protect the SWA's. Also there are already 30ma RCDs in the cabins so why TT the swa and have the cost of another rcd in a already cramped enclosure???

Only other question I have is that if the cabins come pre wired do they come pre bonded ??

My thoughts exactly!

 

[Deleted member 9648]

We do a lot of these....If it is a TNCS we use that to protect the SWA and gland it into an insulated DB where the Tncs earth is separated...(no banjo!)....everything upstream of this is on a rod.........normally prewired units are bonded,but often this has come adrift or corroded so check. We normally carry out a brief PIR on units which dont come with a cert from the hire company...they are often in a state!

 

[malcolmsanford]

We do a lot of these....If it is a TNCS we use that to protect the SWA and gland it into an insulated DB where the Tncs earth is separated...(no banjo!)....everything upstream of this is on a rod.........normally prewired units are bonded,but often this has come adrift or corroded so check. We normally carry out a brief PIR on units which dont come with a cert from the hire company...they are often in a state!

That is a suprise as last year I did quite a bit of work for a company called A1 they are a hire company and hire out quite a lot of these port-a-cabin type offices, and it was carrying out a PIR on the unit for them before they sent it out.

It was just a short term thing as their own sparks was off sick at the time, but I was pleasently suprised how decent they were, compared to the ones you seem to do Wirepuller, and to be honest the ones i'm use to

 

[Deleted member 9648]

The last set we did were all in a state...(hired)....and there were no test labels on the DB's,others have been ok.