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