Supply for building site cabins - a few questions!

[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 .