Supply for building site cabins - a few questions!

[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?