Supply for building site cabins - a few questions!

[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!