TT and TNC-S next to each other

[Toneyz]

We are currently doing a market square this involves in-ground lighting and providing sockets for the market traders. This involves two metered single phase boards The spec asks for the sockets to be on TT and not connected to PME. As the boards are metal. Am I correct that I shouldn't be able to touch both boards at the same time (they are both next to each other) due to the P.D. between each. In the spec, they are referring to the regs for caravan sites I think the thought is that of metal shell burger vans and the loss of neutral although it is all block paving.

 

[Strima]

I think you need to confirm with the designer, I would say the safest option is to have them both on the same earthing arrangement.

 

[Toneyz]

I think you need to confirm with the designer, I would say the safest option is to have them both on the same earthing arrangement.

We are awaiting an answer from the electrical consultant I am hoping that they will say only the pop-up sockets could be used by a burger van or similar that way I can separate the earths there.

 

[Strima]

It would make sense just to TT the lot. Plastic DBs with double pole RCBO OCPDs.

 

[Toneyz]

It would make sense just to TT the lot. Plastic DBs with double pole RCBO OCPDs.

I am floating with the idea of an additional rod

 

[Strima]

I am floating with the idea of an additional rod

What is the current Ra?

 

[Toneyz]

What is the current Ra?

Sorry I meant additional to the PME supply the IET was going to put it into the 18th but is there not now advice we are installing to 17th.

 

[DefyG]

Installing to the 17th Edition! did you start this job last year?
If so seems a bit late in the day to be enquiring about earthing arrangements!
Without further info seems a TT job to me.

 

[Toneyz]

Installing to the 17th Edition! did you start this job last year?
If so seems a bit late in the day to be enquiring about earthing arrangements!
Without further info seems a TT job to me.

It was priced and designed by the electrical consultant last year.
Yes, you're right I should have given the spec more of a read earlier like everything you do as electrical sub-contract labour jobs are given to you they are priced by someone who has little knowledge in an office without any proper design. The spec only asks for the sockets to be on a TT but you have two metered boards next to each other (lighting & power) I know that the two earthing systems need to be kept separate but not 100% that you are not allowed to have two D.B's of two earthing systems next to each other. I have outlined the perceived risk if a metal burger van is being hooked up to one of the sockets in a previous post. There may be mileage regarding the whole installation TT with a timed delayed RCD as the overall main switch and all final circuits with RCBO protection at the moment the spec says MCB for lights. Sometimes I wonder what planet these consultants/designers are on.

 

[Strima]

If you have plastic DBs then there would be no need for an upfront RCD just a standard double pole switch for isolation purposes only.

 

[Toneyz]

If you have plastic DBs then there would be no need for an upfront RCD just a standard double pole switch for isolation purposes only.

The thought had crossed my mind but they want a S.P&N type A metal Memshield 3 D.B. with meters and these are on site now. Thes things are always left for us to sort out no one sits down before handing the job to us and designs these things.

 

[happysteve]

411.3.1.1 is clear that "simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system..." Metal DBs are exposed-conductive-parts. If they're simultaneously accessible, then they need to be on the same earthing system.

 

[DefyG]

411.3.1.1 is clear that "simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system...

But it was designed and being installed to the 17th edition!

I’m sure without checking the old regs (17th) the requirement is the same though.

 

[Toneyz]

Thanks, Steve I thought there was a reg saying that somewhere.

 

[happysteve]

But it was designed and being installed to the 17th edition!

Same reg. Same wording.

I’m sure without checking the old regs (17th) the requirement is the same though.

Well, yes.

 

[UKPN]

We are currently doing a market square this involves in-ground lighting and providing sockets for the market traders. This involves two metered single phase boards The spec asks for the sockets to be on TT and not connected to PME. As the boards are metal. Am I correct that I shouldn't be able to touch both boards at the same time (they are both next to each other) due to the P.D. between each. In the spec, they are referring to the regs for caravan sites I think the thought is that of metal shell burger vans and the loss of neutral although it is all block paving.

Independent earthing only.

The thought had crossed my mind but they want a S.P&N type A metal Memshield 3 D.B. with meters and these are on site now. Thes things are always left for us to sort out no one sits down before handing the job to us and designs these things.

It doesn't matter if the design was to the 14th Ed. A metal fuse board must be protected by a plastic rcd. (on a tt system)