levels of rcd protection on TT system - long post - prepare cuppa first!

[Wilko]

Thanks for the feedback Guys and to OP for tolerating my hijack.
There are animals on site, so I will crack on with a TT installation

 

[suffolkspark]

There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!

 

[Ian1981]

Yes you are quite correct but in agricultural it is the difficulties of bonding extraneous-conductive-parts.

There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!

i find the guidance notes are the most helpful when trying to decipher bs 7671.
The guidance notes do explain in more detail as bs 7671 is just a set of regulations and is not meant to go into details I feel

 

[westward10]

There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!

If you can get a 10.0 cpc across that would be my choice.

 

[suffolkspark]

I can, i can infact more easily fit an inline rcd to protect the conveyors and bin lift, isolate the cpc at the end of the YY cable, and use the structural steel of the warehouse as my earth thus having that as TT, would save crawling through a long loft space!

 

[suffolkspark]

bottom of page 24........ pme not recommended unless a metal grid is in the floor! ffs I think I'm going to price to TT the bitch and be done with it.
Can you chaps just answer me the question of whether I still need to meet my 0.2 sec discon times for the 32A tp panels? as its not in section 705 - just says rcd ratings.
installing 30mA protection on each one would at least reduce the likelihood of the incoming type S tripping - if ever, as it stands now a decent fault on any of the pumps has the ability to trip the entire site.

ps thanks everybody for their input, including the hijacker! all aids the cause

 

[westward10]

Ìt isn't a cattle shed I would leave it as TN-C-S.

 

[Ian1981]

I

bottom of page 24........ pme not recommended unless a metal grid is in the floor! ffs I think I'm going to price to TT the bitch and be done with it.
Can you chaps just answer me the question of whether I still need to meet my 0.2 sec discon times for the 32A tp panels? as its not in section 705 - just says rcd ratings.
installing 30mA protection on each one would at least reduce the likelihood of the incoming type S tripping - if ever, as it stands now a decent fault on any of the pumps has the ability to trip the entire site.

ps thanks everybody for their input, including the hijacker! all aids the cause

It may not be in section 705 but disconnection times are disconnection times.
For TN you have 0.4/5 seconds for 230/400 v
For TT it's 0.2/1 seconds.
You can use the TN disconnect times for TT if you can get your zs values that low but I've never seen it done.

 

[suffolkspark]

might be a herd of funny looking creatures working in there though
Looks like misreading the TN-C not being permitted bit in the regs has got me in a mucking fuddle over almost nothing then! undersized earth to the warehouse will be getting C2, and I'm still going to C3 the fact that a reasonable fault on any of these 3phase items could cause the upfront RCD to trip loosing the entire site which would be a pain in the arse, especially if the cold stores were full!

 

[westward10]

Have you asked about nuisance tripping.