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.

 

[suffolkspark]

yes they have never had any nuisance tripping issues to date, but the chillers are ageing as is the bore hole/irrigation setup, if either did develop a fault then it could be an issue

 

[westward10]

Wouldn't comment on it then, have you done the EICR.

 

[suffolkspark]

I've carried it out but haven't typed it up yet, that's a task for tomorrow

 

[westward10]

I suggest you concentrate on that and worry about remedial works after.

 

[s.osmond]

The RCBO's will not give discrimination over the Selective RCD as they are only single pole therefore a neutral to earth fault on a tripped RCBO will still trip the main RCD if sufficient current is flowing in the system to divert down the neutral earth fault !

 

[s.osmond]

With regard to the tripping times, surely you only need to achieve the 0.2 sec disco time for an earth fault of negligible impedance not the actual rated tripping time .
Not that a times 5 test is required for a 100ma and above RCD but if your tester will permit it try and record the result , I bet it is under 200ms.

 

[Ian1981]

The RCBO's will not give discrimination over the Selective RCD as they are only single pole therefore a neutral to earth fault on a tripped RCBO will still trip the main RCD if sufficient current is flowing in the system to divert down the neutral earth fault !

The RCBO's will not give discrimination over the Selective RCD as they are only single pole therefore a neutral to earth fault on a tripped RCBO will still trip the main RCD if sufficient current is flowing in the system to divert down the neutral earth fault !

Because the Neutral is common and not isolated in the event of a fault it could still could cause s type to trip yes but I'd like to think the 30mA will disconnect any current in the neutral first in the faulty circuit before the time delay trips.

 

[s.osmond]

Yes but the point is the neutral current from the healthy circuits will flow into the neutral earth fault which is not disconnected by the single pole breaker and unbalance the selective RCD. This will happen when heavy loads or equipment with high inrush current are switched on. Believe me out here in the sticks far away from civilisation we know a lot about TT systems and nuisance tripping.

 

[s.osmond]

By the way my personal opinion is that single pole RCBO's should not be allowed on TT systems, after all the regs says that both live and neutral should be treated as live on a TT system !

 

[Ian1981]

By the way my personal opinion is that single pole RCBO's should not be allowed on TT systems, after all the regs says that both live and neutral should be treated as live on a TT system !

Requirements are that the line and neutral both need isolating. A 2 pole rcd or 2 pole mains switch on single phase systems will account for the double pole isolation on a TT as it's an acceptable method and will satisfy the requirements.
Likewise you can have triple pole mains switch on a 3 phase installation isolating a 3 phase db but on a TT the isolation switch would be required to be 4 pole.