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

[suffolkspark]

Hi all, need some opinions on the following please>

Origin is a three phase 60A TN-C-S supply.it goes into a changeover switch (small PTO driven generator across the yard) and then into a wylex three phase board. the incomer is 100A 4pole 100mA type S to provide discrimination to other downstream RCBO's. there are several different outgoing circuits from this board so please bear with me:

2x tp+n supplies go to chiller control panels in the same room on 32A MCB's which cool the adjoined cold store. PVC double insulated tails ref method B
1x sp inside lighting circuit on 6A 30mA RCBO PVC singles ref method B
1x sp outside lighting circuit on 6A 30mA RCBO pvc singles ref method B
1x sp single socket on 16A 30mA RCBO pvc singles ref method B
1x tp+n sub main to a control panel in an irrigation shed 20m away containing bore hole pump and various other pumps. - 32A tp mcb SWA ref metho D + additional 16mm earth
1x single phase sub main to site office container - the cpc is separated for this and linked to an earth electrode - 20A 30mA RCBO - T/E ref method C
2x single phase sub mains to camp sites - 30mA rcd protected boards on the end of these and again separated from the TN-C-S as with the site office and linked onto earth electrode. (there are also earth electrodes at the camp site end) 40A mcb SWA ref method D + additional 6mm earth
1x tp+n sub main to a dis board in an adjacent sorting/packaging warehouse (steel construction + asbestos cladding - structural steelwork is bonded) - within this is another wylex board with 30ma protection via rcbos to everything except a three phase 16A motor running a bin lift and 2x plugged in single phase conveyors linked into the same system though an emergency stop button/contactor in a clearly labelled socket. 32A mcb, 5 core YY cable

As it stands 30mA protection is installed on all single phase circuits (except the camp site sub mains). All sub mains + the irrigation panel, chiller panels, and conveyors only have the 100mA type S to back them up but all Zs values are well within what is allowed, bonding to structural steelwork is good, as is the earth electrode.

Now the spanner in the works is this - its an apple farm, obviously unless you count bees there's no livestock though. BS7671 clearly prohibits the use of TN-C-S in agricultural and horticultural installations. The camp site and office have correctly been separated from the TN-C-S side and are well labelled, infect everything is well labelled. My issue is its probably classed as a farm as it farms apples? but the electricity supplies aren't typically farmy, ie a sorting and packing warehouse and a cold store. with only a forklift running between the 2 buildings. areas with tractors etc actually don't even have power to them.
Should i be recommending the installation be put on TT in its entirety because of reg 705.411.4?

I know the 100mA type S is acceptable for the distribution circuits and it also ensures continuity of service, and discrimination from downstream rcbo's... 705.422.7. but the final circuits, ie the 2x chiller panels, conveyor setup, and irrigation panel require at least 300mA protection for protection against electric shock, does the 100mA type S comply with this? 705.411.1 (iii) I cant find in the regs a yes or a no? The 16A tp+n circuit doing the conveyors does contain a double socket outlet but it is clearly labelled it is just for use of the single phase conveyors, these are wired in SY. an adjacent 30mA protected double socket is labelled general use.

Ra was 17ohms for the earth electrode 0.2 Ze on the TN-C-S side

What are your thoughts please chaps

 

[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.