TT disconnection times

[gselectrical]

Evening all.

I was under the impression that in most cases, in order to meet disconnection times within a TT installation, RCD's must be used unless Ze is very low.

I have come across a 3-phase installation (caravan park) where the setup basically compromises of a panel board feeding 3x 3 phase sub boards. 1 sub board feeds a toilet block (split RCD, no RCD on lighting circuits) as well as a number of streetlights (no RCD). The other sub boards feed various caravan hookups (all with 30mA DP RCD's)

Ze=142ohms Ra=144ohms

PFC=2Ka

The 3 sub boards are fed from 160A 60947-2 MCCB's.

The circuits fed from the sub boards are protected by MCB's (type b ranging from 6A-32A)

So, I cant see how any of the distribution circuits and the circuits feeding the hookups and also the lighting circuits within the toilet blocks can meet their disconnection times?

What are the options regarding this issue, or am I completely missing something?

Cheers

GS

 

[Phoenix]

You're not missing anything, those circuits won't disconnect in the event of a fault, and are will cause a significant touch volatage on earthed metal parts of the installation if such a fault occurs, even parts which are downstream of other RCDs (such as caravan frames!)

I think a danger notice is in order for this mate, unfortunatly it seems to be anything but unusual on caravan sites, a lot seem to be installed by people without a clue!

As to what you can do to fix it, either an upfront RCD, which would probably take the form of an adjustable shunt trip based unit fitted onto the incomming MCCB (still leaves the metal panel board unprotected though, but obviously is an improvement over what you have), or consult with the DNO as to whether they can provide you with TNCS to use for distrubution and site washroom, and isolate it and use the TT for just the pitch supplies with suitable RCD protection

 

[Manator]

The DNO will not provide TN arrangements on caravan sites. The install looks like it was done by some one who has little experiance of TT earthing. All circuits should be RCD protected to enable disconnection times.

 

[Lee321]

hmmm. it is perfectly possible to have disconnection in time without rcd however the rcd will buffer in an event the ze does go high and is the reason for it being preferred method

 

[Andy™]

The DNO will not provide TN arrangements on caravan sites. The install looks like it was done by some one who has little experiance of TT earthing. All circuits should be RCD protected to enable disconnection times.

nothing wrong with TNS on a caravan site

also, you can have a TNCS supply on a caravan site to feed fixed buildings etc.

 

[gselectrical]

You're not missing anything, those circuits won't disconnect in the event of a fault, and are will cause a significant touch volatage on earthed metal parts of the installation if such a fault occurs, even parts which are downstream of other RCDs (such as caravan frames!)I think a danger notice is in order for this mate, unfortunatly it seems to be anything but unusual on caravan sites, a lot seem to be installed by people without a clue!

As to what you can do to fix it, either an upfront RCD, which would probably take the form of an adjustable shunt trip based unit fitted onto the incomming MCCB (still leaves the metal panel board unprotected though, but obviously is an improvement over what you have), or consult with the DNO as to whether they can provide you with TNCS to use for distrubution and site washroom, and isolate it and use the TT for just the pitch supplies with suitable RCD protection

Cheers.

Not sure what is the best option here. Could you give me an example of what you mean by an "adjustable shunt trip based unit"

Either way, it looks like I'll have to get the DNO out. The 50mm tails from the enclosed & sealed 3 phase meter/cutout would need isolating to change the device.

I believe that even if the supply was upgraded/downgraded (opinions differ) to a TNC-S system so that this could be provided to the fixed buildings and 3 phase sub boards, the earth would need isolating at the 3 phase sub boards to where the caravan hookups are fed from. Looking at the condition of some of them, this could be a bit tricky.

So I think the only option is to provide RCD protection throughout.

What is the usual setup where TT installations consisting of various distribution circuits and panelboards require RCD protection to meet disconnection times, but still need to discriminate.

Cheers lads.

GS

 

[Phoenix]

I'll dig up some links later on if you need them, but google vigi units, thats merlin gerins range of earth leackage shunt trip relays, basically it turns an MCCB into an RCBO that has an adjustable RCD part; You have a earth leakage relay with the adjustment controls on, some current transformers to go round the tails/cores and connects into terminals on the suitable MCCB and makes the MCCB trip when it detects earth leakage, with it being adjustable you can set the timings and trip points to get discrimination, so you might for example set the incomming device to 1A / 200ms, then you might have a standard 100ma time delay rcd on the incomming of the pitch supply board and then 30ma ones in the pitch supply units, as I said you still have the issue of the metal panel board, however as long as you fit the earth fault protective device as the incommer (so the busbars are protected), and then take care to have the way the tails come in and connect so that a fault is most unlikely, maybe fabricate a paxalin enclosure within the board to contain the 'unprotected' tails?... not totally ideal, but...