Over the years double pole (DP) disconnection in a TT installation has been discussed many times. However time has moved on with new regulations and books on these regulations, so it may be time to ponder the issue again.

I have been to look at a TT installation that needs work and I am proposing to installing a three way split board with two 30mA RCDs as per the Onsite Guide (OSG) figure 3.6.3(v) page 3. Generally referring to the OSG and to BS7671:2008 Amd1:2011 page 57 411.5 TT System, I am failing to find any reference to a requirement for double pole isolation being required for a protective device being it a RCD, MCB or RCBO, indeed the OSG in fig. 3.6.3 (iii) page shows a consumer unit (CU) for use with either a TN or TT installation with what appears to be single pole RCBOs with the only extra requirement for TT being "for TT installations insulated enclosure or further mechanical protection to meter tails", which itself could be open to discussion, but no mention of DP devices.

I am proposing to install a plastic CU with a all insulated enclosure and as I said a three way split. It is also my intention to install an S-type time delayed 100A/100mA RCD main switch then two single pole 30mA RCBOs with two DP 30mA RCDs having the remaining circuits protected by single pole MCBs divided between them.

I remember Stu Little saying he likes a 100mA RCD upfront for "engineering purposes" and I agree with him, however if I have every circuit then protected by a 30mA RCD or RCBO as well may by some be considered over engineering on my part.

The possibility of a lost neutral with overhead supplies such as this installation has is a real possibility and I have witnessed it personally, so a real issue to consider.

So do I bung another £50 on the price for a 100mA RCD main switch? The customer only actually wants a immersion heater connected, at some time the immersion heater was disconnected and the fuse way used to supply a sub main to a extension on the house. Looking at the equipment though there is a volt operated main switch, undersize fuse board with rewirable fuses and two switch fuses feeding out buildings, which really needs tidying up along with some new earthing. So my quote for connecting a immersion heater is several hundred pounds already without a extra RCD!

Andy

Edited: 04 February 2013 at 07:23 PM by sparkingchip

Are you talking about having a 100mA TD and a 100 mA RCD IN series? If just TD then although a N-E fault on one of the RCBO protected circuits will still take out the whole installation.

I'm leaning towards DP 30mA RCD protecting banks of circuits ( pretty much like a standard TN set up now, although I usually always leave unprotected ways for RCBO's if necessary! ) to avoid the N-E fault remaining scenario. Ideally DP RCBO's but expensive.

I intend to take the main switch out of the CU and replace it with the single 100mA S-type RCD with the mixture of 30mA SP RCBOs and DP RCDs downstream, so the RCBOs would not disconnect the neutral leaving the main switch RCDexposed to the fault if it is on the neutral.

Andy

Do you need to have the RCBO ways?

I'd probably stick with just the 30mA's protecting the outgoing ways. The only reason I used a 100mA TD recently was to protect sub-mains, although if you do use it as a main switch I suppose it's nice to have some back up protection if one of the 30mA fails.

Andy, DP isolation isn't required for each individual circuit, just the installation as a whole, which your main switch will provide. Also although I might have read it wrong are you talking about having a 100m/a S type in front of the two 30m/a ones? if so there's no point, the 30m/a ones will always go first! I've fitted plenty of plastic c/units on TT supplies, when the 17th first appeared there was some scare-mongering on here about something happening to the internal wiring of the c/unit before the RCD's but that seems to have subsided now! As for the tails, if they are going from the meter to the c/unit just next to it as in most domestic situations I see no need for extra protection, different story if they are going some distance of course,

Dave.

If you want to avoid nuisance tripping Change the single pole RCBO's for DP ones if you can get them for the CU and do away with the TD RCD.

Regards

There are two sub boards for out buildings I want to separate from the house RCDs, I'm now thinking I'll ditch the RCBOs for MCBs and put DP RCDs local to the outbuildings as there is one DP 30mA RCD I can reuse, this seems to make more sense as the 100mA will protect the two submains requiring a 1 second disconnection time, all assuming a satisfactory Ra

Andy

That is only one of the faults to consider. It is likely that a phase to earth fault at any other property fed from the same substation that either has no rcd or a failed rcd will cause a voltage between neutral and earth, which could be significant (a hundred volts or so perhaps).

If you are on the same phase as this faulty property, your phase to earth voltage will drop by a similar amount. If you are on a different phase, your phase to earth voltage may increase by this amount instead!


Regards,

Alan.

So, what would be the best way of protecting a TT supplied domestic house?

I'm doing one on Friday, they have already had a surge that took out a TV, but they wont consider fitting a SPD as it will cost them £150 extra.
I was going to fit a 6 way SP RCBO board, with DP main switch, but will take on board anyones thoughts if they think it could be done better with DP RCD protection.
Ta
Alan.

I've done quite a few in my area where there is quite a lot of TT. I've always used the usual twin rcd boards since the 17th came in, before that I done a few using a front-end 30m/a rcd, no ones ever complained to me about nuisance tripping,

Dave.

I've been using the twin RCD consumer units with a 100A main switch as well for TT, however this time I want more separation of circuits across RCDs in this case four RCDs.

Andy

I would tend to agree. I still believe that you need an rcd of some sort that is capable of interrupting the neutral as well. If you want to fit rcbos, perhaps consider a time-delayed rcd up front to give them time to go first.

Regards,

Alan.

Hi Alan, although the regs say that using an RCD is the prefered method for TT, it does allow for there to be no RCD, there could just be a fuse or similar providing earth fault protection..... so in those circumstances would you say the neutral should also be fused?

I suppose the rcd could be dispensed with if the Ra will always be reliably low enough to comply with the max. figure for a tns supply Weirdbeard? (earth loop for the type of mcb I mean)
I don't think fusing the neutral would ever be recommended though,

Dave.

Thanks for the reply dave, though (I think) my question was a rhetorical one.....

A lot of time and money could be spent protecting against all the possible faults in other consumers installations or the suppliers own installations, but at the end of the day unless there is a specific requirement for installations to be guarded against these then BS7671 is the baseline for protection provided - for example in a typical house you could probably protect against the dreaded loss of neutral for less than £100 in parts by fitting a bi-polar contactor in the tails, but no-one does this do they?

Agreed Weirdbeard, there has to be a limit even when safety is involved doesn't there! I'm not sure where this idea that both L + N have to be disconnected by the cpd in a TT system comes from? As far as I understand it the installation is isolatable on both poles if there is a DP main switch present, maybe it comes from some older types of c/unit with SP main switches?

Dave.

You could do it for half that with an RCD, electrode and a resistor - it would be illegal under ESQCR in the UK though

Regards

OMS

-------------------------
Failure is always an option

Hi Dave, the misconception appears to me to derive from some misunderstanding the difference in function between devices for circuit protection and devices for isolation, perhaps it's because sometimes these can be the same device?

Without wishing to disrespect the OP, Andys OP does seem to mix up these functions where he says:

"I am failing to find any reference to a requirement for double pole isolation being required for a protective device being it a RCD, MCB or RCBO"

Probably not helped by BS 7671's muddled terminology - does "disconnection" mean removing wires, operating a switch-disconnector (a.k.a. isolator) or a fuse blowing leaving an unknown gap between supply & load conductors?

Plus an increasing number of special locations are demanding DP ADS/additional protection by RCD - not surprising given Alan's points.

- Andy.

Hmmm.... does ESQCR apply within a BS7671 installation, and an electrode would be only necessary in a TN system if there were no bonded extraneous conductive parts to provide an earth reference?

Where does the resistor go?