RCBOs with TT

[JohnW2]

I have not infrequently seen it asserted (here and elsewhere) that one cannot/should not use SP RCBOs in TT systems. Do I take it that this view derives from the requirement (itself sometimes debated) for DP isolation in TT systems - or is there more to it than that?

I can understand the thinking behind requiring DP isolation, given that neutral can be at a potential above that of the CPCs in a TT system (a little above, normally, but a lot above under some supply-side fault conditions). However, is it not adequate to have a means of DP isolation of a circuit (e.g. CU Main Switch or pre-CU isolator or Type S RCD), even if that involves isolating more than one circuit?

Thoughts?

Kind Regards, John.

 

[Spark123]

The on site guide shows RCBOs which appear to be single width in use with a double poled isolator as acceptable for a TT system as long as the unit is insulated or the meter tails additionally protected, figure 3.3.

 

[JohnW2]

The on site guide shows RCBOs which appear to be single width in use with a double poled isolator as acceptable for a TT system as long as the unit is insulated or the meter tails additionally protected, figure 3.3.

Thanks. Indeed it does - and that's precisely what I have (not many RCBOs, but some - with both insulated CUs and additional protection of tails) - and, as per my original post, I would have expected it to be acceptable. However, as I said, you don't have to look very far to find people asserting that SP RCBOs cannot be used with TT.

Kind Regards, John.

 

[ericmark]

Personally I would use a double pole RCD on main incomer either a 100ma (Because easy to get hold of) or 300ma (As max size for fire protection).

Then I would use 30ma single pole for socket supplies however that would mean using Ali-tube cable where buried in a wall with no 30ma protection.

As it stands at the moment it seems one can only buy Ali-tube by the role which is expensive.

It does seem a bit off to have to use 30ma RCBO for all circuits and use a RCD as main switch.

I suppose a split board with one 300ma RCD and one 30ma RCD the former with RCBO's fitted could work but in real terms one has to ask is it really required?

The only place it says double pole RCD must be used is for caravans and boats and since in both cases often we must also use a TT system it does seem as if twin pole is required for TT.

See 709.531.2 under MARINAS AND SIMILAR LOCATIONS

If wiring to a price I would use a twin RCD board and all MCB's easy way out.

 

[Risteard]

In a domestic installation the main switch is the means of isolation.

 

[sparkiemike]

Personally I would use a double pole RCD on main incomer either a 100ma (Because easy to get hold of) or 300ma (As max size for fire protection).

Then I would use 30ma single pole for socket supplies however that would mean using Ali-tube cable where buried in a wall with no 30ma protection.

Unless the front end RCD is a S type then you won't get any discrimination with the RCDs/RCBOs?

 

[JohnW2]

Eric, I personally have 100mA Type S pre-CU (obviously DP) RCDs, feeding "twin RCD plus Mainswitch" 'high integrity' CUs - with the RCDs protecting most circuits and (SP) RCBOs protecting the (small number of) remaining circuits. Hence all final circuits have RCD or RCBO protection, so the Type S RCDs are actually a bit redundant, except to protect the tails/submains.

I'm personally more than happy with all that, but there are still people who will (and do) say that it is wrong to have those SP RCBOs. I find that rather hard to understand. Whilst, as I said before , I understand the reason for requiring a means of DP isolation in a TT system, I do not think that is meant to imply that one has to have separate DP isolation for each final circuit. Indeed, if that what was intended, SP MCBs would be just as unacceptable (with TT) as SP RCBOs - and I've never heard anyone suggesting that. ... or am I missing something?

Kind Regards, John.

 

[JohnW2]

In a domestic installation the main switch is the means of isolation.

Exactly - hence I don't understand these people who have a problem with SP RCBOs in TT systems - unless I'm missing something. As I've just written elsewhere, if they have a problem with SP RCBOs, they should have the same problem with SP MCBs - which they don't seem to have!

Kind Regards, John.

 

[Paul_C]

if they have a problem with SP RCBOs, they should have the same problem with SP MCBs - which they don't seem to have!

Agree entirely. But then as I mentioned in another thread recently, the whole SP vs. DP for TN vs. TT issue makes so sense to begin with.

 

[JohnW2]

Agree entirely. But then as I mentioned in another thread recently, the whole SP vs. DP for TN vs. TT issue makes so sense to begin with.

Yes, you did, and I think that I more-or-less agreed with you at the time. However, having now thought a bit more about it, I think it probably does make some sense in relation to TT vs.TN-C-S, but less (if any) sense with TN-S vs. TT ....

With TN-C-S, no matter what potential the neutral supply may have relative to true earth (e.g. due to supply-side faults in the ‘right’ place), in the absence of faults within the installation, neutral will always be about the same potential as the installation's CPCs and any bonded metalwork, so there should be no risk to someone working on a circuit which had only had its L isolated. However, with TT, if neutral acquires a high potential relative to true earth, there will also be roughly that same pd between neutral and CPCs/bonded metalwork. Isolation of neutral therefore theoretically becomes desirable - that is, if one feels that the probability of someone working on an isolated circuit at the very time that there is a high neutral potential is something other than 'vanishingly small'!

However, as I see it, with TN-S, there is also a similar theoretical risk (of a high pd between neutral and CPCs/bonded metalwork) in the event that the supply neutral broke at the ‘right’ place (since CPCs and bonding are not connected to N within the premises, but may have a path to true earth via the DNO's earth conductor and/or bonded e-c-ps) – so there might again be a case for advising DP isolation to ‘cover’ the very improbable scenario postulated.

In other words, albeit we’re talking about a very low risk, I think I can see an argument for DP isolation with TT but not with TN-C-S, but it seems to me that the argument for DP isolation would also apply to TN-S.

When we discussed this before, I think we considered an apparently ‘anomolous’ situation in which, say, two adjacent properties have the same TN-C-S supply but that one of them ignored the DNO's earth and TTd their installation - with one 'requiring' DP isolation and the other not. However, I now don’t think that’s really an anomoly. If both premises were ‘enjoying’ a high neutral potential, there would be a high N-CPC pd in the TT house but not in the TN-C-S one.

... or maybe my ‘second thoughts’ are even more flawed than my original ones? I have to say that 'first thoughts' are often right and 'corrected ones' wrong

Kind Regards, John.

 

[Paul_C]

but it seems to me that the argument for DP isolation would also apply to TN-S.

Exactly the way I see it. If the issue is supposed to be about that small chance of a potential difference appearing between the neutral and "earthed" parts within the installation, then a supply neutral fault is just as capable of resulting in that potential difference with TN-S as with TT. Although if we want to look into TN-C-S a little more deeply, there are many other scenarios involving work on the bonding which could result in similar voltages appearing. If the neutral is riding high due to a fault, then what if somebody is altering bonding arrangements and removes a bond to that high neutral? If the metalwork which was bonded already has a substantially low resistance to earth (a few thousand ohms or less) by way of the building fabric, buried pipes, etc., then as soon as the bond to it is removed there will exist a potential between the neutral and that metalwork anyway.

So much of this depends upon the precise nature of the where the fault occurs and what is being done at the time.

However, I now don’t think that’s really an anomoly. If both premises were ‘enjoying’ a high neutral potential, there would be a high N-CPC pd in the TT house but not in the TN-C-S one.

But then it gets a whole lot more complicated in those situations where the bonded metalwork of one installation is within reach of the earthed metalwork of the other!

 

[securespark]

Isn't there also a 0.2s disconnection time requirement for TT?

 

[Paul_C]

Isn't there also a 0.2s disconnection time requirement for TT?

Now you mention it, wasn't that a new requirement with the 17th edition?

 

[securespark]

Think so, but I'm in my usual position (ooh err!) of being scantily-clothed and not being bothered to put on some trews and go down to the van.... LAZY!!!

 

[JohnW2]

Isn't there also a 0.2s disconnection time requirement for TT?

There is, indeed - but I'm not sure how that relates to the matter we are discussing.....

.... Are you perhaps implying a question of why this differs from the 0.4s for TN? If so, that's a jolly good question - any ideas?

P.S. Just as well the nominal voltage is not 231V - TT systems would then require 0.07 secs disconnection time (0.2s for TN)!

Kind Regards, John.