Supplimentry Bonding 17th Edition

[JohnW2]

• "If Ze and R1+R2 are both reasonable/acceptable, then, in a TT installation, the actual value of the sum of them (i.e. Zs) (which is usually going to be too high to achieve required disconnection times with an OPD) is essentially irrelevant."

... I could have added that recording Zs figures in a TT installation can be potentially confusing/misleading. Although I have a TT installation, with the rod itself generally around 75Ω, I actually have a Ze of around 0.25Ω when main bonding is present (and about 0.35Ω without main bonding, presumably due to incidental paths via pipework) - thanks to a water supply pipe and, probably, a neighbour's PME earth/bonding.

As a result, Zs figures measured and recorded with bonding present are all low enough to provide for adequate disconnection times from OPDs. However, if one dark night a man with a lorry and lots of plastic pipe came along and replaced the metal water supply pipe to my property, that situation would presumably change dramatically, and I would be down to reliance on the RCDs.
Kind Regards, John.

 

[davelx]

A common source of misunderstanding is that of either specifying or measuring value of E[F]LI where the circuit also has an RCD fitted. E[F]LI measurement under such circumstances is a futile exercise. The circuit will have been checked for continuity, and this is all that is needed together with, of course, functional checks of the RCD. This criterion satisfies requirements for automatic disconnection."

I totally disagree with that.
Perhaps the common source of misunderstanding is not where the author thought it was.

Can you check for continuity without realising what the value of R1+R2 is?

As stated above, it is just about impossible for a circuit to exceed 1666Ω so we may as well not measure anything.

A little disingenuous to selectively quote my post so that it look as if I said that, or that I agree with it.

 

[EFLImpudence]

Sorry about that. I didn't mean that. That is why I wrote 'the author' and not 'you'.

I shall amend my post.

 

[EFLImpudence]

I don't think it entirely relevant to keep returning to TT situations when the references from the ECA, stating that Zs is irrelevant, was a general statement referring to all installations.

From: ECA Guide to the Wiring Regulations 17th Edition etc. (pub. Wiley) on page 63 section C4.6.5
"C4.6.5 Irrelevant E[F]LI Specification

A common source of misunderstanding is that of either specifying or measuring value of E[F]LI where the circuit also has an RCD fitted. E[F]LI measurement under such circumstances is a futile exercise. The circuit will have been checked for continuity, and this is all that is needed together with, of course, functional checks of the RCD. This criterion satisfies requirements for automatic disconnection."

What, exactly, is the misunderstanding? Again, is it that of the author?

Do we still have to determine PFC - PSCC and PEFC - at all relevant points?
Can we do that without resorting to futile exercises?

'The ECA GUIDE to the wiring regulations' would seem, if this is the standard, to be as reliable and useful as most of the other guides.
Perhaps The IET should publish a guide.

I did not even want to buy the new (green) OSG but was forced to by the scheme.

 

[JohnW2]

I don't think it entirely relevant to keep returning to TT situations when the references from the ECA, stating that Zs is irrelevant, was a general statement referring to all installations.

I think what the author of the ECA guide was probably claiming to be a 'common misunderstanding' was the belief that BS 7671 actually requires that disconnection times be achieved by an OPD (in TN systems,as well as TT ones).

Although most of us agree that RCDs should (in TN systems) be regarded only as 'additional protection', with primary fault protection to be achieved by OPDs, I think you would probably be hard pressed to find anything in the BGB which explicitly prohibits the required disconnection times being achieved with an RCD. Indeed, as I wrote recently, a footnote to Table 41.4 in section 411.3.2 ("Automatic disconnection in case of a fault") starts off "Where compliance with this regulation is provided by an RCD...."

What, exactly, is the misunderstanding? Again, is it that of the author?

As above, I suspect that the author believes that most people are under a 'misunderstanding' that 411.3.2 cannot be satisfied by an RCD, even in a TN installation. Whether that is a 'misunderstanding' or not is open to debate - but, as above, I'm not sure where the BGB actually says he is wrong.

Kind Regards, John.

 

[EFLImpudence]

Although most of us agree that RCDs should (in TN systems) be regarded only as 'additional protection', with primary fault protection to be achieved by OPDs, I think you would probably be hard pressed to find anything in the BGB which explicitly prohibits the required disconnection times being achieved with an RCD.

As far as I am concerned 415.1.2 says it all.
Therefore if you agree that RCDs are only additional protection then surely the initial protection must be verified.

If tests are to be done to ensure protection against overload and short-circuit then why would ensuring the same for earth-fault be described as futile.

Indeed, as I wrote recently, a footnote to Table 41.4 in section 411.3.2 ("Automatic disconnection in case of a fault") starts off "Where compliance with this regulation is provided by an RCD...."

Does that not refer to cases where the 'normal' disconnection times cannot be met?
I think we've been there before.

 

[JohnW2]

Although most of us agree that RCDs should (in TN systems) be regarded only as 'additional protection', with primary fault protection to be achieved by OPDs, I think you would probably be hard pressed to find anything in the BGB which explicitly prohibits the required disconnection times being achieved with an RCD.

As far as I am concerned 415.1.2 says it all. Therefore if you agree that RCDs are only additional protection then surely the initial protection must be verified.

As you say,one might think that 415.1.2 'says it all' - were it not for the fact that 411.4.4 (in the 'TN systems' section) seems to totally contradict it.

Indeed, as I wrote recently, a footnote to Table 41.4 in section 411.3.2 ("Automatic disconnection in case of a fault") starts off "Where compliance with this regulation is provided by an RCD...."

Does that not refer to cases where the 'normal' disconnection times cannot be met? I think we've been there before.

Yes, we've been here more than once before. Many people would, indeed, like to think that it means what you say - but I'm not convinced that the words of the regs actually say that, are you? (plus note 411.4.4, as above).

Kind Regards, John.

 

[EFLImpudence]

As you say,one might think that 415.1.2 'says it all' - were it not for the fact that 411.4.4 (in the 'TN systems' section) seems to totally contradict it.

411.4.4 doesn't contradict 415.1.2 (especially because of NOTE 1) as an RCD may be used for fault protection but 415.1.2 states that the same RCD is not recognised as the sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414.

Yes, we've been here more than once before. Many people would, indeed, like to think that it means what you say - but I'm not convinced that the words of the regs actually say that, are you? (plus note 411.4.4, as above).

Also note NOTE 1, as above.

Well, they quite often don't say what they mean or mean what they say.

Perhaps we do, indeed, need guidance notes from The IET.

 

[JohnW2]

As you say,one might think that 415.1.2 'says it all' - were it not for the fact that 411.4.4 (in the 'TN systems' section) seems to totally contradict it.

411.4.4 doesn't contradict 415.1.2 (especially because of NOTE 1) as an RCD may be used for fault protection but 415.1.2 states that the same RCD is not recognised as the sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414.

It's very confusing, but I think our (attempts at!) interpretations differ.

411.4.1, taken in isolation, appears to be saying that an RCD may be used as fault protection, subject to (NOTE 1) 'overcurrent protection' being separately provided for by an OPD. Since 'overcurrent protection' comprises 'fault protection' and 'overload protection' I see no sensible way of interpreting this other than that the OPD is required to provide only overload protection. Otherwise, 411.4.1(ii) would be ridiculous - what would be the point in saying that an RCD 'may' be used to provide fault protection, so long as something else was also providing fault protection - surely they're not that daft?

As for 415.1.2, I guess it depends upon what it means by 'sole protection'. It does not say what sort of protection - so maybe they mean that an RCD may be used to provide fault protyection so long as an OPD provides overload protection? (i.e. 'the TT situation')

Perhaps we do, indeed, need guidance notes from The IET.

We already do have lots of GNs, and the OSG, don't we? - but more clarity in the regs themselves would also be nice!

Kind Regards, John.

 

[Spark123]

Why the difference, then? Is it perhaps so that RCBOs can use much smaller sensing toriods to enable them to fit into single module width enclosures?

Kind Regards, John.

Possibly, have come across RCCBs with functional earths too - think MEM have them.

 

[Electrifying]

411.4.1, taken in isolation, appears to be saying that an RCD may be used as fault protection, subject to (NOTE 1) 'overcurrent protection' being separately provided for by an OPD. Since 'overcurrent protection' comprises 'fault protection' and 'overload protection' I see no sensible way of interpreting this other than that the OPD is required to provide only overload protection. Otherwise, 411.4.1(ii) would be ridiculous - what would be the point in saying that an RCD 'may' be used to provide fault protection, so long as something else was also providing fault protection - surely they're not that daft?

Kind Regards, John.

I'm guessing you meant 411.4.4 - or are you quoting from a different book?

'Fault Protection' -

Protection against electric shock under single falt conditions.

In other words 'earth fault' - nothing to do with 'Overcurrent' protection.

I think what you meant to say above is:

'Overcurrent' protection comprises 'short circuit protection' and 'overload protection'

..........which now means 411.4.4 makes perfect sense.

 

[JohnW2]

I'm guessing you meant 411.4.4 - or are you quoting from a different book?

You guessed correctly My apologies.

'Fault Protection' - Protection against electric shock under single falt conditions.

In other words 'earth fault' - nothing to do with 'Overcurrent' protection.

Good point. However, you have revealed another potential source of confusion in the regs (at least to my brain when distracted) which explains, even if it doesn't excuse, what I wrote (whilst having at least one eye, and at least one half of my brain, on the Olympics, or was it Big Brother? )

I think what you meant to say above is:'Overcurrent' protection comprises 'short circuit protection' and 'overload protection' .... ..........which now means 411.4.4 makes perfect sense.

Well, no, I actually wrote what I 'intended' to write, even if it shouldn't have been what I wrote - and this is the reason why .... Although, as you say, "fault protection" is defined in Part 2 as you indicate above, throughout Chapter 43 (particularly Section 434) the phrase "protection against fault current" is used to mean what you speak of above as "short circuit protection". The difference between the phrases "fault protection" and "protection against fault current" looks subtle - but, as you point out, the difference in meaning is far from subtle! However, when my one non-TV eye was directed from 411.4.4 to 'Chapter 34', and I found all these references to "protection against fault current", the non-TV half of my brain obvioulsy did not think too deeply, and thought "fault protection"!! Apologies for the confusion.

Kind Regards, John.

 

[JohnW2]

'Fault Protection' - Protection against electric shock under single falt conditions.

In other words 'earth fault' - nothing to do with 'Overcurrent' protection. .... I think what you meant to say above is:'Overcurrent' protection comprises 'short circuit protection' and 'overload protection' .... ..........which now means 411.4.4 makes perfect sense.

I think I was a bit too quick late last night to effectively concede that I had, whilst distracted, written nonsense. At least, in the coolness of day, I can see further confusions here.

Yes, what I should have written is what you say above and, yes, in some senses that means that 411.4.4 makes sense, but I think it actually leaves my original point intact. In fact, I think you are emphasising my point! As you say, 411.4.4 says that fault protection may be provided by an RCD provided that (NOTE 1) provided that the circuit has an OPD per Chapter 43 ('Overcurrent Protection'). As you say, overcurrent protection is nothing to do with 'earth fault' ('L-E') faults. In other words, it looks to me as if 411.4.4 allows fault protection to be provided by an RCD and does not (even in NOTE 1) require that anything else provides disconnection in the case of L-E faults.

So, I'm back to my original thinking - and the fact that, whilst 411.4.4 appears to allow an RCD alone to provide 'fault protection', at first sight that is contradicted by 415.1.2 - which says that "the use of RCDs is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in 411 to 414". However, whatever 415.1.2 was intended to say, as written it seems to be almost meaningless in practice - "one of the protective measures specified in 411 to 414" includes 'basic protection' provided by basic insulation. In other words, whilst 415.1.2 says that an RCD cannot be the 'sole protection', the implication of it (as written) is that an RCD plus basic insulation is not 'sole', and therefore is acceptable. In fact, as written, the only thing it would 'outlaw' would be to use an RCD as the sole means of protection of live exposed parts! At the very least, I still think there is a confusing apparent conflict between 411.4.4 and 415.1.2.

There are also a couple of other related things that I find very confusing. Firstly (and I've never noticed this before), a footnote to Table 41.1 in 411.3.2.2 (disconnection times) says "NOTE: Disconnection is not required for protection against electric shock but may be required for other reasons...". What on earth does that mean? - we are, after all, talking about the chapter on 'Protection against electric shock' (aka 'fault protection'), and disconnection is a pretty useful tool in the protection against electric shock!

Secondly, 411.3.2.6 says that if the required disconnection times can't be achieved, then supplementary bonding is required. It doesn't say anything about why the disconnection times are not met, and says nothing about the requirement for anything other than supplementary bonding in this situation. That seems very odd, and almost 'cancels' the requirements for disconnection times.

I really do think that there are some ongoing confusions!

Kind Regards, John.

 

[stillp]

There are also a couple of other related things that I find very confusing. Firstly (and I've never noticed this before), a footnote to Table 41.1 in 411.3.2.2 (disconnection times) says "NOTE: Disconnection is not required for protection against electric shock but may be required for other reasons...". What on earth does that mean? - we are, after all, talking about the chapter on 'Protection against electric shock' (aka 'fault protection'), and disconnection is a pretty useful tool in the protection against electric shock!

John, the Note is specific to the case where Uo is less than 120 V, in a DC system. I believe that for protection against electric shock, the disconnection times can be many seconds for DC at these voltages.
The 'other reasons' referred to in the Note can include for example the risk of fire, or damage to equipment.

 

[JohnW2]

John, the Note is specific to the case where Uo is less than 120 V, in a DC system. I believe that for protection against electric shock, the disconnection times can be many seconds for DC at these voltages. The 'other reasons' referred to in the Note can include for example the risk of fire, or damage to equipment.

Ah yes. Thanks. If they had used an asterisk or something like that, I would probably have noticed that - but just writing 'NOTE' underneath the Table made me think (without studying the Table itself) that it was just a 'general (seemingly ridiculous) comment'.

However, my main points remain - any views?

Kind Regards, John