Garage supply

[JohnW2]

There's no need for supplementary bonding now though on circuits with RCD protection, you only need to bond extraneous conductive parts to guard against loss of pen.

You're merely indicating what BS7671 currently requires (although main bonding is not 'only' required to deal with the consequences of a lost PEN - apart from anything else, if that were the only reason for main bonding, it would only be required in TN-C-S installations!) - but that does not alter the electrical considerations that I described.

If there is a local L-CPC (or L-exposed-c-p) fault ('of negligible impedance') on a circuit wired with, say, 2.5mm² T+E, then the potential of exposed-c-ps related to that circuit will (until the fault is cleared) rise to roughly 62.5% of the supply voltage (i.e. about 144 V with a 230V supply) above MET potential. In the absence of SB in the location, the potential of exposed-c-ps associated with other circuits, or extraneous-c-ps, will remain at roughly MET potential - so, until the fault is cleared, there would be roughly a 144 V potential difference between some parts within the location - more than enough to kill someone who simultaneously touched 'the wrong two' parts.

That dangerous PD admittedly only persists "until the fault is cleared", and the hope is obviously that the fault would be cleared quickly enough to avoid any death. Although the same remains true in any room/location, BS7671 only addresses this issue in relation to bathrooms and, interesting in terms of the discussion we've been having, it only allows SB to be omitted if the circuits are RCD-protected (which it requires for any circuit supplying a bathroom, anyway!). In other words, it does not seem to regard the MCB/fuse alone as providing a degree of 'fault protection' adequate to justify the omission of SB.

 

[JohnW2]

I suppose it could but that would take some doing.

Sure, but I was merely making the point that an overload current can be very high, so that there is nothing particularly special about the PSCC - both are just 'high currents' that should result in disconnection.

I can only think of - so what?

So .... that's why I have personally never thought of an L-N 'fault' (short-circuit' as being in the same 'category' as an "L-E" one, since no one device will necessarily offer 'fault protection' against both (again, thinking of an RCBO as 'two devices').

 

[EFLImpudence]

Sure, but I was merely making the point that an overload current can be very high,

Are you still not confusing the current drawn by one or more loads rising gradually or in steps - until cleared by the OPD before the current reaches such a high level as PSCC (or PEFC) - and fault current being instantaneously very high - and cleared instantaneously?

so that there is nothing particularly special about the PSCC

No one has suggested there is; just that it is the result of a fault and not an overload.

- both are just 'high currents' that should result in disconnection.

Yes, the electrical current might be the same but the causes are different

 

[trojanhawrs]

If there is a local L-CPC (or L-exposed-c-p) fault ('of negligible impedance') on a circuit wired with, say, 2.5mm² T+E, then the potential of exposed-c-ps related to that circuit will (until the fault is cleared) rise to roughly 62.5% of the supply voltage (i.e. about 144 V with a 230V supply) above MET potential. In the absence of SB in the location, the potential of exposed-c-ps associated with other circuits, or extraneous-c-ps, will remain at roughly MET potential - so, until the fault is cleared, there would be roughly a 144 V potential difference between some parts within the location - more than enough to kill someone who simultaneously touched 'the wrong two' parts.

That dangerous PD admittedly only persists "until the fault is cleared", and the hope is obviously that the fault would be cleared quickly enough to avoid any death. Although the same remains true in any room/location, BS7671 only addresses this issue in relation to bathrooms and, interesting in terms of the discussion we've been having, it only allows SB to be omitted if the circuits are RCD-protected (which it requires for any circuit supplying a bathroom, anyway!). In other words, it does not seem to regard the MCB/fuse alone as providing a degree of 'fault protection' adequate to justify the omission of SB.

I'm sure your maths is on point but to me it's got nothing to do with a dead short situation, otherwise it wouldn't have only been required for special locations e.g. wet places. There won't be much difference in disconnection time between rcds/mcbs at fault currents.

 

[JohnW2]

Are you still not confusing the current drawn by one or more loads rising gradually or in steps - until cleared by the OPD before the current reaches such a high level as PSCC (or PEFC) - and fault current being instantaneously very high - and cleared instantaneously?

I don't think so.

For a start, I would think that the most common situation of (non-contentious) true overload would involve a sudden large step in load which resulted in an overload current appearing essentially 'instantaneously' - e.g. when 'yet another fan heater' was plugged into a sockets circuit or (as often discussed ) if a high-powered vacuum cleaner was plugged into alighting circuit. The resutant current would not be anything like as high as that due to a 'short-circuit', but would nevertheless appear 'instantaneously'.

However, maybe one of the issues is that I'm not sure about your view regarding faults (in an everyday sense) occurring within connected equipment, rather than in the installation. When you and/or I suggest that overload protection is not needed for, say, an oven (because we do not feel it is 'likely that it will result in an overload current'), people like bernard and Sunray come up with arguments about how a fault (everyday sense) in a heating element can (extremely rarely, so you/I would say 'unlikley') result in an overload current. If that (extremely rare) event were to occur, would you regard it as an 'overload current' or a 'fault current'?

No one has suggested there is; just that it is the result of a fault and not an overload.

Yes but, as above, what is a fault and what is an overload? Is your view perhaps that, although it may well have arisen 'instantaneously', the high current due to a bernard/Sunray scenario is 'not high enough' to be a 'fault current'? If so, what if the fault (everyday sense) was extremely close to one end of the element, such that the resultant current was extremely high, the problem ';almost being a short-circuit' - would you still regard that an an 'overload current'?

Yes, the electrical current might be the same but the causes are different

Maybe (dependent on definitions, as above) but one thing is certain - that OPDs will not 'know' the cause, so how can they 'know' whether they are providing fault protection or overload protection when they operate ?

 

[JohnW2]

I'm sure your maths is on point but to me it's got nothing to do with a dead short situation, ...

.... but the "dead short (between L & CPC or L & exposed-c-p) situation" was precisely what my maths was addressing. If the fault had more than 'negligible' impedance, then the touch voltage (prior to clearing of the fault) would be a bit lower than I suggested - but probably still dangerously high ...

otherwise it wouldn't have only been required for special locations e.g. wet places.

You would have to ask the authors of BS7671 why they now require SB (in the absence of RCD protection - which itself would be non-compliant, anyway) only in 'wet places'. I would guess that they probably feel that any given touch voltage (prior to fault clearing) is more of a danger in a 'wet place' than elsewhere. However, that's now. You only have to go back a couple of editions of BS7671 to the days when it seems SB was being installed in all sorts of places, not just 'wet' ones !

There won't be much difference in disconnection time between rcds/mcbs at fault currents.

That's true, but I'm not sure what point you're making.

 

[trojanhawrs]

Think you've misunderstood me, I'll edit my quote to make it clearer.

I'm sure your maths is on point but to me the requirement for supplementary bonding has got nothing to do with a dead short situation, otherwise it wouldn't have only been required for special locations e.g. wet places. There won't be much difference in disconnection time between rcds/mcbs at fault currents.

 

[JohnW2]

Think you've misunderstood me, I'll edit my quote to make it clearer.

Fair enough. Your original was not totally clear and, as you realise, I interpreted it differently from what you had intended., However, as regards your revised version ...

I'm sure your maths is on point but to me the requirement for supplementary bonding has got nothing to do with a dead short situation, otherwise it wouldn't have only been required for special locations e.g. wet places. There won't be much difference in disconnection time between rcds/mcbs at fault currents.

... for a start, as you have said, BS7671 only deals with "dead shorts", so I'm not sure how anything it requires (in the context of faults) can "have nothing to do with the dead-short situation"

As for speculation as to why (now) SB is only required in 'wet places', I offered my thoughts about that in my previous post.

As I've said, I do find it rather odd that 522.6.202 seems to imply that, in a special location, it is 'OK' to have no RCD protection if there is SB, despite the fact that the absence of RCD protection in such a location would itself be a violation of 701.411.3.3 !

 

[trojanhawrs]

You're misquoting me.

... for a start, as you have said, BS7671 only deals with "dead shorts", so I'm not sure how anything it requires (in the context of faults) can "have nothing to do with the dead-short situation"

That's not what I said, what I said was fault protection is basically in consideration of dead shorts and as we've already established, RCDs are not typically considered fault protection, they're additional protection (which is pretty much synonymous with protection against electric shock). If everything was a dead short RCDs would be fully redundant.

Ultimately I suppose it doesn't matter too much though as long as you know what you need to be compliant.

 

[JohnW2]

You're misquoting me. .... That's not what I said, what I said was fault protection is basically in consideration of dead shorts

I was paraphrasing, not "quoting", but what I wrote, what you now write and what you originally wrote all seem remarkably similar to me.

In fact, I went a little further than you, by pointing out that, in general, BS7671 "only deals with what are essentially 'short-circuit' faults", since its definition of (any) 'fault' includes a note which says .... "NOTE: Conventionally, the impedance between live conductors or between live conductors and exposed- or extraneous-conductive-parts at the fault position is considered negligible."

and as we've already established, RCDs are not typically considered fault protection, they're additional protection

I think this is really just playing with words. Since RCDs clearly cannot provide 'additional overload protection' or 'additional short-circuit protection', the only 'additional protection' they can (and do) provide is 'additional fault protection', even if you don't consider them to provide fault protection!

(which is pretty much synonymous with protection against electric shock).

That's what many/most people seem to think - in fact, not that their main value is not to actually prevent electric shocks but, rather (I think mistakenly) that their main value is in rendering electric shocks non-fatal. However, as I've said,they actually do prevent some electric shocks by clearing faults that an OPD would not clear (as quickly, or at all) - before anyone receives a shock.

However, as I think we've discussed, they also offer protection against fire in some circumstances. Indeed, the protection of distribution circuits provided by the up-front RCDs in my TT installation has little, if anything, to do with electric shocks - at least not until (if ever!) the CUs they feed become compliant with the silly requirement for them to have metal enclosures

If everything was a dead short RCDs would be fully redundant.

Only in a TN installation. In a TT installation an RCD is the only thing which can give protection against an ("L-E") fault, even if it is 'of negligible impedance' (i.e. 'a dead short'), since the fault current will never be high enough to cause an OPD to operate.

Ultimately I suppose it doesn't matter too much though as long as you know what you need to be compliant.

Indeed - but as I think I've illustrated by my comments about supplementary bonding, mere compliance with the ('minimum') requirements of BS7671 does not necessarily make an installation as 'safe' as some people might like/want it to be.

 

[ebee]

I think that some regard safety devices such as fuses, MCBs and RCDs with high esteem if designed and installed in accordance with BS7671 and OK I am prepared to go along with that so long as we only view such safety as being relatively safe and not as absolutely intrinsically safe.
There is nothing wrong with feeling safer if such are in use and if more than one such device is affording that result then I will go along with the more the merrier concept too.

What bothers me is the everyday examples we witness of people pushing the boundaries of "standard good practice" because of over reliance of these devices to save the day for us, we actually reach a position where things are less safe than perceived , in fact often they may be, in practice, less safe than they were years ago.
That kind of danger is potentially present not just in electrics but in lots of other things in lifetimes , cars being a good example of this . The Uk driving test is the most difficult its ever been yet we have more drivers than ever before, percentage wise more idiots than ever before and the intense desire to save one hundredth of a second at every turn and ignoring the risks of killing others evermore rendering each car journey a real hazard to life. Less police around, even just to give you that dirty look when you start to transgress does not help the matter. what the heck that does to your blood pressure and other health issues is beyond me.

When i mentioned that one day i might be persuaded AFDD a bit differently to the way I view them now then mainly I am thinking they might technologically improve over the years and , to a lesser degree, we might have more data on them. I am extremely doubtful I will change my mind, but never say never

 

[trojanhawrs]

I think this is really just playing with words. Since RCDs clearly cannot provide 'additional overload protection' or 'additional short-circuit protection', the only 'additional protection' they can (and do) provide is 'additional fault protection', even if you don't consider them to provide fault protection!

It's not me playing with words, that's the definitions they give. They don't consider high impedance faults to be "faults" in the strictest sense, but acknowledge their existence by the requirement for RCDs.

What bothers me is the everyday examples we witness of people pushing the boundaries of "standard good practice" because of over reliance of these devices to save the day for us, we actually reach a position where things are less safe than perceived , in fact often they may be, in practice, less safe than they were years ago.

This is probably another reason they like to keep RCDs in a separate category where possible.

 

[EFLImpudence]

I think this is really just playing with words.

Yes, so stop doing it.

Since RCDs clearly cannot provide 'additional overload protection' or 'additional short-circuit protection', the only 'additional protection' they can (and do) provide is 'additional fault protection', even if you don't consider them to provide fault protection!

No. It is just 'additional'. That is - extra.

 

[JohnW2]

What bothers me is the everyday examples we witness of people pushing the boundaries of "standard good practice" because of over reliance of these devices to save the day for us, we actually reach a position where things are less safe than perceived , in fact often they may be, in practice, less safe than they were years ago.

I'm inclined to agree, primarily because of the complacency which has arisen as a result of the changes over time. Today, because of all the legislation/regulation and the 'technological advances', most people seem to assume that electrical installations and any electrical/electronic products are intrinsically 'safe'. In contrast, a few decades ago (when I was a youngster) anything to do with electricity was regarded as much more potentially 'dangerous', such that anything to do with electricity tended to be treated with much more respect/caution.

When i mentioned that one day i might be persuaded AFDD a bit differently to the way I view them now then mainly I am thinking they might technologically improve over the years and , to a lesser degree, we might have more data on them. I am extremely doubtful I will change my mind, but never say never

As I said, I'm also extremely doubtful. I would need to see a lot of hard data to convince me that (at least in a domestic context) they represent a 'solution' to a significant problem which requires a solution - and I strongly suspect that such data will prove impossible to collect. In the meantime, we will undoubtedly continue to see the appearance of ever more ;'technological advances', essentially on the basis that they have become ';technologically possible', which is certainly not necessarily the same as ;'needed'.

Kind Regards, John

 

[JohnW2]

It's not me playing with words, that's the definitions they give.

I didn't say that it was you who was playing with words - merely that they are being played with. More specifically, as below, the issue appears to relate to a note attached to the BS7671 definition of "a fault".

They don't consider high impedance faults to be "faults" in the strictest sense, but acknowledge their existence by the requirement for RCDs.

Albeit essentially semantic/terminological, and of little actual consequence, I have been very surprised by this discussion. Until a couple of days ago, it would not have occurred to me to regard RCDs as providing anything other than "additional fault protection' (not the least because, as I said, they certainly can't provide 'additional overload protection' or 'additional short-circuit protection').

It really seems to all come down to BS7671's definition of a 'fault', and I suspect that the present definition exists for pragmatic historical reasons, and probably could do with being updated.

In fact, an 'unintended current path'; of non-negligible impedance does satisfy the actual BS7671 definition, per se, of a 'fault' - it is only a 'note' attached to that definition which says that 'conventionally' a fault is considered to be of negligible impedance.

I suspect that, historically, that 'convention' was pragmatic, given that in the pre-RCD days, there was nothing that could be done about faults which were not of negligible impedance. By assuming a negligible (actually zero) fault impedance, one was able to design a circuit that would give the required disconnection times in the case of a (zero impedance) fault, but would not achieve that in the face of faults of finite impedance. Without RCDs, it would have been impractical to design circuits such that the required disconnection times wouyld be achieved with non-negligible fault impedances.

However, as you say, now that RCD protection is required for almost all circuits, BS7671 is implicitly acknowledging that 'unintended current paths' of more than negligible impedance do exist, and now requires that there be (RCD) protection against such 'faults'.

I would therefore suggest that, in my humble opinion, the time has probably come when it would be sensible to consider removing that 'note' from the definition of a "fault". Circuit design would undoubtedly continue to assume negligible (actually zero) fault impedance for ensuring that ('primary') OPD-mediated ADS would provide the required disconnection times when the fault impedance really was zero, but the terminology would become more consistent in accepting that RCDs were also required to provide 'fault protection' when the impedance of the 'unintended path' was greater than zero.