Garage supply

[ebee]

If I am still around in 10 to 20 years time then I might be persuaded tgat AFDDs are a good idea. I, already, do think that surge protection devices might have their merits

 

[trojanhawrs]

You appear to be rather contradicting yourself, given that you previously wrote ...

.. and, in the absence of RCD protection, it is indeed true that the MCB or fuse is the only thing providing any fault protection. As you go on to say, as far as far as BS7671 is concerned, in TN installations RCDs 'only' provide 'additional protection'. To be 'additional', it obviously has to be additional to something else (i.e. the OPD-based ADS).

I'm not sure where the contradiction is.
Yes, it's in addition to overload and fault protection, which is provided by the MCB. A RCD cannot satisfy the requirements for fault protection by itself because it wouldn't trip in a short to neutral. In a TT the RCD would also be classed as fault protection (and potentially additional protection) along with the MCB.

Very true, but it's not Basic Protection which we are talking about.

Well we seemed to be disagreeing on the difference between a fault and an electric shock.

Yes, but coming into contact with" and "having a negligible impedance path to" are, in the real world, not the same thing. You surely must have looked at a (correctly 'zeroed') resistance meter whilst 'touching together' two conductors. One often has to fiddle about quite a lot to get the reading anywhere near zero.

Don't forget that final circuits often have a Zs very close to the maximum that will guarantee ADS with the required disconnection time (e.g. magnetic tripping of an MCB). That means that the impedance of a fault will sometimes have to be only a small fraction of an Ohm to prevent the desired ADS performance. When a 'fault' (even if not such by BS7671 definition ) develops as a result of, say, deterioration/breakdown of insulation or water ingress, the impedance of the path may be of very appreciable impedance - quite possibly still very dangerous but far to high for effective OPD-based ADS. RCDs are obviously particularly valuable in the case of gradually developing (progressive) 'L-E leakages', which they will "stop in the bud".

Yeah, anecdotally in my job RCD protection is pretty scarce and if you open an old panel in a MCC there's an extremely high chance the breaker feeding the anti con heater will be sitting tripped due to failure of the dielectric. Ive tripped more mcbs and blown more fuses than I care to admit messing around inside live equipment. Also anecdotally, I would say it's easier to blow a fuse on a 110 circuit than a 24 which may have something to do with the voltage being able to jump a bit more the higher it is (your meter will be testing with a fairly low voltage). That might be nonsense though, it's obviously a bit more memorable when it goes with a bang.
I'm not knocking RCDs, they're obviously good at what they do. They can be detrimental in an environment where there is little chance of electric shock and some earth leakage is acceptable.

 

[JohnW2]

True we always base OPD short circuit and earth fault protection to be with "bolted faults" (negliable impedance) where we know that real world we might well be faced with lightly touching rusty parts of higher impedance and this RCD action might actually be the device that is saving us from considerable grief.

Exactly and, although I may be wrong, I do strongly suspect that people tend to very much over-estimate the proportion of faults which really are 'of negligible impedance'. As I said, anyone (most of us?) who have played with a resistance meter when 'touching conductors together' ought to realise that.

To what extent this 'matters' is a different question. If the EFLI (including non-negligible impedance of the actual fault) is a bit higher than what we regard as the 'maximum', OPD-based ADS will still 'work', but not quite as quickly (in the case of an MCB, relying on thermal, rather than magnetic, operation), hence disconnection times greater than those 'required' by BS7671. However, given that those 'required disconnection times' are essentially arbitrary, I'm not sure this would be a matter for much concern.

A 30mA RCD is assumed to theorectically be capable of saving 95% of those who might not be saved otherwise.t ... Translate that as a 5% failure to protect and that encourages to be safety conscious in all other respects too.

As I always say, the scope for RCDs to reduce the incidence of death in those who do receive electric shocks has always been very limited, since there were so relatively few domestic electrocutions in the UK even before we had RCDs, and I don't really think this is the 'value' of RCDs that we should concentrate upon (although many do) - to my mind, the real benefit is that RCDs may clear faults that OPD-based ADS will not (as quickly, or ever) before anyone has a chance to get a shock.

As often discussed, it's essentially impossible to get any handle on how many lives RCDs may have saved (and whether, in impassionate terms, the answer 'justifies' the vast amounts that have been spent on RCDs). As regards 'direct life saving' (RCD operating when someone gets a shock), as I've often reported, for a good few years I have taken every opportunity to ask groups of people for reports on incidents in which someone has suffered, and survived, an electric shock which has caused an RCD to operate, and so far have only become aware of a couple of cases - and, even in them, we cannot know whether or not the victims would have survived in the absence of the RCD. Similarly, nor can we know of any cases in which people have died as a result of an electric shock despite an RCD operation. As for 'clearing faults before anyone gets a shock', it is again essentially impossible to get any statistics about that.

In my (limited) experience then RCD failures of two types, the most common failure being failure to act within spec ... (Perhaps stiction? Being the chief contributor) . The other, rarer type I M H O, is complete failure to operate at all. .... Often we are only aware of The most common type because of testing with test gear and ,worryingly, we almost never become aware of any MCB failures outside spec that are not complete failures to operate at all. We take assumptons from type testing alone.. similarly with the humble fuse..

Indeed. As I've said, even if we don't have redundancy, at least we can go some way to testing in-service RCDs (even the householders can {should} to some extent), whereas it is essentially impossible to test MCBs- although I presume that they are not immune from failure (total or partial).

Kind Regards, John

 

[JohnW2]

If I am still around in 10 to 20 years time then I might be persuaded tgat AFDDs are a good idea.

Maybe, but as far as I am concerned, (and although I may be wrong) I rather doubt that I would ever be persuaded!

I, already, do think that surge protection devices might have their merits

Well, that 'merit'; has to be in the eye of the beholder, but that is fair enough so long as they understand that we're talking about protection of equipment, not people, and still decide that they would like SPDs.

As I always say, over the decades I have suffered so few premature failures of electronic equipment (of which I've had a lot) which might conceivably (but probably not!) be attributable to 'surges'/'spikes' that I would not regard it as a sort of 'protection' that I would personally want/need.

Kind Regards, John

 

[JohnW2]

I'm not sure where the contradiction is.

I don't think it particularly important, since I essentially agree with most individual statements you've made, but ...

A MCB or fuse is overload and fault protection,

... and ...

Yes, I realise that's what BS7671 'classes' them as, but that does not alter what I said - namely that they potentially offer much more protection against electric shock than an MCB or fuse ever could.

Absolutely, but that's not what the fuse is there for.

... seem to be saying opposite things as regards whether the fuse (or MCB) is there to provide fault protection. Asyiu say in the first of those statement,in a TN installation the fuse/MCB obviously is there to provide (amongst other things) fault protection.

A RCD cannot satisfy the requirements for fault protection by itself because it wouldn't trip in a short to neutral.

You're talking there about 'short-circuit protection, which is not usually regarded as 'fault protection' - but, yes, of course, an RCD does not provide short-circuit protection, just as it doesn't provide overload protection.

Maybe we should be talking about RCBOs, which can do everything an MCB can do, and more ?

Well we seemed to be disagreeing on the difference between a fault and an electric shock.

I don't really understand that, since faults and electric shocks are clearly two totally different things. An electric shock may be the consequence of (amongst other things) failure of Basic Protection or an ("L-E") fault.

I'm not knocking RCDs, they're obviously good at what they do. They can be detrimental in an environment where there is little chance of electric shock and some earth leakage is acceptable.

... and nor am I intending to promote or advocate RCDs. On the contrary (and as I have recently mentioned) those who have been here for a long time are well aware of the fact that I frequently question ('impassionately') the wisdom of RCDs, since I very much suspect that far more 'lives would have been saved' if the billions spent on buying and installing RCDs in the UK had instead been directed to something else (e.g.road safety, healthcare, medical research etc.).

 

[trojanhawrs]

You're talking there about 'short-circuit protection, which is not usually regarded as 'fault protection' - but, yes, of course, an RCD does not provide short-circuit protection, just as it doesn't provide overload protection.

That's where you're wrong, it is. And that's why RCDs are not usually regarded as fault protection.

 

[JohnW2]

That's where you're wrong, it is. And that's why RCDs are not usually regarded as fault protection.

Interesting. That's contrary to what I've always understood, and also contrary to what eBee wrote when he started this tangential discussion, namely that there are three types of relevant protection ...

1... Overload protection
2... Short-circuit protection
3... Fault protection.

You appear to be saying that (2) is part of (3), which is something I've never previously seen suggested.

As I've observed, BS7671 seems to say almost nothing about short-circuit protection, I imagine because it regards it as simply being an extreme case of 'overload'

 

[EFLImpudence]

PFC is the greater of PEFC and PSCC - so...

 

[JohnW2]

PFC is the greater of PEFC and PSCC - so...

That's true, so again 'interesting', but I must say that I don't think I've ever seen it suggested that L-N short-circuits are addressed by 'fault protection'.

If nothing else, the existence of RCDs makes that seem sensible, particularly in a TT installation. In such an installation, I think we are agreed that the only 'fault protection' is provided by RCDs, but they clearly offer no protection against (L-N) short circuits, that protection being provided by the OPD. Even in a TN installation, the "additional ('fault') protection" offers no 'additional protection' against (L-N) short-circuits.

Do you not agree with eBee in regarding 'short-circuit protection' and 'fault protection' as being two separate things (which cannot necessarily be provided by the same device)?

 

[EFLImpudence]

That's true, so again 'interesting', but I must say that I don't think I've ever seen it suggested that L-N short-circuits are addressed by 'fault protection'.

Well, by definition, they cannot be overloads.
"Overload current. An overcurrent occurring in a circuit which is electrically sound."


Does that make the rest of your post invalid?

If nothing else, the existence of RCDs makes that seem sensible, particularly in a TT installation. In such an installation, I think we are agreed that the only 'fault protection' is provided by RCDs, but they clearly offer no protection against (L-N) short circuits, that protection being provided by the OPD. Even in a TN installation, the "additional ('fault') protection" offers no 'additional protection' against (L-N) short-circuits.

Do you not agree with eBee in regarding 'short-circuit protection' and 'fault protection' as being two separate things (which cannot necessarily be provided by the same device)?

 

[trojanhawrs]

I think we are agreed that the only 'fault protection' is provided by RCDs, but they clearly offer no protection against (L-N) short circuits, that protection being provided by the OPD. Even in a TN installation, the "additional ('fault') protection" offers no 'additional protection' against (L-N) short-circuits.

Do you not agree with eBee in regarding 'short-circuit protection' and 'fault protection' as being two separate things (which cannot necessarily be provided by the same device)?

My understanding is short circuit protection is a part of fault protection. Fault protection requirements can be met by a MCB in most TN systems, in a TT it needs both a MCB and RCD. Equipotential bonding is another requirement of fault protection.

 

[JohnW2]

Well, by definition, they cannot be overloads.

That's true per BS7671 definition. However, electrically-speaking it's only a matter of degree. In fact, there is theoretically no limit to how high an overload current (per BS7671 definition) can be, so it could approach the PSCC.

Does that make the rest of your post invalid?

I don't think so, since I think that everything I wrote was true.

I think the problem is probably that BS7671 is using one word ('fault') to refer to two different things which, in some circumstances, will require two different forms of protection .... for example, in a TT installation, protection against L-E faults can only be ;provided by an RCD, whereas protection against L-N faults (short-circuits) can only be provided by an OPD. Hence, in that situation, no one device (other than an RCBO, which is effectively 'two devices') can protect against both types of 'fault'.

 

[JohnW2]

My understanding is short circuit protection is a part of fault protection. Fault protection requirements can be met by a MCB in most TN systems, in a TT it needs both a MCB and RCD.

I obviously agree with what you're saying, the only issue being about the terminology - which, as I've said, has taken me somewhat by surprise..

Equipotential bonding is another requirement of fault protection.

Yes, it's certainly part of protection against the possible consequences of one of the types ("L-E") of fault (but will obviouskly offer no protection against L-N faults/'short-circuits').

However, in the case of a room/area which has exposed-c-ps relating to more than one final circuit, and/or which has extraneous-c-ps, it's impossible for the room/area to be an equipotential zone unless there is Supplementary Bonding within the room/area. In the event of a local L-E fault on one circuit, the potential of any exposed-c-ps related to that circuit will (until the fault is cleared) rise to considerably above MET potential, whereas exposed-c-ps related to any other ('intact') circuits and/or any extraneous-c-ps will remain at roughly MET potential

 

[EFLImpudence]

That's true per BS7671 definition. However, electrically-speaking it's only a matter of degree. In fact, there is theoretically no limit to how high an overload current (per BS7671 definition) can be, so it could approach the PSCC.

I suppose it could but that would take some doing.

I don't think so, since I think that everything I wrote was true.
I think the problem is probably that BS7671 is using one word ('fault') to refer to two different things which, in some circumstances, will require two different forms of protection .... for example, in a TT installation, protection against L-E faults can only be ;provided by an RCD, whereas protection against L-N faults (short-circuits) can only be provided by an OPD. Hence, in that situation, no one device (other than an RCBO, which is effectively 'two devices') can protect against both types of 'fault'.

I can only think of - so what?

 

[trojanhawrs]

However, in the case of a room/area which has exposed-c-ps relating to more than one final circuit, and/or which has extraneous-c-ps, it's impossible for the room/area to be an equipotential zone unless there is Supplementary Bonding within the room/area. In the event of a local L-E fault on one circuit, the potential of any exposed-c-ps related to that circuit will (until the fault is cleared) rise to considerably above MET potential, whereas exposed-c-ps related to any other ('intact') circuits and/or any extraneous-c-ps will remain at roughly MET potential

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.