Spot The Problem

[mfarrow]

When I wrote of " 'additional protection' in the normal sense", what I was talking about was that, in the normal situation of a Class I fitting/accessory with earthed exposed-conductive parts, a fault between L and that exposed-c-p of too high an impedance to result in operation of an OPD would usually result in an RCD (if there was one!) operating

Right, gotcha. Do we know any statistics for operation of RCDs provided for Additional Protection ?

 

[JohnW2]

Right, gotcha. Do we know any statistics for operation of RCDs provided for Additional Protection ?

I'm not quite sure what you mean, but the answer is probably 'no' - since any decent statistics about such things are few and far between, if/when they exist at all!

If a Class I fitting/accessory with its exposed-c-ps connected to an adequate CPC, and protected by an RCD (as well as OPD), develops a "BS7671 (negligible impedance) L-CPC fault" then, in a TN installation, I imagine that it's probably pot luck as to whether the RCD, OPD or both operate. In a TT installation, one will usually be reliant on the RCD - even if called 'additional' protection, it's likely to actually be the 'only' protection - even in a lighting circuit protected by a B6, it's fairly unlikely that the average domestic earth electrode would result in a low enough Zs for even a B6 to operate.

What sort of statistics did you have in mind?

Kind Regards, John

 

[mfarrow]

What sort of statistics did you have in mind?

Just numbers, how the RCD provided for Additional Protection has tripped. Is it:

- L-E fault, caused by failure of insulation in an otherwise sound circuit (so breaker/fuse operates)
- L-E fault, caused by failure of insulation and fault protection
- L-E fault, not causing RCD to trip but tripping by subsequent event (i.e. touch)
- Other loss of fault protection, e.g. loss of double insulation of cable by rubbing on a pipe.
- Accidental damage to the circuit/appliance, e.g. mower through cable, hammer through socket, where an breaker/fuse operated also.
- For the above, but where no fuse or circuit breaker operated.

These are all statistics which I'm assuming (naively?) have been used to decide on changes to BS 7671, and how/where/why to insist on Additional Protection by applying a 30mA RCD.

 

[knowall]

camera is not focused and obviously, circuit 2 has no rcd protection.

Kind regards,

KA

 

[JohnW2]

What sort of statistics did you have in mind?

Just numbers, how the RCD provided for Additional Protection has tripped. Is it:
- L-E fault, caused by failure of insulation in an otherwise sound circuit (so breaker/fuse operates)
- L-E fault, caused by failure of insulation and fault protection
- L-E fault, not causing RCD to trip but tripping by subsequent event (i.e. touch)
- Other loss of fault protection, e.g. loss of double insulation of cable by rubbing on a pipe.
- Accidental damage to the circuit/appliance, e.g. mower through cable, hammer through socket, where an breaker/fuse operated also.
- For the above, but where no fuse or circuit breaker operated.

As I said/implied, I would be extremely surprised if even poor versions of such statistics exist! Indeed, I doubt that there are any half-reasonable statistics about the even the number of operations of RCDs, let alone the circumstances/reasons!

The RCD operations we tend to hear about (in TN installations) are 'nuisance' ones (often no cause found), those due to wiring errors (wrong neutral bar, borrowed neutrals etc.), those due to faulty appliances and those due to water ingress into outdoor accessories etc. As I said, I've never heard of an RCD tripping as a result of a current though a person. Chopping through mower etc. cables rarely causes an RCD to operate, since cables are usually 2-core.

These are all statistics which I'm assuming (naively?) have been used to decide on changes to BS 7671, and how/where/why to insist on Additional Protection by applying a 30mA RCD.

I fear that you're probably being very naive. I suspect it is much more likely that the decisions about BS7671 were based on gut feelings about things which 'sounded like a good idea'. It's probably on a par with the 'very interesting' number of deaths and serious injuries that the ESC seems to think would be avoided if 100% of UK domestic electrical installations were RCD protected. How often, for example, do you think that an RCD, but not an OPD, would operate as a result of putting a screw or nail through a cable buried <50mm deep in a wall?

I suspect that many of the requirements for RCD protection exist more because of fears of the criticisms/consequences that could result from their not requiring them than from any statistic-based evidence that the requirement will have any significant effect (on injury/death etc.) - but maybe I'm just too cynical!

Kind Regards, John

 

[JohnW2]

.. and obviously, circuit 2 has no rcd protection.

How clever of you [I presume you mean the circuit in position 3] - take a prize.

Kind Regards, John

 

[knowall]

No. as clearly stated circuit 2.

 

[mfarrow]

I fear that you're probably being very naive.

Thanks

I suspect it is much more likely that the decisions about BS7671 were based on gut feelings about things which 'sounded like a good idea'.

I suspect something similar - an identified hazard for which there is a clear and easily applied mitigating measure. I should imagine the hazards were all identified years ago, and they're slowly been eliminated in order of perceived risk.

How often, for example, do you think that an RCD, but not an OPD, would operate as a result of putting a screw or nail through a cable buried <50mm deep in a wall?

I imagine it depends on what cores you hit, and the width of the screw/nail.

 

[Iggifer]

SOME of the metal accessories had CPC's borrowed from the socket radials but by no means all of them.

And apologies, the photo is perfectly clear on my computer, it got resized uploading and didn't like it too much it seems

 

[JohnW2]

I fear that you're probably being very naive.

Thanks

You're welcome . Seriously, though, I was merely replying to the question which you effectively asked when you wrote;

...statistics which I'm assuming (naively?) have been used..

...which I assume meant that you already suspected that it was a naive assumption.

I suspect it is much more likely that the decisions about BS7671 were based on gut feelings about things which 'sounded like a good idea'.

I suspect something similar - an identified hazard for which there is a clear and easily applied mitigating measure. I should imagine the hazards were all identified years ago, and they're slowly been eliminated in order of perceived risk.

Probably, yes. However in the (I strongly suspect) absence of (useful, or any!) decent statistics about either the magnitude of the identified hazard or the the extent to which the 'mitigating measure' would mitigate the hazard, I think it still comes down largely to a 'gut feeling', rather than in any way 'evidence based'. As I said, I suspect that those who create the regulations will have been as mindful of the 'risk' (to them) of not introducing measures as they were of an evidence-based estimate of the probabe magnitude of the benefit.

How often, for example, do you think that an RCD, but not an OPD, would operate as a result of putting a screw or nail through a cable buried <50mm deep in a wall?

I imagine it depends on what cores you hit, and the width of the screw/nail.

Well yes, of course, but I was asking you how often you think it (RCD would operate when an OPD didn't, at the time of cable penetration) actually happens. Remember that, at the time of penetration, if the cores hit and the width of the nail/screw are such that an RCD would operate, so normally should an OPD operate (in a TN installation). The only situation in which and RCD would operate but an OPD would not operate (at the time of penetration) would be if the nail/screw hit L and CPC but, for some reason, the impedance of that fault was not low enough for an OPD to operate - a scenario which I strongly suspect would be extremely rare. That only leaves the scenario in which the nail/screw hits L, but not CPC, and some extremely unlucky person manages to touch it at the same time as touching something earthed. We all know that happened in the tragic case of the MP's daughter - but I wonder whether that might prove to have been the only such case this century!

However, I'm necessarily guessing/speculating, since I am ignorant of any useful statistics (if there are any), so I'm not knocking the requirement for RCD protection of buried cables. What I am suggesting is that I strongly suspect that those who introduced the requirement probably did not have much more evidence than I do that the requirement would prevent a significant amount of serious harm.

Kind Regards, John

 

[securespark]

Board clearly isn't 50+ years old so suspect BG conned the lady into a board change

As far as I am aware, BG came into that market much later than the generation of board suggests.

 

[plugwash]

As I said, I've never heard of an RCD tripping as a result of a current though a person.

Many years ago I was swapping the speaker fuses in an audio amplifier which happened to be fed via a plug in RCD (afaict there was no RCD protection in the fixed insall) and I was too lazy to unplug it first. after replacing the fuse I accidently brushed my arm over the mains input connections, received a shock and tripped the RCD.

The amplifier in question appeared to be class 0 (no earth wire in the mains flex and I don't think it could be considered double insulated given the lack of any insulation or barriers on the mains input connections) but the case may have been indirectly earthed as a result of connections to other equipment.

 

[JohnW2]

As I said, I've never heard of an RCD tripping as a result of a current though a person.

Many years ago I was swapping the speaker fuses in an audio amplifier which happened to be fed via a plug in RCD (afaict there was no RCD protection in the fixed insall) and I was too lazy to unplug it first. after replacing the fuse I accidently brushed my arm over the mains input connections, received a shock and tripped the RCD.

Well, that's one case, then! As I've said, it obviously must happen sometimes, but I've personally never experienced it or heard of it happening in circles I move in. Can you remember whether that was a 30mA RCD?

Kind Regards, John

 

[aesmith]

Shortly after we bought our house the 30A fuse for the cooker circuit blew, and I found that had been caused by mouse damage. There was quite an area of scorching, and I think we were lucky it didn't cause a fire. Surely an RCD would have reduced the damage and risk from that sort of issue, as there's a good chance it would have tripped before the damage was extensive enough to cause a full-blown short.

I suppose saving the mouse from getting blown up would have been a mixed blessing.

 

[JohnW2]

Shortly after we bought our house the 30A fuse for the cooker circuit blew, and I found that had been caused by mouse damage. There was quite an area of scorching, and I think we were lucky it didn't cause a fire. Surely an RCD would have reduced the damage and risk from that sort of issue, as there's a good chance it would have tripped before the damage was extensive enough to cause a full-blown short.

Not impossible but, I would say, pretty unlikely.

If the mouse damage resulted in and L-N short, then an RCD would never have noticed that. Even with an L-E short, it's not all that usual for a 'progressive' short to arise (i.e. one which starts off high impedance {but enough to trip an RCD} and then gradually reduces until it gets low enough to blow a fuse or trip an MCB) - with rodent attack, it's usually either 'all or nothing' (i.e. no short or low impedance short), in which case an RCD would not operate any sooner than would a fuse or MCB.

If the mouse exposed the bare L conductor and then peed onto that and the earth/CPC, then I suppose that might have caused an RCD (had there been one) to operate before a short arose!

Kind Regards, John