Cost-effectiveness of RCDs,SPDs, AFDDs etc., in perspective

[JohnW2]

In another thread (in a forum in which I choose not to participate) ....

... then we might as well completely abolish all regulation/regulations relating to electrical work (and probably also gas work and building in general), and get rid of RCDs (let lone SPDs and AFDDs!), since the amount of reduction in morbidity and mortality which that regulation achieves undoubtedly fades into total insignificance in comparison with what we seem to be 'accepting' as a result of Covid!

It could be said that the regulations for RCDs have been (and probably SPDs and AFDDs will be) a waste of time and money as there are no fewer deaths now than without them.

Interesting thought, but it's not going to happen - there are few, if any, examples of the relentless forward march of 'Nanny' ever having being reversed - on the contrary things keep moving progressively more in the same direction.

In terms of 'big pictures' (comparisons with other 'risks'), I would have thought that it would have been 'obvious' that RCDs could not possibly have any appreciable impact on 'saving lives', given that there were so few domestic electrocutions even pre-RCDs, far from all of which could/would have been prevented by an RCD, anyway.

As for SPDs and AFDDs, I really don't fully understand what people think they are going to 'save' (I would say certainly not an appreciable number of lives)!

Perhaps a closer comparison with the covid restrictions than might at first be thought.

... except that, as above, the "waste of [vast amounts of] time and money" on RCDs etc. is almost certainly never going to go away (probably will 'get worse' as new 'technological possibilities' show their faces). So, if 'we' feel that such a vast and ongoing cost (in all senses) is justified in the name of perhaps saving a tiny number of lives, then that brings me back to 'wondering' about 'our' attitude to Covid.

Kind Regards, John

 

[ericmark]

As you say death by electrocution in domestic is rare, and injuries don't tend to be put in the news. So death by electrocution reported only one which comes to mind is Emma Shaw, and she would have been saved had there been RCD protection.

But if we look at loss of PEN this it seems is reported and we are told

According to HSE statistics (table 1), each year there are around 400 reported incidents of broken Protective Earth and Neutral (PEN) conductors on TN-C-S (PME) earthed electrical installations with around 10% of these causing an electric shock.

so it seems 40 electric shocks per year, of them how many are fatal?

So at the moment we still have TN-C-S supplies. So it seems some deaths per year are considered within limits.

The AFDD tested with John Ward did seems to raise questions as to one if they will trip, and two will they trip in error. The same problem with a RCD, it tripping in error can result with loss of food in fridge, or danger from trips, but in both cases seems to be a lack of data on how many times injury has resulted due to RCD's tripping. I am sure insurance companies could say how often there is a claim for loss of food.

I was surprised when my freezer failed, what I had not considered was if it failed just after a defrost cycle how goods in the top could defrost, but goods lower than the vents in the back could remain frozen solid. Since freezer motor failed it was transshipped to another freezer so I was aware. However with a RCD tripped would one even know food had defrosted and frozen again? So any food poisoning would not likely be attributed to RCD failure.

On the other hand we have no idea of how many fires they have prevented.

Personally I would not fit AFDD but have fitted RCD, however the RCD did not stop my getting a shock when I put a hacksaw blade through a horizontal cable which was outside the safe zones.

The SPD is completely different, unlike the AFDD and RCD I can't see any case where fitting them could cause danger or loss of supply. So can see good reason for lights and freezer not to have AFDD and RCD, but not SPD.

I would agree case not proved, I have lost very few electronic devices, I have an under ground supply, and the transformer does not supply any industrial premises, as to if a SPD can stop LED lights from being taken out with a surge or spike is guess work.

So, if 'we' feel that such a vast and ongoing cost (in all senses) is justified in the name of perhaps saving a tiny number of lives, then that brings me back to 'wondering' about 'our' attitude to Covid.

Same can be said about returning to a man with red flag walking in front of road vehicles. Our local railway is limited to 15 MPH but is a direct route to Welshpool, the work about to start on the A458 on the Golfa bank means for once in a long time the railway will be faster than the bus, but the railway is seen as a tourist attraction, not public transport, so people have to book a bubble, i.e. group of 4 seats, each bubble is segregated, so a train able to carry 200 plus passengers is limited to around 50, but there are no unused seats on the bus. If the bus worked on same rules as railway it could carry around 6 passengers.

I went on holiday around November to Cornwall, I was led to understand because of Colvid we would not have the room cleaned daily, and it would be waiter service, however we did not get rooms cleaned and waiter service had been stopped a few days before we arrived. Seems English rules different to Welsh, in England bring a bottle parties are OK if you claim for business.

Rules for them and us it seems in England.

However there is a big difference between recommended and law. And I see no law to say we must have RCD, or SPD, they may be recommended, it says

A way of satisfying the fundamental principles would be to follow:
(a) the technical rules described in the body of BS 7671:2001 as amended or in
an equivalent standard approved by a member of the EEA; and
(b) guidance given in installation manuals that are consistent with BS
7671:2001, such as:
(i) the IEE (Institution of Electrical Engineers) On-Site Guide;
(ii) the series of IEE Publications, Guidance Notes Nos 1 to 7.

And my copy of BS 7671:2001 does not require RCD protection for all, so although I have fitted it, nothing says I must.

 

[JohnW2]

As you say death by electrocution in domestic is rare, and injuries don't tend to be put in the news. So death by electrocution reported only one which comes to mind is Emma Shaw, and she would have been saved had there been RCD protection.

Indeed - and, as you say, the fact that you have to look back over 14 years to find a case to cite lends weight to the fact that it's a very rare event.

But if we look at loss of PEN this it seems is reported and we are told "...each year there are around 400 reported incidents of broken Protective Earth and Neutral (PEN) conductors on TN-C-S (PME) earthed electrical installations with around 10% of these causing an electric shock..." .... electric shocks per year, of them how many are fatal?

I must be missing something here, because, on the face of it, I can't seen why 'lost PENs' on TN-C-S supplies should, per se, result in electric shocks. Provided that adequate required bonding is in place, there should be no risk of a significant shock within the building - and I can't believe that many people would be, say, outside and touching an 'earthed' outdoor tap at the very time that there was a lost PEN'! What am I missing?

In any event, I would imagine that, even with inadequate bonding, any such shocks would usually be of relatively low voltage, so maybe many/most of the claimed shocks are non-fatal?

As you will be aware, useful data on non-fatal electrical shocks is almost non-existent. Even statistics on serious non-fatal shocks are very poor, since many are documented (e.g. in 'Hospital Activity Analysis') as 'burns', without any indication of the cause of the burn.

have fitted RCD, however the RCD did not stop my getting a shock when I put a hacksaw blade through a horizontal cable ...

It goes without saying that no protective device could prevent you getting a shock under those circumstances. The only way they can prevent shocks is if they operate, due to the emergence of a fault, before anyone has had a chance to get a shock.

Kind Regards, John

 

[plugwash]

Indeed - and, as you say, the fact that you have to look back over 14 years to find a case to cite lends weight to the fact that it's a very rare event.
I must be missing something here, because, on the face of it, I can't seen why 'lost PENs' on TN-C-S supplies should, per se, result in electric shocks. Provided that adequate required bonding is in place,

I suspect there are a non-negligible number of houses where adequate bonding is not in place

And what is adequate bonding anyway? damp masonry can certainly conduct enough electricity to give someone a shock but we don't typically bond floors in this country.

 

[bernardgreen]

'lost PENs' on TN-C-S

What am I missing?

The "Earthed" back boxes pulling surrounding material ( damp masonry ) up to a dangerous potential above true Ground which the bottom of the wall likely to be.

A voltage gradient across a damp wall is possible

damp masonry can certainly conduct enough electricity to give someone a shock but we don't typically bond floors in this country

 

[ericmark]

I must be missing something here, because, on the face of it, I can't seen why 'lost PENs' on TN-C-S supplies should, per se, result in electric shocks. Provided that adequate required bonding is in place, there should be no risk of a significant shock within the building - and I can't believe that many people would be, say, outside and touching an 'earthed' outdoor tap at the very time that there was a lost PEN'! What am I missing?

With a PME as the letters suggest you have multiple earth rods, so if a section of 10 homes lose their PEN there should be earth rods which mean the amount the supply earth and the real earth voltage varies should be less than 50 volt. But TN-C-S is not PME, even if PME is TN-C-S, there is nothing to say how many earth rods with a TN-C-S and also no current limit to any true earth other than the fuses in the transformer, as my friend found enough to melt his earth wires to his shack.

So there are two dangers, shock and fire. In the main loss of PEN seems to result in fire, where the gas supply has been earthed wrong side of electrical isolation and it has melted the gas pipe.

The thing is the reporting seems flawed

The theft of copper cabling may have been responsible for a large gas explosion that ripped through a row of houses in Castleford.
Around 30 people were forced to leave their homes on Rhodes Street, Hightown, in the early hours after the incident in which two fire officers suffered minor injuries.

more here it blames the fire on thieves, and one has to ask where were the multiple earths that thieves can steal them all?

I was dismayed with my parents house to find no earth. Now as a lad I know I made an error and the fuse ruptured when I allowed line to touch earth. So there was without any question a good earth to the house.

Some 30 years latter I find the electrician fitting a consumer unit has used the party line earth wire from the telephone to earth the whole house, around 3 mm² bare copper going around the outside of the house, and my hunt for an earth rod failed to find any earth rod or cables going to an earth rod other than the old GPO one which was put in for the party line.

The ELI was around the 200Ω mark the steel rod was clearly not good enough for a supply earth. Home was made TN-C-S but the question was where did the old earth go? House built 1954.

There were multi faults with that house, and I breathed easy once required, I could not find an earth fault, but a RCD would not hold in, I had gone around the house many times trying to find the fault, but it seems when built my dad was very proud of the new electric system, he had 8 sockets, where my grand fathers house only had two, by time I came to fault find there were around 50 sockets where my dad had spured off spurs off spurs. Since original was 7/0.029 which was thicker that 2.5 mm² only overload damage found was new PVC cables.

But it was the failed attempt to fit a RCD which flagged up the faults. I thought my dad knew what he was doing, he was the technical power station superintendent before he retired, and I had got the don't try teaching grandmother to suck eggs so many times, I never looked at what he had done until he died.

Had the house had an EICR once every 10 years I am sure all faults would have been found, at a time when my dad could have remembered when he did things, but by the time I came to look, I did not want to disturb the rubber cables. It was clear a rewire was well over due. Which was done in the end after dad died.

But for the RCD in the main they high light problems before anyone gets a shock, in this house leaking roof caused water into back of socket, and on replacing the old socket was clearly warped due to heat inside the socket one assumes, so the RCD likely stopped a fire. But I have not filled a near miss report, so only I and now anyone reading this, knows the RCBO likely stopped a fire, and of course it may have blown a fuse before it went on fire, who knows?

So the RCD well worth while.
The AFDD likely not required.
The SPD who knows.
What about loss of PEN detection?
And also types of RCD?

 

[JohnW2]

I suspect there are a non-negligible number of houses where adequate bonding is not in place

I'm sure you're right but
(a) there's a limit to how far one can/should consider installations which are 'non-compliant',
(b) in many (quite possibly most) cases and situations, absence of main bonding probably would not make much difference. Metal pipework (and hence most relevant touchable metal things) will commonly be connected, directly or indirectly, to the MET through multiple CPCs ('in parallel') the combined CSA of which will often be comparable with (if not greater than) the CSA of 'compliant' bonding conductors.

And what is adequate bonding anyway? damp masonry can certainly conduct enough electricity to give someone a shock but we don't typically bond floors in this country.

The "Earthed" back boxes pulling surrounding material ( damp masonry ) up to a dangerous potential above true Ground which the bottom of the wall likely to be. A voltage gradient across a damp wall is possible

All true, but I'm not at all sure how common it is for damp/wet masonry to provide a sufficiently low impedance path to earth to result in a 'serious' electric shock under any circumstances.

It's also worth remembering that, even in the absence of any bonding (explicit or 'incidental') and the presence of soaking wet walls/floors, a shock will only occur in the (presumably very unlikely) that a person simultaneously touches two relevant things (or one thing whilst standing on a soaking wet floor with bare feet) at the very time that there is a 'lost PEN' - which I would think would be an extremely improbable co-incidence.

Anyway, can we perhaps agree that in a house in which there is 'adequate' (per regs) bonding, and no soaking wet walls/floors, that loss of a PEN will not result in significant electric shocks? Getting back to how this started, can you believe that such a (seemingly improbable) co-incidence results in electric shocks in 10% of all cases of "lost PENS"?

Kind Regards, John

 

[JohnW2]

With a PME as the letters suggest you have multiple earth rods, so if a section of 10 homes lose their PEN there should be earth rods which mean the amount the supply earth and the real earth voltage varies should be less than 50 volt.

As you may recall, the late-lamented westie often told us that it was common for the "M" or PME supplies to only be 2 - with one rod at the transformer and the other at the end of the cable run.

Kind Regards, John

 

[bernardgreen]

can you believe that such a (seemingly improbable) co-incidence results in electric shocks in 10% of all cases of "lost PENS"?

A gas meter reader told me the reason he wore gloves was the (seemingly improbable) risk of a serious shock when touching an external meter having had a few tingles over the years.

And personal experience of significant differences in potential between "Earth" derived from the Neutral and Earth derived from a Ground rod. When the feeder cable faulted the Neutral potential was bouncing for several days until the cable was repaired.

 

[JohnW2]

A gas meter reader told me the reason he wore gloves was the (seemingly improbable) risk of a serious shock when touching an external meter having had a few tingles over the years.

My suspicion is that, if the "10%" figures (percentage of 'lost PEN' incidents that resulted in 'electric shocks') is remotely correct, it may well relate to 'tingles', rather than anything approaching a 'serious shock'.

In any event, touching 'external meter', whilst outside of the property, is obviously a different matter. For someone inside the property, in addition to the other (improbable) possibilities we have discussed, it's necessary for a 'victim' to be in simultaneous contact with two relevant things at the very time when there is a 'lost PEN' - which, again, would seem very improbable. Off the top of my head, I really can't think of when I was last in simultaneous contact with two conductive things which, under rare circumstances, could possibly be 'a problem'!

Kind Regards, John

 

[ericmark]

In the main, items used outside, so one can touch true earth and appliance earth at same time, are normally class II. There are exceptions, patio heater for example, and one does question if out door equipment should be class I or should the rules say class II only, specially for domestic.

So the canal boat is limited by where the canal goes, so the only items connected for an extended time are caravans, motor homes and electric cars.

I really can't work out why the electric car is not class II? But neither can I see why rules for caravans and motor homes do not apply to electric cars.

The whole idea of disconnecting earth seems wrong, did not like the idea of putting diodes in the earth connection either, however even if fresh water the current even if diodes fail should be limited.

If I want an electric car, I have enough room to charge it well away from house so no problem with a TT supply. But some charge points are close to buildings but although I am told that one is TT, a metal frame building like that is not likely to go far from true earth even if the PEN is lost. Drawing on all three phases even with loss of PEN, there should not be too much imbalance, however it seems many cars only use a single phase, so large sign saying 22 kW is only valid if the car can take it.

But my house is not steel framed, and it only has a single phase supply, but although I have seen the damage due to loss of PEN when it happened my digital meter could not measure the voltage it was varying that fast, measured supply not DNO earth to true earth, so below 200 and above 300 but as to voltage to earth likely more than a tingle, but not sure if enough to kill.

Earth cables have been a problem many places where I have worked, I have seen even 6 mm² earth cables melt where some one did not have a good earth when welding, the problem is we don't tend to test the earth that often, in the workshop I would not use anything smaller than 16 mm² and the Oxford oil filled welder was only used in the workshop, I banded its use anywhere else, due to the damage which could be caused.

Not only earth cables of course, many a bearing trashed due to earth in wrong place when welding.

 

[plugwash]

Class 2 works well for certain sorts of appliance.

It's not a magic bullet though, as the size of an "isolated" power supply increases, so generally does it's leakage. Beyond a certain size it gets very difficult to meet the Class 2 leakage requirements while maintaining high efficiency. I suspect this is why electric cars are not class 2.

I'd imagine heaters are also challenging to make class 2 because of concerns about heat degrading insulation.

 

[JohnW2]

In the main, items used outside, so one can touch true earth and appliance earth at same time, are normally class II.

To my thinking, the main problem with items commonly used outside (e.g. hedge trimmers, mowers, trimmers, chainsaws etc.) is that, because they are (as you say) usually Class II, they usually come with 2-core cables.

The most common 'incident' with these things is the tool itself cutting through its cable. That usually will results in the circuit's OPD (at CU, and/or fuse in the plug) operating, but that is not inevitable - and if it doesn't happen (and although a circuit supplying such an item has, for many years, been required to be RCD-protected) the chopping through the cable, per se, cannot result in any RCD operating - that will only occur if the conductor of the live cut end happens to come into reasonable contact with the ground. If neither OPD nor RCD operate, then a person touching the cut end (whilst standing 'outside') is quite likely to receive a shock (possibly even a L-N shock, which no RCD could have noticed).

I therefore do rather wonder why 2-core cables are allowed fr such items. There's obviously no corresponding problem with 'outdoor' Class I items (rare though they are), since they will necessarily have a 3-core cable, with a CPC.

Kind Regards, John

 

[JohnW2]

Class 2 works well for certain sorts of appliance. It's not a magic bullet though, as the size of an "isolated" power supply increases, so generally does it's leakage. Beyond a certain size it gets very difficult to meet the Class 2 leakage requirements while maintaining high efficiency. ...

I probably need some educating here, since I'm not sure what you mean by an " 'isolated' power supply", what "leakage" you are talking about or what are the "Class II leakage requirements" to which you refer.

Also, of course, many Class II items of equipment (such as the outdoor/garden tools to which eric referred) do not have a 'power supply' of any sort.

I'd imagine heaters are also challenging to make class 2 because of concerns about heat degrading insulation.

One might think so but, having just had a scout around, a good few of the heat-producing things ariound here are marked as Class II - including fan heaters, hot air guns, hair dryers and coffee machines etc.

This rather take me back to a question I have asked more times than I care to recall (which really ever getting much of an answer). If (as with many items I have) there are no exposed-c-ps to earth (which presumably means it can't be Class I) and if it is not designed to be (and marked as) Class II, then "what is it?" - is it perhaps, 'not allowed', or what?

Kind Regards, John

 

[plugwash]

I probably need some educating here, since I'm not sure what you mean by an " 'isolated' power supply"

Something which takes power from the mains and carries it across an isolation barrier.

what "leakage" you are talking about

Current that can flow between input and output when someone or something grounded touches said output.

or what are the "Class II leakage requirements" to which you refer.

I don't know what the numbers are for regular commercial devices offhand. I remember for medical devices, limits of 100uA of touch current under normal condition and 500uA of touch current under single fault condition.

Also, of course, many Class II items of equipment (such as the outdoor/garden tools to which eric referred) do not have a 'power supply' of any sort.

Absoloutely, and so they are relatively easy to make class 2. You just have to include an insulating barrier somewhere in the motor or it's output shaft.

One might think so but, having just had a scout around, a good few of the heat-producing things ariound here are marked as Class II - including fan heaters, hot air guns, hair dryers and coffee machines etc.

I'm speculating a bit here, but AIUI patio heaters are halogen heaters which run at much much higher temperatures than the other appliances you mention.

This rather take me back to a question I have asked more times than I care to recall (which really ever getting much of an answer). If (as with many items I have)

Can you name some of these items?

The main thing I can think of is lamps, I'm sure I remember references historically to "non metal table lamps or class II insulated devices" when discussing things that didn't need an earth, suggesting that lamps were treated specially and not regarded as regular appliances at least at one point.

Certainly there is no way that a traditional UK table lamp would pass muster under the rules for most other types of appliance, but honestly that is less to do with the lack of insulation and more to do with the fact that live parts can be accessed without a tool. OTOH I don't think i've seen a traditional UK table lamp in some time.

there are no exposed-c-ps to earth (which presumably means it can't be Class I) and if it is not designed to be (and marked as) Class II, then "what is it?"

At least according to Wikipedia there are actually five classes.

Class 0 - protection by basic insulation only.
Class 01 - protection by basic insulation, plus earthing but with the earth connected via a seperate earth terminal rather than a wire in the mains cable
Class 1 - protection by basic insulation plus earthing through the mains cable.
Class 2 - protection by "double or reinforced" insulation.
Class 3 - protection by an external SELV power supply.

Wikipedia claims class 0 appliances were banned in the UK in 1989 but I can't seem to find a precise reference in the linked law.

Not every piece of electrical equipment is an appliance designed to an IEC appliance class though.