Do RCDs save lives

[ericmark]

As I said I googled to see how many deaths per year in the UK domestic with LV supplies. I was rather surprised I expected more than 22. So we are looking at something like 0.000034% chance of death for electrocution there are twice as many people killed in the US by electrocution intentionally than killed in the UK by accident. Well maybe they use lethal injection now? But anyway the figures are so low for electrocution it seems we are going OTT with protection if the RCD is just to stop deaths by electrocution.

 

[stillp]

I wonder if anyone has actually lost his life through its presence leading him into carelessness and an erroneous belief that he can't be hurt by an RCD-protected circuit?

Or thinking they can't be electrocuted because all their metalwork is earthed? Or because their neon screwdriver told them a circuit was dead? etc, etc. We can never know, we can only strive to reduce risks where we can.

 

[kai]

Another safety measure when working on a circuit, is to move the Phase/Live Conductor for that circuit out of the Breaker, and place it into the earth bar on the distribution board whilst works are being carried out. This is a safeguard against accidential re-energisation. It also comes in handy when you need to measure R1+R2 at the end of the job, to check that the earthing is indeed continuous before returning the circuit to use.

 

[JohnD]

I was once redecorating, and turned off the circuits for the whole floor before removing sockets. I tested for dead at the start, and was surprised to hear a "click" from the tripping RCD when my screwdriver touched something in one of the sockets. I discovered it was fed from the circuit on another floor. Should have been more careful. Lucky for me all circuits were protected.

 

[JohnW2]

And this is where I feel that some people have been misled by much of the "hype" surrounding the RCD, and really do believe that the presence of one somehow completely prevents the possibility of dangerous electric shock or electrocution.

Indeed, and I think that "some people" may well be a big understatement. There is also a widely-held misconception, even amongst some electricians, about what RCDs "do" - many believing that they limit the magnitude of the shock current (not just its duration) to 30mA (or whatever).

So as a side question to that of how many people have been saved by the RCD, I wonder if anyone has actually lost his life through its presence leading him into carelessness and an erroneous belief that he can't be hurt by an RCD-protected circuit?

Quite - but, of course, even more "unmeasurable" than the other things we have been discussion. Also, as I recently mentioned, there's also the question of the number of serious injuries (and maybe deaths) due to the secondary consequences of 'nuisance' RCD trips.

Kind Regards, John

 

[JohnW2]

it won't protect you from a shock between live and neutral. but if its just the live itself that you come in contact with or the live and earth you will definitely get a shock you will probably feel a lot, but most of the time the rcd will trip out before anything too serious happens just leaving you shaken and feeling like an idiot ....

That is what we hope, and have been led to believe. However, another point to bear in mind is that, despite the seemingly 'hard' information published about the effects of electric shocks of different magnitudes, durations, and routes on the human body are far from as well-established as one might think - since (for fairly obvious reasons) they are derive primarily from experiments on animals other than human beings and/or are extrapolations from experiments undertaken on 'already-dead' human beings.

... bear in mind though an rcd DOES NOT guarantee protection against electrocution. there is always a possibility so don't take any chances.

That is obvioulsy the most important point.

Kind Regards, John

 

[JohnW2]

As I said I googled to see how many deaths per year in the UK domestic with LV supplies. I was rather surprised I expected more than 22. So we are looking at something like 0.000034% chance of death for electrocution ...

Indeed, we have very frequently discussed that figure, which clearly has (or should have) a major impact on discussions about measures/regulations designed to "further reduce" fatal electric shocks. Quite apart from comparisons with the number of intentional electrocutions in the US (which really needs to be divided by about 5, to take into account the relative population sizes), in the UK the figure for domestic electrocutions is only around 5 times less than the annual number of deaths due to people being struck by lightning.

As I've often said, a conservative estimate of the cost of providing every UK domestic electrical installation would be in the ballpark of £1billion - £2billion, and I cannot help but wonder how many 'lives could saved' if that same investment were put into some other safety-critical area (road safety?).

Despite some of the things we see and hear, people in general must have a high degree of 'respect' for the potential dangers of electricity, since the number of deaths we are seeing in the UK is almost 'unbelievably' low.

Kind Regards, John

 

[JohnD]

I cannot help but wonder how many 'lives could saved' if that same investment were put into some other safety-critical area (road safety?).

I know the answer to that!

Where the safety rules are made by the people or organisation that has to pay for them (e.g. working hours for hospital doctors; hard shoulders on motorways), a cost-benefit analysis is used and must show that considerably more money would be saved than is spent.

Where the safety rules are made by the people or organisation that do not have to pay for them (e.g. working hours for lorry drivers, RCDs in houses), the rules can be totally absurd in costing vast amounts for a negligible improvement.

it's not just a government thing. Factory owners and truck companies will lobby for public spending on roads that they don't have to pay for themselves.

 

[stillp]

I wonder how common it is (in a TN installation) to get an L-E fault of sufficiently low impedance to result in a lethal shock, but of a sufficiently high impedance not to cause an OPD to operate?

I investigated two cases that would meet that criterion, where a fire had resulted. An RCD would have prevented both. They were both in industrial premises though.

 

[JohnW2]

I wonder how common it is (in a TN installation) to get an L-E fault of sufficiently low impedance to result in a lethal shock, but of a sufficiently high impedance not to cause an OPD to operate?

I investigated two cases that would meet that criterion, where a fire had resulted. An RCD would have prevented both. They were both in industrial premises though.

It's obviously a possibility, so it bound to happen at least occasionally - but, as I said, I wonder how common it actually is. Some people put a lot of weight on this function of RCDs in clearing faults before a shock (or fire) results, but I suspect that it is pretty rare for this situation to arise, in which an OPD would not have cleared the fault pretty quickly even in the absence of an RCD.

I'm a little surprised that you say that RCDs would have prevented fires in your two cases - an OPD will not allow particularly high currents to flow for an appreciable time before operating. Indeed, 'by definition', they should not allow sufficient current to flow for sufficient time to damage the cable, let alone start a fire.

Kind Regards, John

 

[JohnW2]

I cannot help but wonder how many 'lives could saved' if that same investment were put into some other safety-critical area (road safety?).

I know the answer to that! ....

I'm sure that what you say has some relevance, but there are lots of other factors as well.

The 'cost' side of a cost-benefit analysis is not all financial. The financial cost of a reduction in national speed limit would be 'minimal', yet I feel sure that it would not require much of a (enforced) reduction to save at least 22 lives per year, something which is never going to be possible in terms of domestic electrocutions (I'm sure that there will always be some, no matter what measures are taken). However, the 'cost' in terms of inconvenience etc. of a reduction in NSL would probably not be acceptable to the population.

Public expectations are also often unrealistic. If it were known that some change in electrical regulations could "halve" the number domestic electrocutions, there could well be a strong public demand to introduce it - even though the present risk they would be 'halving' is only around 1 in 3 million per year, similar to the statistical risk of being struck by lightning.

Kind Regards, John

 

[JohnD]

they use a "cost" per road death which I think is based on police, ambulance, autopsy etc.

I had an idea it was not based (as life or accident insurance might be) on the cost of supporting the family for 20 years, or lost earnings.

however I may be wrong as I found this:

Accident/casualty type Cost per casualty Cost per accident


Fatal .. . . . . . . . . . . . . 1,742,988 . . . . . . 1,953,783


Serious . . . . . . . . . . . . . 195,863 . . . . . . . . 223,870


Slight . . . . . . . . . . . . . . . .15,099 . . . . . . . . 23,544


Average for all severities . . . 52,529 . . . . . . . 74,280


Damage only - 2,096


1 The costs were based on 2013 prices and values


2 The number of reported road accidents were based on 2013 data

on page 225 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/359311/rrcgb-2013.pdf

 

[stillp]

I wonder how common it is (in a TN installation) to get an L-E fault of sufficiently low impedance to result in a lethal shock, but of a sufficiently high impedance not to cause an OPD to operate?

I investigated two cases that would meet that criterion, where a fire had resulted. An RCD would have prevented both. They were both in industrial premises though.

It's obviously a possibility, so it bound to happen at least occasionally - but, as I said, I wonder how common it actually is. Some people put a lot of weight on this function of RCDs in clearing faults before a shock (or fire) results, but I suspect that it is pretty rare for this situation to arise, in which an OPD would not have cleared the fault pretty quickly even in the absence of an RCD.

I'm a little surprised that you say that RCDs would have prevented fires in your two cases - an OPD will not allow particularly high currents to flow for an appreciable time before operating. Indeed, 'by definition', they should not allow sufficient current to flow for sufficient time to damage the cable, let alone start a fire.

Kind Regards, John

In both cases the fault loop impedance was too low for the OCPD to operate. The faults occurred at the end of a long cable, in one case an extension lead, in the other an addition to the original installation.

 

[JohnW2]

they use a "cost" per road death which I think is based on police, ambulance, autopsy etc.

Yes, I know that, but you've got it back to front - although you call that "cost", it's actually the "benefit" side of a cost-benefit analysis (for potentially 'life saving' measures). As I said, the (financial) cost of, say, reducing the NSL would be minimal - so, given those "cost per death" figures they work with (the potential 'benefit'), a purely financial cost-benefit analysis would, for quite a modest NSL reduction, very probably come out in favour of such a reduction. However, as I said, the non-financial cost ('inconvenience' etc.) would probably (certainly?!) not be acceptable to the population (of road users).

People can be 'funny'. Many of those who would support, maybe even campaign for, tightening of regulations regarding 'electrical safety' would probably (because it would represent an 'inconvenience for them) not support a NSL reduction, even though the potential for reducing deaths by the latter would be considerably greater.

Kind Regards, John

 

[JohnW2]

In both cases the fault loop impedance was too low for the OCPD to operate. The faults occurred at the end of a long cable, in one case an extension lead, in the other an addition to the original installation.

I still don't really understand. Assuming that the OPD was appropriate for the cable, the design should be such that fault current (of whatever magnitude) could not flow for long enough for the cable to be damaged - so I find it hard to understand how fires could have resulted from the cable.

Are you perhaps saying that the In of the OPD was too high to give adequate protection to the cable? ... or (I suspect more likely) are you perhaps saying that what was not adequately protected (and caught fire) was 'what was connected to the end of the cable' - in which case the manufacturer of the equipment should have specified a maximum OPD rating or, better (BAS would say 'necessarily'), provide appropriate over-current protection within the equipment.

Kind Regards, John