Can you get a shock with a 30mA RCD in place

[JohnW2]

Interesting question and responses. I tend to agree that the RCD appears to have gained a sort of false cult status protecting against fatal electric shock no matter what else is wrong with an electrical installation. A catch all.

I agree essentially with most of what you go on to say, but you don't actually mention the two worrying misconceptions which I've previously mentioned:

• 1...That a high proportion of the general public do not realise that an RCD offers no protection if a victim contacts both L and N, but not E. The public need much more education about this limitation of RCDs.
  
  2...Many people, including a good few electricians, seem to believe that a 30mA RCD will restrict current flowing through a victim to 30mA.

To address just one of your specific points:

- A fatal electric shock may occur well below 30mA. There are many variables and no guarantees.
I have been told about a lab medical study which shows that a current exceeding 50mA as measured across the longitudinal axis of the heart can induce VF but I suspect that theory bares little resemblance to the real world.

That figure is too high. Fibrillators used for deliberately inducing VF (ventricular fibrillation) during cardiac surgery (when electrode paddles are applied direct to the surface of the heart) usually operate at 6-12V and many have a maximum current of only 15mA.

It is certainly true that very low currents through the heart can induce fatal VF in some situations. The figures we hear about relate to people with healthy hearts. However, some of us (even some who are totally unaware of any heart disease) have a heart which is already in an electrically precarious situation, and even a tiny current, for a very short time, can be 'the last straw' which induces VF. Of course, the current through the body and the current through the heart are two very different things, and we generally will not know the latter. Needless to say, arm to arm (e.g.hand to hand) or arm to leg (particularly right arm/hand to left foot) currents tend to go more through the heart than others.

Kind Regards, John.

 

[ericmark]

My house has had RCD protection on all circuits at 30ma for around 20 years and yes you do get a shock. When I in error of course put a pair of hack saw blades through the cable joining two two way switches it was enough to knock me out.

However they did ensure anyone coming to my aid would not suffer a similar fate.

Not sure max current in 40ms but likely the max allowed with earth loop impedance as unlikely MCB will trip quicker than 40ms.

 

[sparkticus]

I agree essentially with most of what you go on to say, but you don't actually mention the two worrying misconceptions which I've previously mentioned:


That a high proportion of the general public do not realise that an RCD offers no protection if a victim contacts both L and N, but not E. The public need much more education about this limitation of RCDs.

Many people, including a good few electricians, seem to believe that a 30mA RCD will restrict current flowing through a victim to 30mA.[/list]To address just one of your specific points:

I agree with both of your points. If a person contacts L and N then the majority of current will flow from L - N via the mass of human tissue between the L - N contact points. Some current will flow from L - E which may eventually reach the 30mA threshold but with little impact on the final outcome - the person will receive a severe electric shock.

Yes, an RCD limits the time that a person is exposed to a live conductor assuming the person forms a path to earth causing a differential current to be seen by the RCD which will be the case in the majority of circumstances. It does not limit the current to 30mA since damp human tissue may draw much more than 30mA in the time it takes an RCD to disconnect - 200ms is 10 cycles of AC at 230V RMS---- ouch.


With that said, I think this is too complex and a difficult point to convey to people that may not be well informed about electrical principles. It may be a better idea to promote the rapid disconnection features of an RCD without raising the discussion of electric shock. A controversial point but I would like to see the requirements for competent persons (domestic electrical electricians) increased to include at least a certificate level in electrical science (not just the regs) but I doubt that will happen any time soon.


My son recently fitted a dual RCD consumer unit in a 1920s house where he had to partially replace some wiring since it would have caused the RCDs to trip (poor insulation resistance) The customer was an electronics engineer and understood the primary need for RCDs but insisted he only wanted them to save money because they detect leakage currents that register on the electricity meter! Can't argue with that point

 

[JohnW2]

I agree with both of your points. If a person contacts L and N then the majority of current will flow from L - N via the mass of human tissue between the L - N contact points. Some current will flow from L - E which may eventually reach the 30mA threshold but with little impact on the final outcome - the person will receive a severe electric shock.

Significant current flow to earth is surely far from inevitable. I certainly don't suggest you do the experiment with a 'high voltage' (still LV, for the pedants!) IR tester, but I imagine that with shoes and floor coverings, the resistance of the path to earth from a person in an average house is usually very high.

It does not limit the current to 30mA since damp human tissue may draw much more than 30mA in the time it takes an RCD to disconnect ...

Exactly my point - but, as I said, even some electricians have not thought this one through and believe that the RCD will limit the current through a human body to 30mA.

With that said, I think this is too complex and a difficult point to convey to people that may not be well informed about electrical principles.

I agree that the discussion we're having here would not be appropriate for the general public, but I feel sure that more should be done, in both promotion and 'instructions' to stress to the public that there are limitations of what RCDs can protect one against and that it remains possible to receive fatal electric shocks even with adequate RCD protection in place. If you look at some of the promotion, particularly for RCD sockets and adapters, it's not difficult to see why many members of the public might come away with the belief that they can behave as foolishly as they like without any risk if an RCD is being used.

My son recently fitted a dual RCD consumer unit in a 1920s house where he had to partially replace some wiring since it would have caused the RCDs to trip (poor insulation resistance) The customer was an electronics engineer and understood the primary need for RCDs but insisted he only wanted them to save money because they detect leakage currents that register on the electricity meter! Can't argue with that point

An interesting reason for wanting an RCD ... but, crikey, with a low enough IR to trip a 30 mA RCD (only about 7.7K&#937 did not this electronics engineer realise that he had far more to worry about than his electricity bills?

Kind Regards, John

 

[martinxxxxxx]

This reminds me of a comedian on the telly the other day who said;
he once told his younger brother that with automatic doors - no matter how fast you run at them , the doors will always open before you get there - wasn't you was it

 

[JohnW2]

This reminds me of a comedian on the telly the other day who said;
he once told his younger brother that with automatic doors - no matter how fast you run at them , the doors will always open before you get there - wasn't you was it

Nope - but I've seen people (probably on Saturday nights) who don't believe that is normally true doing an experiment to find out if it becomes true after one has had a few beers

Kind Regards, John.

 

[sparkticus]

Significant current flow to earth is surely far from inevitable. I certainly don't suggest you do the experiment with a 'high voltage' (still LV, for the pedants!) IR tester, but I imagine that with shoes and floor coverings, the resistance of the path to earth from a person in an average house is usually very high.

That's true, the earth fault current will have many variables including shoes, floor coverings etc. I once (when I was very much younger) held a live cable while standing on a rubber mat to demonstrate the lack of potential difference.

Exactly my point - but, as I said, even some electricians have not thought this one through and believe that the RCD will limit the current through a human body to 30mA.

Yes, I have too and encourage them to spend a little more time reading up on electrical science in general. I usually see their eyes glaze over at that point.

I agree that the discussion we're having here would not be appropriate for the general public, but I feel sure that more should be done, in both promotion and 'instructions' to stress to the public that there are limitations of what RCDs can protect one against and that it remains possible to receive fatal electric shocks even with adequate RCD protection in place. If you look at some of the promotion, particularly for RCD sockets and adapters, it's not difficult to see why many members of the public might come away with the belief that they can behave as foolishly as they like without any risk if an RCD is being used.

I absolutely agree and wish that the likes of Cowboy Builders, Rogue Traders etc would do more to inform people. The likes of the IET, NICEIC, ELECSA, NAPIT BRE and others out to spend a little of their revenue on informative TV sponsership. Not to mention the government.

Much of the misconceptions are spread by ill informed competent people. When I say ill informed I mean those that don't put a little time aside to understand the underlying theory. It's a subject that find myself ranting about quite often.

 

[JohnW2]

That's true, the earth fault current will have many variables including shoes, floor coverings etc. I once (when I was very much younger) held a live cable while standing on a rubber mat to demonstrate the lack of potential difference.

Exactly. It's not something I'm proud of, but I'm sure that I have come into contanct with a line/phase conductor (alone) without feeling anything in my time - but it's a definite case of 'don't try this at home'!!

Exactly my point - but, as I said, even some electricians have not thought this one through and believe that the RCD will limit the current through a human body to 30mA.

Yes, I have too and encourage them to spend a little more time reading up on electrical science in general. I usually see their eyes glaze over at that point.

I don't apportion much blame to the individuals, in this or many other fields. So many subjects seem today taught without adequate emphasis on the underlying principles. Apart from anything else, I think this makes learning much more arduous - if one understands the underlying theory/principles, so many things become 'obvious', or can easily be worked out, rather than having to be learned 'parrot-fashion'.

I absolutely agree and wish that the likes of Cowboy Builders, Rogue Traders etc would do more to inform people. The likes of the IET, NICEIC, ELECSA, NAPIT BRE and others out to spend a little of their revenue on informative TV sponsership. Not to mention the government.

I agree up to a point, but the efforts of all those you mention would be much less required if the manufacturers of the devices were a bit more responsible in their promotion and instructions. I honestly think that the RCD sockets and plugs should probably come with a cigarette-like 'health warning', along the lines of "This device will not necessary prevent you suffering a fatal electric shock. Electricity must always be respected"

Much of the misconceptions are spread by ill informed competent people. When I say ill informed I mean those that don't put a little time aside to understand the underlying theory. It's a subject that find myself ranting about quite often.

As above, agreed. However, I don't mean to imply that the most of those in this, or any other, field have an inadequate understanding of underlying theory - merely that, as everywhere in life, that is true of a few ... like the electrician who, not too long ago, tried to tell me that I should install an additional RCD close to my {RCD protected} shower "because a 30mA fault current which would trip a local RCD might have fallen to a sufficiently low level not to trip the RCD in the CU by the time it got back to the CU"!! ... and the worrying thing is that he was both offended and seemingly unable to understand my explanation when I tried to tell him that I didn't think that was quite right!

Kind Regards, John.

 

[sparkticus]

I don't apportion much blame to the individuals, in this or many other fields. So many subjects seem today taught without adequate emphasis on the underlying principles. Apart from anything else, I think this makes learning much more arduous - if one understands the underlying theory/principles, so many things become 'obvious', or can easily be worked out, rather than having to be learned 'parrot-fashion'.

Yes, I took the 17th edition update in 2008 and C&G2391 in 2009. It had been a while since I last did any formal study and I was very surprised at how both were taught. Both courses included a sort of intensive "get through the exam" revision session shortly before the exams. The 17th update was simply multiple choice and heavily coached. The 2391 was a written paper but the class was heavily coached at the revision session just before the exam. Both courses did assume prior experience but some people on the course had quite low experience and what I would regard as "alarming gaps" in their electrical science knowledge. To balance that out, the practical assessment was quite comprehensive and I know that only around 60% of the class passed both the written exam and the practical.

As above, agreed. However, I don't mean to imply that the most of those in this, or any other, field have an inadequate understanding of underlying theory - merely that, as everywhere in life, that is true of a few ... like the electrician who, not too long ago, tried to tell me that I should install an additional RCD close to my {RCD protected} shower "because a 30mA fault current which would trip a local RCD might have fallen to a sufficiently low level not to trip the RCD in the CU by the time it got back to the CU"!! ... and the worrying thing is that he was both offended and seemingly unable to understand my explanation when I tried to tell him that I didn't think that was quite right!

Oh dear, he appears to have confused something like voltage drop with current flow perhaps. An accurate understanding of these things should be second nature to an experienced electrician. Even on a bad day I would expect a good electrician to know the theory inside out. There is no excuse - what would that person do in an electrical emergency or write on a PIR?

 

[JohnW2]

... like the electrician who, not too long ago, tried to tell me that I should install an additional RCD close to my {RCD protected} shower "because a 30mA fault current which would trip a local RCD might have fallen to a sufficiently low level not to trip the RCD in the CU by the time it got back to the CU"!! ... and the worrying thing is that he was both offended and seemingly unable to understand my explanation when I tried to tell him that I didn't think that was quite right!

Oh dear, he appears to have confused something like voltage drop with current flow perhaps. An accurate understanding of these things should be second nature to an experienced electrician. Even on a bad day I would expect a good electrician to know the theory inside out. There is no excuse - what would that person do in an electrical emergency or write on a PIR?

Goodness knows what he was thinking, but I was startled that a practising (and 'qualified') electrician appeared totally unable to grasp the concept, even when I tried to explain, that the same current inevitably flows along a single length of a conductor!. As you say, one can but assume that he was in some contorted way getting muddled up with something like voltage drop - but such a fundamental lack of understanding of what must be sub-GCSE electric principles was just plain frightening!

Kind Regards, John.

 

[Paul_C]

A controversial point but I would like to see the requirements for competent persons (domestic electrical electricians) increased to include at least a certificate level in electrical science (not just the regs) but I doubt that will happen any time soon.

I've often been amazed at some of the comments from "qualified" electricians on other forums, which demonstrate quite clearly that while they might have a good knowledge of BS7671 and of "typical" installations, their understanding of some of the most fundamental electrical principles is poor to non-existent.

 

[Astra99]

I am sure everyone will agree that Ohm's Law and Kirchoff's Laws (plus some input from Michael Faraday and others) are the bases upon which electrical science is built. In my day we had to learn, and quote these laws in written exams. Nowadays the "youngsters" seem to have no comprehension of these laws, they just know V=IR and punch numbers into their pocket calculators without understanding the "why"

 

[JohnW2]

Nowadays the "youngsters" seem to have no comprehension of these laws, they just know V=IR and punch numbers into their pocket calculators without understanding the "why"

...or, even worse, find an on-line 'calculator' with boxes into which they type the values of I and R, then press 'calculate' to get V (which was E in my day!), without even knowing how the calculation was done!

Kind Regards, John.

 

[sparkticus]

I am at least happy to see that I am not alone in my frustration over this subject. Is it a problem with the "system"? but there also appears to be a lack of interest in the theory these days by many new entrants to the industry. City & Guilds courses are absolutely solid and have been for as far back as I can remember but they were designed to be taught over a reasonable period of time with periodic assessments and lab/workshop practice. You can now get a level three after about 4 days in class!! Many of the trade schools appear to just push people through with intensive coaching just prior to the exams. Is that right? There are trade schools popping up everywhere, in fact I received a letter a few weeks back asking I was interested in setting up a trade school and becoming accredited by City & Guilds and EAL all fast tracked of course. No thank you, I am not interested in cattle herding. I taught at a CFE back in the early 80s and there was at least time to put effort into the training at that time. I suspect it is now all about the throughput!

Having said all of that, you can now get a fast track medical degree in four years which will probably reduce to three in years to come. The additional requirements for electrical installations in hospitals will be the least of an electricians problems then

 

[JohnW2]

Having said all of that, you can now get a fast track medical degree in four years ....

Totally off topic (apologies) but I think that many people are probably trying to read too much into that. 40 years ago, the norm was 4.5 years for most medical students (unless they studied at Oxford or Cambridge). Even then, many took 6 years from start to finish, but only because they chose to 'take out' 1.5 years in the middle of their medical studies and thereby get a BSc in a medically-related science subject (e.g. physiology, biochemistry etc.) in addition to their medical degree.

Kind Regards, John.