Rcd and electric shocks

[stuartR1963]

Hi all, am i correct in thinking if you were to receive an electric shock from an rcd protected circuit it would not limit the current through you to 30ma? Obviously there is so many variables on how someone can receive an electric shock, but am i correct in thinking if your body resistance is low to pass 50ma through your body (which i imagine would be an extremely painful shock) the rcd would do nothing to reduce that current, but only reduce the time you are exposed to it, so you would only feel the shock briefly, in other words it should trip fast enough to prevent you being killed potentially but it won't limit the current? So it doesn't matter if the current through you is 31ma or 100ma you will still feel the severity of that shock and all the rcd would do is trip fast enough to save your life.
Is this correct in how an rcd works.
The above scenario excludes a shock between live and neutral i would think in that case you would be deadmeat as there is no imbalance for the rcd to detect.
Thanks

 

[JohnW2]

Hi all, am i correct in thinking if you were to receive an electric shock from an rcd protected circuit it would not limit the current through you to 30ma? Obviously there is so many variables on how someone can receive an electric shock, but am i correct in thinking if your body resistance is low to pass 50ma through your body (which i imagine would be an extremely painful shock) the rcd would do nothing to reduce that current, but only reduce the time you are exposed to it, so you would only feel the shock briefly ...

All totally correct, provided you are talking about a current flowing through the person from live/line to earth.

... in other words it should trip fast enough to prevent you being killed potentially but it won't limit the current? So it doesn't matter if the current through you is 31ma or 100ma you will still feel the severity of that shock and all the rcd would do is trip fast enough to save your life.

Again, all true, other than that you should say "hopefully fast enough to prevent you being killed". In some people, particularly those with (often unknown) pre-existing heart disease, can be killed by just a few mA.

Is this correct in how an rcd works.

As above, essentially yes.

The above scenario excludes a shock between live and neutral i would think in that case you would be deadmeat as there is no imbalance for the rcd to detect.

Indeed so. Absolutely nothing could do anything about an L-N shock, since no technology would be able to distinguish between current from from L to N through a person and that flowing from L to N through an 'intended' load.

I would add that, as often discussed here, we really don't have much of a clue as to how many (if any!) lives may possibly have been saved by RCDs - reports of people experiencing (and surviving) an electric shock which resulted in an RCD tripping are rare, and even in those few cases, it is far from certain that the person would have died in the absence of an RCD (even without RCDs, it is fortunately the case that most electric shocks do not result in death).

Kind Regards, John

 

[stuartR1963]

Thanks i thought thats how they worked thanks for confirming. I did get a shock some years ago from a socket i loosened for decorating which did operate the rcd thankfully, not sure how much current had passed through my body but it was painful even though the shock was only for a split second. That is the only shock i have ever had and ever will,since then i always treat electricity with upmost respect, I also test the rcd every few months as i hear they have a tendency to stick.

 

[bernardgreen]

A short duration shock is seldom fatal, it may stun the heart into stopping but when the current ceases then the heart will re-start itself. This is how a de-fribrillator works. It stuns the heart to stop the in-effective rapid contractions ( fibrillations ) and allow the heart to then re-start itself into an effective rhythm.


Deaths from electric shocks are most often the result of a prolonged current through the body which has caused internal burning and/or chemical reactions in the body tissues. These deaths can occur some considerable time after the shock hence the advice to seek medical advice after a serious ( across the body shock ).


EDIT the point is that an RCD will ( should ) limit the duration of the current to avoid any damage to body tissues

 

[Calonbarth]

Hi, came across this thread while waiting for a reply from another forum and it brought back memories. In the days long before Elf n Safety 65+ years ago my brothers and I shared a bedroom which had a brass switch to operate the light. The wires were 'protected' by a brass domed cover through which a brass toggle switch emerged, we payed a game which involved us trying to remove the cover without touching the toggle switch because if you did you got a shock and were out! We played it for several years on wet weekends before we had a television. Luckily we all grew up to be hale and hearty, and incidentally none of us grew up to be an electrician!

 

[JohnW2]

A short duration shock is seldom fatal, it may stun the heart into stopping but when the current ceases then the heart will re-start itself. This is how a de-fribrillator works. It stuns the heart to stop the in-effective rapid contractions ( fibrillations ) and allow the heart to then re-start itself into an effective rhythm.

It's actually relatively unlikely that an AC shock will cause the heart to 'stop' briefly in the way you suggest ('asystole' - no heart activity, electrical or mechanically, at all) - if it does anything, it's far more likely to induce a rhythm disturbance, at worst ventricular fibrillation (the 'ineffective rapid contractions to which you refer) - which will usually be fatal unless treated (very rarely can sort itself out). Defibrillators use DC shocks, which (hopefully) have the effect you describe. AC shocks can be used to deliberately induce ventricular fibrillation for certain purposes (and, ironically, around 50Hz is the optimal frequency for achieving that).

Deaths from electric shocks are most often the result of a prolonged current through the body which has caused internal burning and/or chemical reactions in the body tissues. These deaths can occur some considerable time after the shock hence the advice to seek medical advice after a serious ( across the body shock ).

All theoretically true, albeit the 'late' heart attacks and deaths in the days/weeks following an electric shock are extremely rare.

However, as I warned people in a recent thread here, whilst it probably makes sense to follow the NHS advice and 'seek medical advice' following a serious electric shock, people should be warned that they should not derive any reassurance from the fact that they have been to A/E following an electric shock and had a 'heart check' which detected no problems - as I said in that other thread, my personal experience is that those attending A/E following an electric shock nearly always have perfectly normal ECGs, no abnormalities detected by clinical examination, and no abnormalities of any other tests which may be done.

In other words, whilst it is unfortunately true that there is a (very slightly) increased risk of heart attack or cardiac death in the days/weeks following a serious electrical shock, such occurrences are usually totally unpredictable, and remain essentially just as likely even if a 'medical check up' soon after the shock reveals nothing of concern.

Kind Regards, John

 

[bernardgreen]

Defibrillators use DC shocks,

 

[JohnW2]

<graph of output current from three defibrillators>

I would dare to suggest that you are 'trying to be clever'

What those curves show is, as expected, a period of DC current flow followed by a period of reversed-direction DC current flow - and that is 'the end of it'. Whilst it might be argued that it is literally true that that is an 'alternating current' (since it 'alternates' just the once - hence, if you want, one cycle of asymmetric AC), the physiological effects of 'continuous AC' (i.e. multiple cycles of AC, following one after another) are totally different.

As I wrote before, 'true' alternating current will generally induce (not terminate) fibrillation - which has been done, both as one of the methods of 'stopping the heart' (or sometimes just the atria) during cardiac surgery and also as a means of inducing fibrillation in animals to produce models of fibrillation to be used for studying treatments. In fact, one of the very first papers I wrote, in 1979, was about just that (albeit in relation to atrial fibrillation, not ventricular fibrillation - but the mechanism is identical).

I will share the summary of this paper with you by PM but, to illustrate that this is a 'reliable and repeatable' effect of AC (not just an occasional chance happening), I will quote this bit here ....

".... 50 Hz sine-wave electrical stimulation of the left atrium reliably and repeatedly produced an arrhythmia which was indistinguishable from atrial fibrillation in terms of its ECG appearance, the statistical properties of the ventricular response and responses to a number of pharmacological agents....

Kind Regards, John

 

[ericmark]

Hi all, am i correct in thinking if you were to receive an electric shock from an rcd protected circuit it would not limit the current through you to 30ma?

No the RCD does not limit the current, it only limits time for current over 30 mA to 40 mS.

Obviously there is so many variables on how someone can receive an electric shock, but am i correct in thinking if your body resistance is low to pass 50ma through your body (which i imagine would be an extremely painful shock) the rcd would do nothing to reduce that current, but only reduce the time you are exposed to it,

Yes

so you would only feel the shock briefly, in other words it should trip fast enough to prevent you being killed potentially but it won't limit the current?

No it can still kill you.

So it doesn't matter if the current through you is 31ma or 100ma you will still feel the severity of that shock and all the rcd would do is trip fast enough to save your life.
Is this correct in how an rcd works.
The above scenario excludes a shock between live and neutral i would think in that case you would be deadmeat as there is no imbalance for the rcd to detect.
Thanks

No the idea is the RCD trips before you touch something. So if for an example the immersion heater thermostat fails and water is released which could result in a fault in the wiring causing your body to bridge between line and earth the RCD would trip before she tried to turn off the stop tap, I am referring to the death of Emma Shaw. Had RCD's been fitted in 2004 it would have saved her life.

Yes it can give you a fighting chance by limiting the time, but be it 30 mA or 1 amp rating the current until it trips is the same. However if the fault would let say 100 mA flow and you have a 1 amp trip it would not trip, so the 30 mA means it will trip if there is a current likely to do damage to a healthy human body.

 

[EFLImpudence]

I think all that will just confuse Stuart. It did me.

 

[JohnW2]

I think all that will just confuse Stuart. It did me.

I'm inclined to agree - particularly in relation to the first sentence - where I think that "Yes" would probably have been more appropriate than "no" - although an answer which didn't use either of those words would probably have been even better/clearer.

Kind Regards, John