Table 41.1 Assumed Touch Voltage

mapj1:

The fact I say 'current' and not 'voltage' is significant.  Skin resistance of 5k-50k ohms per square cm of skin means that only a large area contact will get anywhere near the 30mA of the RCD threshold, in turn based on the lethality date. Shock current in terms of contact resistance is the single biggest variable in all this.

Skin conductance per area should be treated with caution when getting to small (few mm²) areas and large proportions of 230 V across the skin. When applied voltages are over some threshold, typically below 230 V (e.g. 200 V) skin can quickly become more conducting. This is included in some plots in IEC 60479-1. For example Fig.5 (2018) shows the 1 mm² contact resistance collapsing to around 7 kΩ around 220 V (50 Hz) compared to over 200 kΩ at 200 V. The quick transition is labelled 'rupture of skin'. Ok, that figure is only based on 'one living person': a lot else about impedance with substantial voltages is on larger numbers of corpses. But I can verify tripping a 30 mA RCD from contact of a 1.5 mm² conductor-end (from a cut-off wire, insulated except the 1.5 mm² tip) on a palm, with 230 V,  when the other body connection was a good one; it left a white spot. As long as there are fairly poor contacts at both the 'entry' and 'exit' then the split of 230 V between them prevents this level of skin voltage being achieved.

(Following a recent discussion of units, how should that skin 'resistance' be described?  It may be 5 to 50 kilohms for a 1 cm square [for a given voltage and condition], but saying e.g. "50 kΩ per cm²" suggests 500 kΩ for 10 cm².  Instead, writing as 50 kΩ·cm² would be the strange-sounding way that shouldn't be able to be misunderstood. Alternatively 20 µS/cm². Yes, I know no one who thinks about the subject would misunderstand anyway...)

Nathaniel:

ProMbrooke:

Any idea as to why IEC TR 1200-413 is retired?

No. I've wondered too. I was surprised even when I first saw it, as I've not come across other cases of such a detailed 'technical report' devoted to explaining why certain choices were made in a standard.  Perhaps it's not fashionable now to bother with public explanations. The IEC and national standards bodies appear happy to carry on selling this retired publication at a hefty price. (IEC says 'withdrawn').

 

Well, I'm prepared to show my inference in great length, for the world to see, for the benefit of all humanity. :) Hiding the technical reasoning is both a crime and unethical when used to make what is used as law all over the globe. And I know why the IEC has gone into hiding technical aspects: because AFDDs are just the start to a long marketing concept. If you can mandated it, then it will sell. But that is for another thread.


Me personally I've been compelled to come up with my own code and my own standards. I would very much like something that will one day supersede all IEC and NEC based codes.


The IEC TR 1200-413 makes a very good argument for faster disconnection times in wet locations in annex C but for some reason this has not made itself into BS7671. The US NEC also goes to great lengths to put GFCIs into kitchens, laundry rooms, unfinished basements, garages, ect all considered low skin conductivity locations in the eyes of the NFPA.


Because BS1363 sockets will not open without an earth pin, and are not likely accept schuko sockets, in my view this counts as assurance of the CPC path. Hence why RCDs in the UK were delayed into the 90s early 2000s. As such RCDs are not intended to be the primary means of protecting against indirect contact.And in general that should hold true for any supply. The primary means in my opinion should be in the ADS, achieved only though loop impedance ie high conductance. The voltage at the point of fault to remote earth determining the maximum duration which an MCB or fuse is permitted to stay closed.  


I'd be ok with a smaller CPC provided faster disconnection (which is usually the norm with most MCBs), however I personally think this should be avoided on circuits 63 amps and below on the count high Ze, such as that of a generator hook up, can cause annealing and in turn future compromising of the CPC path.      


   

mapj1:

The disconnection time needs to be a fraction of a heartbeat period to significantly reduce the risk of fibrillation.

Animals with faster heartbeats, like mice and lab rats can be killed by shorter current impulses than humans..


The fact I say 'current' and not 'voltage' is significant.  Skin resistance of 5k-50k ohms per square cm of skin means that only a large area contact will get anywhere near the 30mA of the RCD threshold, in turn based on the lethality date. Shock current in terms of contact resistance is the single biggest variable in all this.


The fast disconnection will work with fuse or MCB, if the fault path is mostly metallic and the human is in parallel with some of that 'wire wound' resistance all will be well, RCD or not. The time an RCD is most useful is when the the human is in series with the fault path, or something else limits the current,  like  a liquid path rather than a metal one, as in that case the fault current will be utterly incapable of operating even the smallest fuse or breaker.

M.

 

Well, perhaps. AC-3 in Figure 1 of IEC-61200 does not indicate ventricular fibrillation, 5% at curve c2.


I fully agree with that rest though.

 Hence why RCDs in the UK were delayed into the 90s early 2000s. As such RCDs are not intended to be the primary means of protecting against indirect contact.And in general that should hold true for any supply. The primary means in my opinion should be in the ADS, achieved only though loop impedance ie high conductance.   

   

The U.K. has had the benefit of Earth Leakage Circuit Breakers for many decades before the 90s. Now called R.C.D.s in the case of the current operated types. Initially Voltage Operated types then current operated types were used. Or have I taken your comments out of context.


Of course B.S. 7671 requires R.C.D.s to be the only source of protection from shock/earth faults in special locations like caravan parks, marinas and farms etc. where TT earthing is specified.


Of course if I am in contact with a severed flex, with L in contact with my left hand, and N in contact with my right hand at the same time, I am a gonner despite what type of protective device is used, if I am wearing isolating shoes.


Z.

Zoomup:

 Hence why RCDs in the UK were delayed into the 90s early 2000s. As such RCDs are not intended to be the primary means of protecting against indirect contact.And in general that should hold true for any supply. The primary means in my opinion should be in the ADS, achieved only though loop impedance ie high conductance.   

   

The U.K. has had the benefit of Earth Leakage Circuit Breakers for many decades before the 90s. Now called R.C.D.s in the case of the current operated types. Initially Voltage Operated types then current operated types were used. Or have I taken your comments out of context.


Of course B.S. 7671 requires R.C.D.s to be the only source of protection from shock/earth faults in special locations like caravan parks, marinas and farms etc. where TT earthing is specified.


Of course if I am in contact with a severed flex, with L in contact with my left hand, and N in contact with my right hand at the same time, I am a gonner despite what type of protective device is used, if I am wearing isolating shoes.


Z.

Yes, but these were to deal with TT supplies. My understanding is that RCDs were not required on TN supplies until the late 90s.

My understanding is that RCDs were not required on TN supplies until the late 90s.

AJJewsbury:

My understanding is that RCDs were not required on TN supplies until the late 90s.

RCDs for supplementary protection have been gradually required for more and more situations over time. For sockets supplying equipment outdoors the 30mA RCD requirement goes back at least to the 15th Ed - so from about 1981.

   - Andy.

Ah, thank you for clearing this up :)


What was the IETs reasoning for outdoor sockets originally, if you know?

Zoomup:

Of course if I am in contact with a severed flex, with L in contact with my left hand, and N in contact with my right hand at the same time, I am a gonner despite what type of protective device is used, if I am wearing isolating shoes.

Z.

Not so in my experience; in my early years as a shift maintenance electrician in the steel industry, I had 2 severe shocks at 250V; one hand to hand across the chest, and 1 arm through leg. After the first one, I experienced muscular pain for about 3 days in the arms!. My labourer mate standing beside me, didn't even know I was being shocked; (one of the reasons why the electrical trade union insisted on an assistant helper for electricians at that time).


Jaymack.    ..