Table 41.1 Assumed Touch Voltage

ProMbrooke:

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 (IEC says 'withdrawn').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.      


   

BS 1363 socket-outlets, unfortunately, can be made to supply a Shuko or similar European plug, as I've seen many times over the past 30 years. Unfortunately, when someone does manage to make it work, the protective conductor connection is not made.


BS 7671 has now for many years permitted RCDs to provide the disconnection time required in Chapter 41. For that to happen, it must be the primary means of protection against electric shock. This was not really spelled out fully until the 17th Edition (BS 7671:2008). In many TT systems, there isn't an option, as fault currents may in theory be less than 1 A, so there is no choice? In TT systems, the touch voltage is more likely to be more, especially for equipment outdoors, or where what we used to know as an equipotential zone can't be established (e.g. plastic service pipes)


The issue of touch voltage has never been hidden. There is no limit to the touch voltage, only disconnection time. It's also true that you can touch hazardous live in a fault for up to 5 seconds in TN systems if the circuit is a distribution circuit, or rated > 63 A with socket-outlets, or rated > 32 A for fixed equipment.


The basic fact is that the likely scenario in TN systems is that the touch voltage will be much lower than U0. Not so in TN systems, but also recognised that what we used to know as an equipotential zone doesn't often exist in new installations. This is why by the time we get to the 2005-2007 IEC, TT systems have half the disconnection times (roughly) as TN systems based on what is Table 41.1 in BS 7671, and 2 s where TN systems have disconnection times of 5 s.


Explanations of what is going on with ADS have been published in a few places ... and amended accordingly over the years ... not least more recently in IET Guidance Note 5 Protection Against Electric Shock and the Commentary on the IET Wiring Regulations. But these are not the only places.




 

and 2 s where TN systems have disconnection times of 5 s