ProMbrooke:

gkenyon:

There is no limit to touch voltage in IEC 60364-4-41 for TN systems (as today)


For TT systems, same disconnection times, but touch voltage limited to 50 V by equipotential bonding. This is not the same as today.


The disconnection times in Table 41 A (TN systems) are AC only but otherwise as they are for TN systems today, way back to 1992 at least, perhaps before ... As below for the 1999 Ed for example:


U0        Disconnection time (s)

120           0.8

230           0.4

277           0.4

400           0.2

>400         0.1

So where are they getting wet locations from?

I don't understand.


Compare then with now (Section 701, or IEC 60364-7-701).

Then (pre BS 7671:2008): Disconnection times as per Table 41A, PLUS supplementary local equipotential bonding [to achieve local touch voltage up to 50 V]

Now (BS 7671:2008 onwards): Disconnection times as per Table 41.1, PLUS additional protection of all circuits serving, and passing through zones 1 and 2 of the location but not serving the location, by 30 mA RCD either:

• supplementary local protective bonding [to achieve local touch voltage up to 50 V];
  
  OR

• main protective bonding PLUS confirmation that all extraneous-conductive-parts of the location are connected to the main protective bonding PLUS additional protection of all final circuits serving the location by 30 mA RCD (but this last provision doubles up on 701.411.3.3 a little; however, consider additions and alterations to existing installation).

ProMbrooke:

Yes ... these ignore the supplementary protective measures in Sections in Part 7. As per previous post.


So Section 701, before 2008, limit of touch voltage = 50 V, so disconnection time = 5 s as per your table for humid condition above.  After 2008, either limit of touch voltage = 50 V, or RCD protection = better disconnection time. Mains not permitted in bathtub (zone 0) itself.

mapj1:
Hang on ... in 2001 edition, Table 41C (48A) is NOT an equivalent of what we have now in Table 41.1.


My apologies - I do not have the  copies to compare here, and the notes implied it was the same.  (and given even the 1977  version is 100 CHF it will be some time before we do have all possible standards here, an up to date copy of a few of them is quite enough...) However, it is interesting to know where the figures come from, as there are clearly some assumptions hidden . I will edit my post to delete the implication that it is the same.

Mike.

There appear to be assumptions, some of which I may humbly not agree with.


IMO these values need to be revisited and discussed further. 

gkenyon:

ProMbrooke:

Yes ... these ignore the supplementary protective measures in Sections in Part 7. As per previous post.


So Section 701, before 2008, limit of touch voltage = 50 V, so disconnection time = 5 s as per your table for humid condition above.  After 2008, either limit of touch voltage = 50 V, or RCD protection = better disconnection time. Mains not permitted in bathtub (zone 0) itself.

Would these be for mainland Europe where bathrooms may not have supplemental bonding?


I personally would not rely on bonding alone in wet locations, in particular exterior sockets where earth mat is not practical for a user.

ProMbrooke:

mapj1:
Hang on ... in 2001 edition, Table 41C (48A) is NOT an equivalent of what we have now in Table 41.1.


My apologies - I do not have the  copies to compare here, and the notes implied it was the same.  (and given even the 1977  version is 100 CHF it will be some time before we do have all possible standards here, an up to date copy of a few of them is quite enough...) However, it is interesting to know where the figures come from, as there are clearly some assumptions hidden . I will edit my post to delete the implication that it is the same.

Mike.

There appear to be assumptions, some of which I may humbly not agree with.


IMO these values need to be revisited and discussed further. 

There is definitely no problem doing that.


The basic standards for protection against electric shock, used by product and installation standards committees, are IEC 61140 (BS EN 61140), and IEC 60479 series (BS IEC 60479 series, although two of these standards are Published Document/Technical Reports rather than full standards).


It's probably best to discuss comments/suggestions in that direction.


You are fully correct that the provisions in Chapter 41 are dry conditions only. BS 7671 then has Part 7 for locations of increased shock risk.


Hence, you will not find anything to do with humid and wet location in Chapter 41.


In terms of hazard treatment to reduce risk in Part 7, in exactly the same way that we manage other risks, we use the Hierarchy of Control, which is per the following order:

1. Eliminate

• Substitute

• Engineering controls

• Administrative controls

• PPE

mapj1:
...IMO these values need to be revisited and discussed further.


I'd like to know why you say that, though you may well be right, after all 

"because another fancy document you have not read says so" is no guarantee of validity.  (*)


Ideally such tables should indicate their sources, but that is not much good if that too is another reference or unavailable.

In the favour of the current situation, there are not enough electrocutions to indicate that the current assumptions are widely off the mark, or if they are, then not in a dangerous direction.


Mike.


*

I am reminded of the fiasco of "coefficient of friction", the idea being that the friction force is proportional to contact area and pressure there is a constant factor for any  given material.

Some numbers were tabulated by some lazy experimenters in the late 1800s, and copied without imagination into many engineering textbooks well into the 2nd half of the 20th century..

For nearly a century school children grew up learning about this "fact", and also learning that in practical labs, that in some way they could never re-create the data book values except for one very contrived experiment.  Not until the late 1960s and part of the work for the US space program were tests re-done and the whole idea of linearly pressure dependent friction largely discredited. It is just vastly more complex than that. Some things are sticky with no applied pressure (jam and glue come to mind), and some are almost independent of it up to some critical value then bind, almost anything with an oily feeing surface, most things take more to get them moving than to keep them in motion, once the starting 'stiction' is overcome.. Oh there are many errors to the simple model.


 

These values seem to be based on 0.8 multiplier relying on the sag at the trafo's terminals plus the MET being bonded to any conductive parts within the premises. However, the calculations I'm running show only a few volts difference of exposure between remote earth vs bonded metal like pipes and rebar.


It is funny you mention hidden inference- AFDDs come to mind but that is a different thread altogether.


 

gkenyon:

ProMbrooke:

mapj1:
Hang on ... in 2001 edition, Table 41C (48A) is NOT an equivalent of what we have now in Table 41.1.


My apologies - I do not have the  copies to compare here, and the notes implied it was the same.  (and given even the 1977  version is 100 CHF it will be some time before we do have all possible standards here, an up to date copy of a few of them is quite enough...) However, it is interesting to know where the figures come from, as there are clearly some assumptions hidden . I will edit my post to delete the implication that it is the same.

Mike.

There appear to be assumptions, some of which I may humbly not agree with.


IMO these values need to be revisited and discussed further. 

There is definitely no problem doing that.


The basic standards for protection against electric shock, used by product and installation standards committees, are IEC 61140 (BS EN 61140), and IEC 60479 series (BS IEC 60479 series, although two of these standards are Published Document/Technical Reports rather than full standards).


It's probably best to discuss comments/suggestions in that direction.


You are fully correct that the provisions in Chapter 41 are dry conditions only. BS 7671 then has Part 7 for locations of increased shock risk.


Hence, you will not find anything to do with humid and wet location in Chapter 41.


In terms of hazard treatment to reduce risk in Part 7, in exactly the same way that we manage other risks, we use the Hierarchy of Control, which is per the following order:

1. Eliminate

• Substitute

• Engineering controls

• Administrative controls

• PPE

Consider Section 701 example given above. The controls applied are Eliminate (no mains and limits on SELV/PELV voltages in Zone 0), and either eliminate (bonding) or engineering controls (RCD) in Zones 1 and 2. Hence, disconnection time need not enter into it.

Right, but what has me perplexed are the lack of tables in part 7. Exception being medical locations however this should apply to all kitchens, bathrooms, laundry areas, garages, unfinished basements, exterior sockets, pool circuits and the like- anywhere skin conductivity is lowered typically through moisture or large earthed surfaces.


Bonding and RCDs should not be the primary means to achieve protection in these areas. Rather a pyramid approach should be taken imo. Remote earth is impossible to avoid in all scenerios. RCDs have much higher failure rates compared to MCBs. Yet ADS remains the most simple, assured and reliable way of protecting life and property.

ProMbrooke:

Would these be for mainland Europe where bathrooms may not have supplemental bonding?


I personally would not rely on bonding alone in wet locations, in particular exterior sockets where earth mat is not practical for a user.

Roughly harmonized across Europe, minor differences as socket-outlets are permitted in Zone 1 etc. - RCD protection gets you to 0.04 s, so OK. A slightly more cautious approach here in the UK, but there all the same.


I don't really see an issue for "wet" or "humid" as we either apply bonding, or use RCD in the UK.



Exterior locations are not classed as "wet" particularly in BS 7671. But your point is fair and definitely considered. RCD protection is used for outdoor socket-outlets and many items of outdoor equipment, so disconnection time is 0.04 s (Regulation 411.3.3). Larger circuits with Class I products, expect larger CPC and perhaps lower touch voltages ... and so on with that sort of thinking.