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Topic Title: What is the significance of 32 A and 20 A in automatic disconnection for shock protection?
Topic Summary: Regulation 411.3
Created On: 06 November 2017 08:40 AM
Status: Read Only
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 06 November 2017 08:40 AM
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Nakhil

Posts: 46
Joined: 11 April 2014

It says:

411.3.2.2 The maximum disconnection time stated in Table 41.1 shall be applied to final circuits not exceeding 32 A.

And:

411.3.3 Additional protection

In a.c. systems, additional protection by means of an RCD in accordance with Regulation 415.1 shall be provided for:

(i) socket-outlets with a rated current not exceeding 20 A that are for use by ordinary persons and are intended for general use, and
(ii) mobile equipment with a current rating not exceeding 32 A for use outdoors.


Why are the disconnection times only applicable to circuits not exceeding 32 A? Is there anything in the times or the protection is itself not needed for circuits exceeding 32 A?

Why RCD is required only for circuits not exceeding 20 A, and for mobile equipment whose rating does not exceed 32 A?
 06 November 2017 09:35 AM
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gkenyon

Posts: 4954
Joined: 06 May 2002

My understand of the idea of 411.3.2.2 cutoff, is that for circuits rated more than 32 A, there is a balance between the fact that it's more difficult to get loop impedances low enough to operate protective devices in the lower time, and the fact that the c.s.a of the protective conductor will be greater (and hence R2 lower).

With 411.3.3, it's important to understand that the requirement relates to a 30 mA RCD. Circuits rated greater than 32 A may well have protective conductor currents that will cause operation of a 30 mA RCD. So, a 30 mA RCD can't be mandated for circuits exceeding 32 A for reasons of practicality. This is again balanced against a theoretically lower R2 for higher-rated circuits, so this is my understanding of why an RCD with higher residual current rating has not been required.

The selection of 20 A socket outlet rating in this Regulation means that 13 A, 16 A and 20 A socket outlets are covered by 30 mA RCD, but 32 A commonly used for things like computer racks etc., that have high protective conductor currents, are not.
Interestingly, the Draft for Public Comment for the 18th Edition, aligned both parts of 411.3.3 to 32 A (although this does not mean this will be the actual 18th Edition wording ... we will have to wait until it is published to see that).

These ratings and disconnection times for these particular requirements are agreed internationally - I believe they are the same in the CENELEC Harmonized standards, and IEC 60364-4-41.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com

Edited: 06 November 2017 at 06:16 PM by gkenyon
 06 November 2017 10:05 AM
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tattyinengland

Posts: 987
Joined: 23 November 2006

My thoughts are:

See the note at the bottom of the table 41.1 - disconnection is not required for protection against electric shock but may be required for other reasons, such as protection against thermal effects.

So this table is for more that just protection against electric shock......

I think some your confusion around the additional protection requirement is that an RCD shall be provided for SOCKET OUTLETS not exceeding 20Amp. (You put circuits in your question).

This would encompass all 13 Amp normal 3 pin UK plugs and 20 Amp or less commando sockets (Single and three phase) that you generally could plug something small and movable into. Easily moved equipment. Multiple plugs may be found on a single circuit quite commonly.

The next plug available commercially in the UK I think is a 32 Amp and 63 Amp commando plug - which is unlikely to be for general purpose plug and play use, more likely it'll be a fixed bit of bigger kit, bigger earths, a single plug on a single circuit - - in which case the disconnections times can be a little longer - in a TN system this is 5 seconds - see 411.3.2.3.
 06 November 2017 10:08 AM
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tattyinengland

Posts: 987
Joined: 23 November 2006

wow I took a long time to reply - sorry - gkenyon was way faster than I - I got distracted half way through replying. His answer is probably better....
 06 November 2017 03:20 PM
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mapj1

Posts: 9553
Joined: 22 July 2004

In the old days, back when 3rd phase was green not blue, and earth leakages from electronics were yet to be invented (No, I;m not that old, but I have relatives who are, and many old books in the collection) there was an assumption that it was not reasonable for more than about 1 in 1,000 of your electrons to go astray on low pressure suppliesm, and 1 in 10,000 was a more normal running level.- this being essentially due to cable capacitances, and some insulation leakage, and the dimensions of cables and insulation thicknesses of the day. (being based on 1000 amps per square thumb, and gutta-percha probably.)

So a 100A supply may have a maximum of 100mA of leakage, but 10mA is more likely, a 30A supply 30mA of leakage but 3mA is a good result, etc.

This has all long since been washed overboard and replaced by more scientifically grounded considerations of shock currents and body resistance based on some dubious animal research, and then later based on papers based on giving shocks to American soldiers and students in the 1960s. (Charles F Dalziel and colleagues)

However, especially with capacitors acting as mains filters in electronics, the expected leakage has not gone down, and in some cases has gone up.
The real question is what is the largest circuit, or group thereof, on which you dare put a 30mA RCD. 32A seems to be the current answer. And I think the intention is also as noted above, a desire to catch most likely hand held devices under a 30mA or less RCD umbrella.

-------------------------
regards Mike
 06 November 2017 06:27 PM
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gkenyon

Posts: 4954
Joined: 06 May 2002

Originally posted by: mapj1
The real question is what is the largest circuit, or group thereof, on which you dare put a 30mA RCD. 32A seems to be the current answer. And I think the intention is also as noted above, a desire to catch most likely hand held devices under a 30mA or less RCD umbrella.
There's still sort of a disconnect between the standards for products, and the UK Wiring Regs.

e.g. BS EN 60950-1 permits a 16 A plug to be fitted to a product with high protective conductor currents (> 10 mA) ... but BS 7671 Reg 411.3.3 (in AMD 3 2015) says you have to have an RCD on the socket ... which may well trip ... unless, of course, you have a "risk assessment".

A little bit crazy really, to have one British Standard that permits a manufacturer to do something, and another British Standard that makes it difficult for the purchaser to use the product that complies with the first British Standard in the way the manufacturer intended ....

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
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