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Topic Title: No more DP Switches??
Topic Summary: including 14th Ed rant!
Created On: 27 November 2014 10:53 AM
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 27 November 2014 10:53 AM
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Shortercircuit

Posts: 21
Joined: 01 July 2011

I was brought up under the 14th Ed. when life was oh so simple! the Regs book itself was handy sized and written as an instruction manual by those on high telling the likes of me exactly what to and what not to do.
You were expected to memorise as much of the contents as possible - how well I remember our instructors ('board training school) barking the question at some poor individual (at any juncture) 'name the first 5 sections of the wiring regs laddy' (none of your christian names stuff) - we could soon all recite them at the drop of a hat. You were certainly not allowed to take the book into exams.
Compare that instruction manual with todays edition - gradually rewritten as a book so complicated that most of the content of your C&G up-date course is based on navigation of the book rather than understanding the content. Surely it speaks volumes when over 40% of the book are appendices and the Contents do not even refer to page numbers??
I delve into this most expensive paperback infrequently usually to confirm that long held beliefs are still valid and to check the new latin terminology for things I like to call earth and make sure my CPCs have not reverted back to ECCs or perish the thought just plain earth wires.

Anyway enough of the ranting and back to DP switches! - whilst seeking to confirm that every 'appliance & luminaire 'should still have a (separate) switch which in the case of heating elements (touchable inc silica sheathed) should be DP etc etc - surprise I cant find the Reg I used to know. Does 537.2.1.1 realy do away with the need for any DP switches in most 'regular' installations other than the main switch? 537.1.4 (nice to see those on high still writing some Regs and not just 'ordinary' people) Have I missed somthing? is there a new latin term for double pole that got past me? are we over engineering our sw/spurs, cooker switches, fan isolators etc - could be usefull in the case of shower pull cords - we could link the DP making a 90A SP which could reduce the burn outs!!!
Seriously though is the old requirement for double pole switching lurking somewhere in the 17th - any thoughts welcome. thanks
 27 November 2014 11:08 AM
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AJJewsbury

Posts: 15849
Joined: 13 August 2003

Seriously though is the old requirement for double pole switching lurking somewhere in the 17th

Not for TN systems as far as I know - but still there for TT (and the main switch for single phase domestics, as you say).

Appliance things are now covered by separate appliance standards - if a particular appliance did require DP isolation, I expect it would be in the manufacturer's instructions - which BS 7671 does oblige us to follow. Many appliances instructions (e.g. boilers) demand DP isolation regardless - although there is a school of thought that suspects that's just down to the manufacturers not wanting to have to explain the difference between TN and TT systems.

- Andy.
 27 November 2014 11:17 AM
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Parsley

Posts: 1353
Joined: 04 November 2004

As you have alresady stated In a domestic set up there is 537.1.4 that requires a DP main switch.
If the installation's earthing system is TN and non domestic there is no need to switch the neutral conductor see 537.1.2 and 537.2.1.1 and even emergency switching doesn't require it 537.4.1.2. Functional switching of neutral isn't required see 537.5.1.2.

So yes there doesn't seem to be any need for it in 7671. But appliance manufacture's instructions may require it.

Regards
 27 November 2014 01:31 PM
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Zoomup

Posts: 3286
Joined: 20 February 2014

Originally posted by: Shortercircuit

I was brought up under the 14th Ed. when life was oh so simple! the Regs book itself was handy sized and written as an instruction manual by those on high telling the likes of me exactly what to and what not to do.

You were expected to memorise as much of the contents as possible - how well I remember our instructors ('board training school) barking the question at some poor individual (at any juncture) 'name the first 5 sections of the wiring regs laddy' (none of your christian names stuff) - we could soon all recite them at the drop of a hat. You were certainly not allowed to take the book into exams.

Compare that instruction manual with todays edition - gradually rewritten as a book so complicated that most of the content of your C&G up-date course is based on navigation of the book rather than understanding the content. Surely it speaks volumes when over 40% of the book are appendices and the Contents do not even refer to page numbers??

I delve into this most expensive paperback infrequently usually to confirm that long held beliefs are still valid and to check the new latin terminology for things I like to call earth and make sure my CPCs have not reverted back to ECCs or perish the thought just plain earth wires.



Anyway enough of the ranting and back to DP switches! - whilst seeking to confirm that every 'appliance & luminaire 'should still have a (separate) switch which in the case of heating elements (touchable inc silica sheathed) should be DP etc etc - surprise I cant find the Reg I used to know. Does 537.2.1.1 realy do away with the need for any DP switches in most 'regular' installations other than the main switch? 537.1.4 (nice to see those on high still writing some Regs and not just 'ordinary' people) Have I missed somthing? is there a new latin term for double pole that got past me? are we over engineering our sw/spurs, cooker switches, fan isolators etc - could be usefull in the case of shower pull cords - we could link the DP making a 90A SP which could reduce the burn outs!!!

Seriously though is the old requirement for double pole switching lurking somewhere in the 17th - any thoughts welcome. thanks


Nice rant Shorter Circuit. You have my vote. I must admit to never having considered single pole switching being right for cookers, showers and switched fused connection units generally.

BUT, I like the idea of double pole switching for cleaning and maintenence purposes of appliances. Cos if I was to disconnect say, a cooker, to undertake a repair by switching off the cooker's single pole miniature circuit breaker then I may still find a neutral to earth potential inside the cooker, if a single pole isolator only is used.

Bye,

Z.
 27 November 2014 01:46 PM
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AJJewsbury

Posts: 15849
Joined: 13 August 2003

I may still find a neutral to earth potential inside the cooker, if a single pole isolator only is used.

Indeed - but it shouldn't be hazardous in a TN system. Even worst case TN-S, taking in account v.d. along the N of both the supply and installation it should be well below 25V.

DP switching does have advantages from the nuisance point of view though where there are upstream RCDs - if either maintenance or an appliance fault risks shorting N to PE.

- Andy.
 27 November 2014 03:27 PM
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rogerbryant

Posts: 979
Joined: 19 July 2002

As I think I have said before here in Switzerland the circuits have single pole MCBs but mounted next to them is a half width neutral isolator. The handle is arranged so that the neutral isolator can only be opened when the MCB is open.

These are very helpful in avoiding spurious RCD trips when changing light fittings and sockets.

Best regards

Roger
 27 November 2014 05:59 PM
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weirdbeard

Posts: 3116
Joined: 26 September 2011

Single pole switching is sometimes better......from Wiring Matters 2006:


12. Double-pole switching. Doublepole
switching within the fixed wiring
is known to trip an RCD when
switching off or on due to capacitive
effects. Changing over from doublepole
to single-pole switching can
overcome the problem, where such
replacement is permissible and safe


http://electrical.theiet.org/w...tand-rcds.cfm?type=pdf

-------------------------
:beer)
 13 January 2015 09:34 AM
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TMHajaSahib

Posts: 35
Joined: 02 September 2014

A DP (isolator) unlike a MCB (miniature circuit breaker) affords a degree of protection during high voltage fault events such as a voltage surge in that the spark may not be able to jump across the gap in the DP in open position and do the damage to the equipment connected to the DP.

Edited: 13 January 2015 at 10:03 AM by TMHajaSahib
 13 January 2015 10:12 AM
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mapj1

Posts: 9371
Joined: 22 July 2004

Hmm. If I've got the back off that cooker after have 'isolated' it and then drop power to half the house when the pliers bridge N to the case and an RCD pops, I don't think we have won very much.

Furthermore if the only way to make it safe and break L and N is at the CU incomer, then suddenly we need to work by torchlight to install a cooker.

Equally in PME world you might prefer a 3 pin plug and socket !!

I don't see DP switching for isolation being removed any time soon, whatever the regs writers think.

That RCD article is interesting, and actually I think I may well disagree with it to a degree.
I shall have a bit more of a think through possible faults first though.

-------------------------
regards Mike
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