IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Disconnection times
Topic Summary: just wondering
Created On: 10 February 2011 09:59 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 10 February 2011 09:59 PM
User is offline View Users Profile Print this message



PatHammond

Posts: 215
Joined: 04 December 2008

Why do final circuits greater than 32A have a 5s disconnection time in TN systems whereas those less than 32A have to disconnect in 0.4s

-------------------------
4.5 day wonder...had to start somewhere
 10 February 2011 10:27 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

Because such circuits are not generally used for mobile equipment.
Or what used to be known as portable equipment.
 10 February 2011 10:31 PM
User is online View Users Profile Print this message



daveparry1

Posts: 6226
Joined: 04 July 2007

That's the only reason I know of too Pat! >32amp wouldn't normally be used for hand-held or portabvle equipment.
 10 February 2011 10:44 PM
User is offline View Users Profile Print this message



PatHammond

Posts: 215
Joined: 04 December 2008

so why include lighting circuits and or immersion circuits; why not just make it applicable to circuits incorporating socket outlets

-------------------------
4.5 day wonder...had to start somewhere
 10 February 2011 10:52 PM
User is online View Users Profile Print this message



daveparry1

Posts: 6226
Joined: 04 July 2007

Who knows Pat, i'd guess it was to make things simpler, easier to say everything up to 32a - 0.4sec than having differing disc times for different circuits. (don't forget TT- 0.2 secs)
 10 February 2011 11:07 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

Perhaps lighting circuits are now included because ordinary persons change light bulbs not knowing whether the light is on or off.
Or it could be that it is quite acceptable to have mobile equipment connected to lighting circuits.
 10 February 2011 11:41 PM
User is offline View Users Profile Print this message



PatHammond

Posts: 215
Joined: 04 December 2008

thinking about it I'm sure I saw some 1960s kitchen sink drama with some housewife with an iron plugged in to a lighting bayonet dangling down above the ironing board. Perhaps this sort of malarkey still goes on up North

-------------------------
4.5 day wonder...had to start somewhere
 11 February 2011 12:10 AM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

I wouldn't know, as I live in London.
Quite posh in London dontc'ha know.
They even have floor lamps (I believe they're refered to as standard),
and these are plugged into 3A or 5A round 3 pin socket-outlets.
 11 February 2011 09:36 AM
User is offline View Users Profile Print this message


Avatar for gkenyon.
gkenyon

Posts: 4478
Joined: 06 May 2002

Originally posted by: PatHammond

so why include lighting circuits and or immersion circuits; why not just make it applicable to circuits incorporating socket outlets
BS7671 (17th Edition) removed requirements fully mandating bathroom bonding, providing certain other conditions are met. Since the type of circuits you describe here are used in domestic or similar situations, I agree, with BS7671 as it is today, they should be on the lower disconnection time. Reason is as follows:

I'm a little skeptical about the situation where you've an existing installation, with some circuits on 5s, and decide to refurb the bathroom. So you RCD protect circuits serving the new bathroom, install to 17th, but leave in the existing copper pipes. BS7671 says you don't need to bond, provided all the conditions are met (including adequate main bonding).

I think there's still a possibility that under some conditions, a fault on one of these 5s disconneciton time circuits outsite the bathroom, but with earthed parts in contact with copper piping, could transfer a potential of above 50 V ac for the duration of the fault (5s) into the bathroom.

So personally, I'd prefer as far as practicable, for all circuits in a dwelling at least to be on short disconnection times. And I'd caution to leave bathroom bonding in (you've still got the option to do so) in copper-piping installations, where there's a possibility of other circuits with 5s disconnection time in the property, that may be in contact, even if only fortuitously, with the copper piping.

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
 11 February 2011 09:47 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19693
Joined: 23 March 2004

So personally, I'd prefer as far as practicable, for all circuits in a dwelling at least to be on short disconnection times. And I'd caution to leave bathroom bonding in (you've still got the option to do so) in copper-piping installations, where there's a possibility of other circuits with 5s disconnection time in the property, that may be in contact, even if only fortuitously, with the copper piping.


Not just in a domestic Graham - I suspect the potential for 5 second disconnection times to be even greater in a commercial environment (if only due to the loads involved or the distances being considered) - I guess the classic example would be a HWS system that uses a large electric immersion for summer use when the boilers are down. You wouldn't want to be in an unbonded bathroom with a faulty central immersion protected by fuses regardless if the circuits of the location were on an RCD.

Regards

OMS

-------------------------
Failure is always an option
 11 February 2011 01:51 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11502
Joined: 13 August 2003

I'm a little skeptical about the situation where you've an existing installation, with some circuits on 5s, and decide to refurb the bathroom. So you RCD protect circuits serving the new bathroom, install to 17th, but leave in the existing copper pipes. BS7671 says you don't need to bond, provided all the conditions are met (including adequate main bonding).

I think there's still a possibility that under some conditions, a fault on one of these 5s disconneciton time circuits outsite the bathroom, but with earthed parts in contact with copper piping, could transfer a potential of above 50 V ac for the duration of the fault (5s) into the bathroom.

That was one of my fears when the DPC came out - however one of the conditions in the final version is not just that main bonding is in place (i.e. connected where they enter the building), but that extraneous-conductive-parts of the location (as distinct from the installation) are effectively connected to the main bonding. Along with the note that "effectiveness" means complying with the requirements for supplementary bonding - i.e. R <= 50/Ia. So in theory all should be well.

But for this to work, it needs to be understood that metallic parts that perhaps aren't extraneous-conductive-parts from the point of view of the installation might still be extraneous-conductive-parts from the point of view of the bathroom. Classic example would be hot water pipework that's isolated from the bonded cold pipes by a plastic header tank.

Ia relates to any device which could impose a touch voltage in the location - even if the circuit itself never enters the location. E.g. a water heater in another room.

- Andy.
 12 February 2011 09:56 AM
User is offline View Users Profile Print this message


Avatar for gkenyon.
gkenyon

Posts: 4478
Joined: 06 May 2002

Originally posted by: AJJewsbury
That was one of my fears when the DPC came out - however one of the conditions in the final version is not just that main bonding is in place (i.e. connected where they enter the building), but that extraneous-conductive-parts of the location (as distinct from the installation) are effectively connected to the main bonding. Along with the note that "effectiveness" means complying with the requirements for supplementary bonding - i.e. R <= 50/Ia. So in theory all should be well.
Agreed - but I'm still not 100% happy - read on.

But for this to work, it needs to be understood that metallic parts that perhaps aren't extraneous-conductive-parts from the point of view of the installation might still be extraneous-conductive-parts from the point of view of the bathroom. Classic example would be hot water pipework that's isolated from the bonded cold pipes by a plastic header tank.
Yes - and what happens when some copper is replaced with plastic after you've gone (or vice-versa). The designer needs to think about what may reasonably happen in maintenance, surely?

It's generally taken as read that the plumbing isn't part of the electrical installation: but in this special case, it definitely is.

Hopefully, however, the number of installations that will be affected will be very low.

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.