IET logo
 
IET
Decrease font size
Increase font size
Topic Title: fuse discrimination
Topic Summary:
Created On: 15 February 2013 08:22 AM
Status: Post and Reply
Linear : Threading : Single : Branch
<< 1 2 3 Previous Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 15 February 2013 03:31 PM
User is offline View Users Profile Print this message



misterben

Posts: 415
Joined: 11 June 2007

Ok will do that LOL
 15 February 2013 03:36 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Post him a link to this thread - that should do it -

OMS

-------------------------
Failure is always an option
 15 February 2013 03:44 PM
User is offline View Users Profile Print this message



rogersmith7671

Posts: 886
Joined: 04 November 2004

I see, a 100A BS 1361 supply fuse at the supply head, then a supply meter, which feeds a Wylex BS 5419 (BS88) switch fuse and then a single consumer unit to supply a single installation within the domestic property, yes?
 15 February 2013 03:52 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Originally posted by: rogersmith7671

I see, a 100A BS 1361 supply fuse at the supply head, then a supply meter, which feeds a Wylex BS 5419 (BS88) switch fuse and then a single consumer unit to supply a single installation within the domestic property, yes?


Thats it Roger - supply fuse in series with the consumers switch fuse (fuse) - no branches or other services, so basically either fuse operating causes the black out - to achieve discrimination you'd have to reduce the consumers fuse to 63A or below, and you wouldn't achieve anything as the consumer would still be off - and dropping that fuse to 63A is probably going to caue a real discrimination problem with the consumers downstream Type B or C 32A MCB's and even more so if there's a 45A shower MCB.

Regards

OMS

-------------------------
Failure is always an option
 15 February 2013 04:07 PM
User is offline View Users Profile Print this message



rogersmith7671

Posts: 886
Joined: 04 November 2004

Why bother with the Wylex BS 5419 (BS88) switch fuse? Just whip it out, then fuse discrimination would be moot and the installation would be just as hundreds of thousands of domestic installations in the UK. Problem solved,Eh?

regards
 15 February 2013 04:41 PM
User is offline View Users Profile Print this message



misterben

Posts: 415
Joined: 11 June 2007

The switch was already installed due to repositioning of the consumer unit. This is approx 6 metres from the main cutout, so it needs to stay in.

regards
Misterben
 15 February 2013 06:11 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Originally posted by: rogersmith7671

Why bother with the Wylex BS 5419 (BS88) switch fuse? Just whip it out, then fuse discrimination would be moot and the installation would be just as hundreds of thousands of domestic installations in the UK. Problem solved,Eh?

regards


I'm guessing the switch fuse is there as a DNO requirement as the sub main exceeds XXX metres - ie the Dist Bd is remote fromthe utility intake and meter position

Regards

OMS

-------------------------
Failure is always an option
 15 February 2013 06:40 PM
User is offline View Users Profile Print this message



rogersmith7671

Posts: 886
Joined: 04 November 2004

Now that the situation is clear, it's hard to see what it is that misterben's QS has a problem with. If the switch fuse (and presumably it's rating) is a DNO requirement then he should be aware of that. And not asking for changes to an EIC which are either not required or justifiable.

Regards
 18 February 2013 09:57 AM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1407
Joined: 11 July 2008

Really - try looking at ESQCR and then read regulation 434.3


So I have looked at the above and am struggling to find it in ESQCR, would you be so good as to point me to the reg number.

434.3
iv states that the distributor has to agree his device will provide protection.

This is approx 6 metres from the main cutout, so it needs to stay in.



I seem to remember a 3M rule of thumb with this, but the op states the switch is 6 meters away from cutout

Maybe this installation doesn't comply but for other reasons.

So shouldnt the DNO be contacted to confirm that thier protective device will provide protection for the 6M tails.
 18 February 2013 05:52 PM
User is offline View Users Profile Print this message


Avatar for UKPN.
UKPN

Posts: 462
Joined: 17 January 2012

-try reading ESQCR and then 434.3

why?

Regards
 19 February 2013 12:33 AM
User is offline View Users Profile Print this message



alancapon

Posts: 5745
Joined: 27 December 2005

Originally posted by: rogersmith7671
I think your QS is right to ask for discrimination in this case, as pointed out, the supply fuse is in series with your sub-main fuse . Your best approach should have been to provide adequate discrimination for the sub-main, at say, a junction between the supply fuse and any appropriate part of the main installation. At present the design cannot confirm that in the event of a fault the supply fuse will not disconnect before the sub-main, with all the obvious and possibly dangerous consequences for the main installation being plunged into darkness at a minimum.

I would disagree. Your QS is wrong. DNOs usually state that their cutout fuse will protect the first 3m of insulated and sheathed meter tails (some slightly more length, some less). After this distance, customer's overcurrent is required to protect the extended tails. This makes it easier for the DNO, as they do not have to do the calculations for the extended tails. If you design your submain for 100A, then fuse it at 100A. It doen't need to discriminate, it is only fitted due to the DNO's rules.

Regards,

Alan.
 19 February 2013 05:48 AM
User is offline View Users Profile Print this message



Thripster

Posts: 635
Joined: 22 August 2006

Do a google search for Bulrush Charts. Two topics under V......m's website will come up; one of which having a downloadable PDF file of the charts required.

Regards
 19 February 2013 10:38 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Originally posted by: UKPN

-try reading ESQCR and then 434.3

why?



Regards


Because Regulation 24 of ESQCR will tell you the distributor is responsible for protecting part of the consumers installation (ie it sets the legal requirement for DNO's to provide fuses or circuit breakers subject to the limits they ascertain (usually 3m tails length to avoid calculation) and 434.3 tells the consumer that the DNO has given permission for that protection as he's legally obliged to provide it

But you know that anyway.

Regards

OMS

-------------------------
Failure is always an option
 19 February 2013 10:44 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Originally posted by: zeeper

Really - try looking at ESQCR and then read regulation 434.3


So I have looked at the above and am struggling to find it in ESQCR, would you be so good as to point me to the reg number.

Regulation 24 (1) (c)


434.3

iv states that the distributor has to agree his device will provide protection.

Which is what Reg 24 does

This is approx 6 metres from the main cutout, so it needs to stay in.


I don't disagree - my comment was in response you stating that the suppliers fuse doesn't provide protection - Reg 24 indicates that clearly it does.

I seem to remember a 3M rule of thumb with this, but the op states the switch is 6 meters away from cutout

He does now


Maybe this installation doesn't comply but for other reasons.

I doubt it - the QS is clearly wrong


So shouldnt the DNO be contacted to confirm that thier protective device will provide protection for the 6M tails.

Why, there's a consumer fuse doing that - the DNO are only protecting the bit up to the switch fuse as they are legally obliged to do


Regards

OMS

-------------------------
Failure is always an option
 19 February 2013 11:06 AM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1407
Joined: 11 July 2008

Why, there's a consumer fuse doing that - the DNO are only protecting the bit up to the switch fuse as they are legally obliged to do


But the consumers fuse will only be providing overload protection.
 19 February 2013 11:29 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Originally posted by: zeeper

Why, there's a consumer fuse doing that - the DNO are only protecting the bit up to the switch fuse as they are legally obliged to do


But the consumers fuse will only be providing overload protection.


How so - if I put the silver stake through L and N 100mm downstream of the consumers fuse won't is see the same fault current that the DNO fuse does - of course it provides short circuit protection which is why we have the OP asking the question about discrimination ? Ditto for that silver stake across line and PE

Regards

OMS

-------------------------
Failure is always an option
 19 February 2013 01:56 PM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1407
Joined: 11 July 2008

How so - if I put the silver stake through L and N 100mm downstream of the consumers fuse won't is see the same fault current that the DNO fuse does - of course it provides short circuit protection which is why we have the OP asking the question about discrimination ? Ditto for that silver stake across line and PE


So you think that meets the requirements of 411.3.2.1, I think not.

If there was a fault at 5 meters alone the six meter tail its not going to pop the consumer fuse.

And the dno fuse may not provide protection because we didnt check.
 19 February 2013 02:06 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

Originally posted by: zeeper

How so - if I put the silver stake through L and N 100mm downstream of the consumers fuse won't is see the same fault current that the DNO fuse does - of course it provides short circuit protection which is why we have the OP asking the question about discrimination ? Ditto for that silver stake across line and PE


So you think that meets the requirements of 411.3.2.1, I think not.

If there was a fault at 5 meters alone the six meter tail its not going to pop the consumer fuse.

And the dno fuse may not provide protection because we didnt check.


Are you reading things differently to me Zeeps - DNO fuse, meter, consumers switchfuse with BS 88 100A fuse, 6m run to client consumer unit is what we have.

Regards

OMS

-------------------------
Failure is always an option
 19 February 2013 03:02 PM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1407
Joined: 11 July 2008

If the customer has a 100amp cutout and the submains has a 100 fuse then you would want the submains to blow first, but surely downgrading it to say an 80amp could potentially leave the customer short of 20amps.



The switch was already installed due to repositioning of the consumer unit. This is approx 6 metres from the main cutout, so it needs to stay in.


Yeah looks like it

I was thinking, cutout, 6m run, switch fuse,
 19 February 2013 04:00 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19430
Joined: 23 March 2004

OK - so now we agree on the installation arrangement, clearly the DNO fuse(100A BS 1361) is providing protection up to the switch fuse (ESQCR Reg 24 and BS 7671 432.3(iv)) subject to limits on length and conductor size.

The BS 88 fuse (100A) is protecting the sub main

Discrimination under overlaod, short circuit and earth fault cannot be achieved, and it's pointless anyway - regradless of which device operates, the installation is off supply

To achieve discrimination woud mean reducing the consumer capacity to at most 63A (to grade with the 100A BS 1361) and to do that would also cause a real discrimination problem with the consumers distribution board circuit breakers (536.2) - a 32A Type C is probably not going to clear before the 63A BS 88 has gone beyond the pre arcing I2t

Regards

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » fuse discrimination

<< 1 2 3 Previous Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



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