IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Undersized cables
Topic Summary: Atlas Kablo
Created On: 03 October 2017 01:21 PM
Status: Read Only
Linear : Threading : Single : Branch
1 2 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.
 03 October 2017 01:21 PM
User is offline View Users Profile Print this message



rogerbryant

Posts: 980
Joined: 19 July 2002

This article was on the BBC web page this morning:

http://www.bbc.com/news/uk-41391343

I remember reading a little about it at the time, probably on here. How much undersized were the cables, is it a real problem? In most domestic installations the cables are not fully loaded for long periods. Would a Zs test show if it was a problem? This should confirm that the OPD is correctly sized to protect the circuit. (I know that most installations would be just big bang tested ;-) )

Best regards

Roger
 03 October 2017 02:29 PM
User is offline View Users Profile Print this message



KFH

Posts: 567
Joined: 06 November 2010

I thought it interesting that the expert believed the danger of a fire was 8/10. I don't think I put any of this stuff in but how would I know? I heard something about it at the time and thought then that I had not installed any. But I know I have put in some of their cable, possibly SWA, was that also affected?

I think someone should say by how much the cables were undersized, I would expect that if it was significant someone may have noticed when installing. Interesting that the retailer did not think it was much of an issue.
 03 October 2017 02:30 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16104
Joined: 13 August 2003

Would a Zs test show if it was a problem?

Unlikely, unless you happened to know the precise length of the cable (and had a better than average loop tester, especially if on a non-trip setting).

This should confirm that the OPD is correctly sized to protect the circuit.

If might be protected against faults - but not necessarily against overloads (i.e. loads above the actual (reduced) CCC). Even faults might be a bit dodgy as there's a min c.s.a. needed to pass the adiabatic as well as Zs to consider.

- Andy.
 03 October 2017 03:55 PM
User is offline View Users Profile Print this message



sparkingchip

Posts: 10180
Joined: 18 January 2003

On the face of it this is just disgraceful scaremongering by the BBC.

In seven years how many issues have any of you found with over heated cables as working electricians and inspectors?

There is absolutely no way there can be a recall in this country as no one knows where this cable is and a general inspection of a installation without dismantling the fabric of the building will not even reveal who the manufacturer of the installed cables were.

Let the BBC report again when they have actually identified an installation where the has been a issue with these cables.

Andy Betteridge.
 03 October 2017 04:03 PM
User is online View Users Profile Print this message



broadgage

Posts: 2452
Joined: 07 August 2007

IIRC the reduced conductor size will not be a problem in the majority of cases, most cables in domestic use are indeed lightly loaded if compared to their nominal rating.

For example domestic lighting is commonly wired in 1.0mm on a 6 amp MCB. If the allegedly 1.0mm cable is in fact 0.8mm, then that is still good for 6 amps.

Domestic small power is commonly wired in 2.5mm, as either a 20 amp radial or a 32 amp ring final. If the alleged 2.5mm is actually 2.0mm, then that should be good for over 20 amps.

So MOST installations should be fine, but the worry is the small percentage that are already marginal and could be pushed "over the edge"
Consider for example two ring finals bundled together and both slightly overloaded to 35 amps. The 32 amp OCPD will pass 35 amps almost forever, and four cables bundled together and each carrying 17 or 18 amps will get very warm and perhaps dangerously hot.
Even worse would be a circuit that is ALREADY wrongly installed, but goes from a "bit rough but probably ok in practice" to "dangerous" as a result of undersized cable.
For example it is regrettably common to use 2.5mm twin with earth on a 32 amp MCB to supply a cooker or electric shower.
Incorrect in most cases, but generally OK in practice. If however the cable was only 2.0mm then an already poor choice could become actually dangerous.
 03 October 2017 04:04 PM
User is offline View Users Profile Print this message



sparkingchip

Posts: 10180
Joined: 18 January 2003

 04 October 2017 10:49 AM
User is offline View Users Profile Print this message



davezawadi

Posts: 3955
Joined: 26 June 2002

It is scaremongering in my opinion too. Whilst there is a risk of higher conductor temperatures, the difference will not be very large, and suggesting this is a serious fire risk is ridiculous. A 20% reduction in conductor area does not lead to 20% greater temperature rise, and even if it did then 84 degrees C maximum will not cause any kind of problem except accelerated degradation of the PVC, which happens anyway.

If BASEC does not realise this too, they should not be in business!
However the BBC should be sanctioned for poor reporting, exaggeration and political bias now seem to be the order of the day!

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 04 October 2017 02:06 PM
User is offline View Users Profile Print this message



mikejumper

Posts: 2466
Joined: 14 December 2006

I'm not convinced the fire risk is as great as the 'experts' make out.
 04 October 2017 04:26 PM
User is offline View Users Profile Print this message



davidwalker2

Posts: 306
Joined: 29 April 2009

I agree. The media seem to wheel out these tame "experts" to make the point they wish to make. Last night it was suggested that hot cable would ignite plasterboard. Last time I put a blow-lamp to plasterboard resulted in scorching of the paper, but certainly not fire.

David
 04 October 2017 04:30 PM
User is offline View Users Profile Print this message



davidwalker2

Posts: 306
Joined: 29 April 2009

It would help if BASEC would specify exactly what was wrong with the cable so that professionals could decide if it was a hazard in any particular installation. "Insufficient copper" is hardly helpful.

David
 04 October 2017 04:46 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16104
Joined: 13 August 2003

It would help if BASEC would specify exactly what was wrong with the cable so that professionals could decide if it was a hazard in any particular installation. "Insufficient copper" is hardly helpful.

Agreed. A lot depends on the "shape" of the missing copper - if it's missing evenly from the whole length of the conductors (i.e. deliberate attempt to save copper by using a smaller c.s.a.) then the risk is probably small and detection reasonably easy (by resistance testing or weighing), but if it's an occasional thinning of the wire in relatively short lengths (e.g. by poor quality machines occasionally over-stretching the wire) - giving say 99m of full size copper then 1m of half-size then a similar missing overall percentage would be much more significant and harder to spot.
- Andy.
 05 October 2017 06:55 AM
User is offline View Users Profile Print this message



rogerbryant

Posts: 980
Joined: 19 July 2002

Thank you for the replies and thoughts. I wondered if there had been an incident in the UK that had prompted the BBC piece.

Best regards

Roger
 05 October 2017 09:40 AM
User is online View Users Profile Print this message



broadgage

Posts: 2452
Joined: 07 August 2007

I am not aware of any serious consequences from installing and then using this undersized cable, but would we know about such incidents ?

There are many house fires every year and some are fatal, but AFAIK samples of electric cable are not taken from the scene and examined for correct CSA, so how would we know ?

And if a reasonably competent DIY er found undersized cable due to it become alarmingly hot, I suspect that many would simply replace it and tell no one for fear of the part pee police.
 05 October 2017 11:06 AM
User is online View Users Profile Print this message



broadgage

Posts: 2452
Joined: 07 August 2007

By "insufficient copper" they might mean cable of the correct cross section, but manufactured from impure copper.
Even a small percentage of other metals greatly increases the resistance. For this reason, copper for electrical purposes SHOULD be pure "electrolytic" copper that is very pure.

I suspect some cheap cable might be made from scrap copper that has been melted in a furnace, cast into ingots, and then drawn into wire without ever being electrolyticaly refined.
It would take only small traces of other metals such as brass terminals, soldered joints, or tinned copper wire in the mix for the resulting wire to have a significantly higher resistance than pure copper.
 08 October 2017 09:14 AM
User is offline View Users Profile Print this message



Zoomup

Posts: 3430
Joined: 20 February 2014

Originally posted by: broadgage

IIRC the reduced conductor size will not be a problem in the majority of cases, most cables in domestic use are indeed lightly loaded if compared to their nominal rating.



For example domestic lighting is commonly wired in 1.0mm on a 6 amp MCB. If the allegedly 1.0mm cable is in fact 0.8mm, then that is still good for 6 amps.






Domestic small power is commonly wired in 2.5mm, as either a 20 amp radial or a 32 amp ring final. If the alleged 2.5mm is actually 2.0mm, then that should be good for over 20 amps.



So MOST installations should be fine, but the worry is the small percentage that are already marginal and could be pushed "over the edge"

Consider for example two ring finals bundled together and both slightly overloaded to 35 amps. The 32 amp OCPD will pass 35 amps almost forever, and four cables bundled together and each carrying 17 or 18 amps will get very warm and perhaps dangerously hot.

Even worse would be a circuit that is ALREADY wrongly installed, but goes from a "bit rough but probably ok in practice" to "dangerous" as a result of undersized cable.

For example it is regrettably common to use 2.5mm twin with earth on a 32 amp MCB to supply a cooker or electric shower.

Incorrect in most cases, but generally OK in practice. If however the cable was only 2.0mm then an already poor choice could become actually dangerous.


If the cable is sold as a certain size then it should be a certain size, not smaller. It is illegal to mis-describe goods sold, and we can not assume that it will be lightly loaded.

Z.
 08 October 2017 09:18 AM
User is offline View Users Profile Print this message



Zoomup

Posts: 3430
Joined: 20 February 2014

Originally posted by: davezawadi

It is scaremongering in my opinion too. Whilst there is a risk of higher conductor temperatures, the difference will not be very large, and suggesting this is a serious fire risk is ridiculous. A 20% reduction in conductor area does not lead to 20% greater temperature rise, and even if it did then 84 degrees C maximum will not cause any kind of problem except accelerated degradation of the PVC, which happens anyway.



If BASEC does not realise this too, they should not be in business!

However the BBC should be sanctioned for poor reporting, exaggeration and political bias now seem to be the order of the day!


I disagree, if a cable is sold as a certain size then it should legally be that physical size. It is not up to the cable maker to assume a certain "diversity". 1.0mm2 cable should be in fact 1.0mm2 C.S.A.

Would you like to get short changed at the petrol pumps and only get 0.8 litres instead of the declared 1.0 litre for which you pay? Or get a water filled fire extinguisher only half filled with water?

Z.
 08 October 2017 09:19 AM
User is offline View Users Profile Print this message



Zoomup

Posts: 3430
Joined: 20 February 2014

Originally posted by: davidwalker2

I agree. The media seem to wheel out these tame "experts" to make the point they wish to make. Last night it was suggested that hot cable would ignite plasterboard. Last time I put a blow-lamp to plasterboard resulted in scorching of the paper, but certainly not fire.



David


Yep the expert said: Sam Gluck, technical manager at electrical fire consultants Tower Electrical Fire and Safety, said this approach had "planted a bomb in the system".
He explained: "If it overheats, it will ignite anything that touches it. If it's against a plasterboard wall that will ignite.

Z.
 08 October 2017 09:21 AM
User is offline View Users Profile Print this message



Zoomup

Posts: 3430
Joined: 20 February 2014

Originally posted by: AJJewsbury

It would help if BASEC would specify exactly what was wrong with the cable so that professionals could decide if it was a hazard in any particular installation. "Insufficient copper" is hardly helpful.


Agreed. A lot depends on the "shape" of the missing copper - if it's missing evenly from the whole length of the conductors (i.e. deliberate attempt to save copper by using a smaller c.s.a.) then the risk is probably small and detection reasonably easy (by resistance testing or weighing), but if it's an occasional thinning of the wire in relatively short lengths (e.g. by poor quality machines occasionally over-stretching the wire) - giving say 99m of full size copper then 1m of half-size then a similar missing overall percentage would be much more significant and harder to spot.

- Andy.


I would use a micrometer to check the copper wire diameter accurately.

Z.
 08 October 2017 09:25 AM
User is offline View Users Profile Print this message



Zoomup

Posts: 3430
Joined: 20 February 2014

The BBC report said: "The British Approvals Service for Cables (Basec) had found the cable had too little copper, meaning it was at risk of overheating." This clearly refers the cross sectional area not the length of the copper cables.

Z.
 08 October 2017 09:33 AM
User is online View Users Profile Print this message



broadgage

Posts: 2452
Joined: 07 August 2007

Originally posted by: Zoomup

Originally posted by: broadgage



IIRC the reduced conductor size will not be a problem in the majority of cases, most cables in domestic use are indeed lightly loaded if compared to their nominal rating.







For example domestic lighting is commonly wired in 1.0mm on a 6 amp MCB. If the allegedly 1.0mm cable is in fact 0.8mm, then that is still good for 6 amps.
Domestic small power is commonly wired in 2.5mm, as either a 20 amp radial or a 32 amp ring final. If the alleged 2.5mm is actually 2.0mm, then that should be good for over 20 amps.
So MOST installations should be fine, but the worry is the small percentage that are already marginal and could be pushed "over the edge"
Consider for example two ring finals bundled together and both slightly overloaded to 35 amps. The 32 amp OCPD will pass 35 amps almost forever, and four cables bundled together and each carrying 17 or 18 amps will get very warm and perhaps dangerously hot.
Even worse would be a circuit that is ALREADY wrongly installed, but goes from a "bit rough but probably ok in practice" to "dangerous" as a result of undersized cable.
For example it is regrettably common to use 2.5mm twin with earth on a 32 amp MCB to supply a cooker or electric shower.
Incorrect in most cases, but generally OK in practice. If however the cable was only 2.0mm then an already poor choice could become actually dangerous.




If the cable is sold as a certain size then it should be a certain size, not smaller. It is illegal to mis-describe goods sold, and we can not assume that it will be lightly loaded.



Z.


Agree entirely, I was not defending the sale of undersized cable, goods should indeed be correctly described when offered for sale.
I was however suggesting that in MOST cases it wont be a problem in practice.
There is certainly the potential for overheating in the case of continual loads that are right at the limit.
IET » Wiring and the regulations » Undersized cables

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..