IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Cable derating-how grouping factor comes?
Topic Summary: grouping factor logic
Created On: 06 December 2013 02:19 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.
 06 December 2013 02:19 PM
User is offline View Users Profile Print this message



abrahamjp

Posts: 8
Joined: 20 September 2007

iec 60287 says, Thermal resistance is the only means ,a cable needs to get derated which is understandable in terms of physics behind it - heat conduction.

But for low voltage cable,we use some factor called 'grouping
factor'-which is not related to temprature directly.
If anybody can say how idea of grouping factor derived from,
it would be very helpful.
 06 December 2013 02:29 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19605
Joined: 23 March 2004

Any cable is effectively an internally heated cylinder (basically the I2t).

That cylinder is being internally heated and also is cooling by a combination of conduction, convection and radiation so that it reaches a steady state of heat gain and loss and stabilises at the operating temperature. This stable limit temperature is effectively tyhe maximum current that cable can carry without overheating

If you now add further cylinders (ie you group the cables), then thier ability to lose heat reduces and as such will overheat if the same steady state current is applied to them. To overcome this overheating, and consequent insulation damage risk the cable current carrying capacity is reduced (ie less I2r is allowed) - so effectively that's the derating for grouping.

Other factors would apply for ambient temperature, method of installation, effect of insulative materials and for the type of overload protection offered

regards

OMS

-------------------------
Failure is always an option
 06 December 2013 02:42 PM
User is offline View Users Profile Print this message



abrahamjp

Posts: 8
Joined: 20 September 2007

Dear OMS,
Thanks for reply.I see a grouping factor till 6 cables mentioned in regulation,If I accomodate more than 6 cables-how would be grouping factor get fluctuated?

regards

Abraham
 06 December 2013 03:17 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19605
Joined: 23 March 2004

It depends on what cables you are installing and how - if you take a group of 6 armoured cables on horizontal tray, then assume a cable 3 or 4 to be the index.

That cable is being heated by, and has diminished heat loss caused by the cable next to it, ditto for the cable next to that - from that point on, working outwards from the index, then further cables are not really influencing that index cable - so any further derating isn't really required.

If however you have single cables in trunking, adding furher cables creates a mutually heating mass so you would want further derating as you increase the number of circuits.

If you take a look at Table 4C1 in BS 7671, which has factors for up to 20 circuits, you should see the significant difference with cables installed as Row 1 of the table compared to cables installed as row 2, 3 or 4

I can't quite make out what data you are looking at or what cable type you have in mind - but I suspect the factor for 6 cables in your information is based on the fact that beyond 6 cables laid flat on caontainment basket, tray or cabble ladder, there is almost no further derating needed

Regards

OMS

-------------------------
Failure is always an option
 06 December 2013 03:29 PM
User is offline View Users Profile Print this message



abrahamjp

Posts: 8
Joined: 20 September 2007

Thanks,excellent
 06 December 2013 03:36 PM
User is offline View Users Profile Print this message



abrahamjp

Posts: 8
Joined: 20 September 2007

I have one more question,Is there any special derating factor for taking acount of solar radation ,for cables directly exposed to sun light
 06 December 2013 04:29 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19605
Joined: 23 March 2004

Originally posted by: abrahamjp

I have one more question,Is there any special derating factor for taking acount of solar radation ,for cables directly exposed to sun light


Well they would be covered by ambient air temperature and a conversion of direct solar irradiance to an equivalent Sol air temperature (effectively the assumed air temperature that accounts for the direct radiant heating of the sun (sol) ) - or you could calculate from first principles.

For the wiring regs, ambient temperature derating is usually sufficient but if you are running exposed cables in very high irradiance areas then you might have to put in a bit of adjustment yourself.

Regards

OMS

-------------------------
Failure is always an option
 08 December 2013 05:25 AM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3481
Joined: 17 December 2004

A slightly different tack regarding grouping of cables

Take a 3 core flexible cable and pass a current through it. The heat generated by the I^2R in both the phase and neutral conductors and lost through heat loss through the insulation should reach a stable operating temperature and balance itself out.
Now, roll the cable up onto a drum and pass the same current through it. You will see that the heat lost through the insulation will be a contributing factor to the I^R of the next section of cable lying next to it. Therefore there will be a net production of heat and so a reduction of heat loss.
You have in effect the same cable, same current but with greater production of heat and less cooler air surrounding each section of cable.
The operating temperature will therefore rise to a new state until the heat generated is beyond the operating temperature of the insulation. It will first melt and then, if the insulation is combustible, burst into flames.

I once had an orange extension cable where the insulation and sheath had melted together. Cut through it with an angle grinder for demonstration purposes. It looked like 'Blackpool rock'

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
Statistics

See Also:



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