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Topic Title: Cable Core Temps
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Created On: 11 July 2013 08:29 PM
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 11 July 2013 08:29 PM
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MrOther

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What depends if a cable is to be run at 70C or 90C - the type of cable or the load?

I would of thought the cable myself.
 11 July 2013 08:33 PM
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OMS

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Cable insulation actually - followed by termination methods and equipment rating (for temperature)

Regards

OMS

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 11 July 2013 08:38 PM
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MrOther

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Can you give me an example please OMS?
 11 July 2013 11:03 PM
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AJJewsbury

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What depends if a cable is to be run at 70C or 90C - the type of cable or the load?

May run at, or actually runs at?

May run at - the maximum temperature rating of what surrounds the conductor (insulation etc) as OMS says.

Actually runs at - is a balance between the heat generated within the cable (I2R - so for a given cable, how much current it's actually carrying) and how easily that heat is lost to the environment (ambient temperature, thermal resistance of the insulation, sheath and the immediate surroundings, shape, and so on). With everything else constant, the heat loss depends on the temperature difference between the conductor and the environment - the bigger the difference the more heat is lost.

For example, run 30A through a 2.5mm2 (round) 2-core cable in free air in a 30-degree environment and it'll gradually get up to about 70 degrees before heat loss matches heat generated and the temperature levels off; put 36A through it and the extra heat generated will means it will stabilise at about 90 degrees instead.

- Andy.
 12 July 2013 07:05 AM
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kj scott

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Originally posted by: MrOther

What depends if a cable is to be run at 70C or 90C - the type of cable or the load?

I would of thought the cable myself.


The designer.

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 12 July 2013 04:19 PM
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OMS

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Originally posted by: MrOther

Can you give me an example please OMS?


OK - if I put 77A down a 1 x 4 x 16mm2 PVC/SWA/PVC submain clipped direct it will get to 70C when installed in an ambient temperature of 30C. That 70C will also exist at terminations etc

If I swap out the cable for 1 x 4 x 16mm2 XLPE/SWA/PVC submain clipped direct I could put 77A down it and keep the conductors at 70C or I could over run the cable to 90C and get 94A through it without exceeding a 90C limit.

The thermoplastic PVC is only good for 70C , the thermosetting XLPE is good for 90C - but, and it's a big but, the terminations are also at 90C and most equioment only has an equipment temperature rating of 70C.

If you have switchgear that can accept termination temperatures of up to say 105C then you could get more amps down the same size cable essentially by using XLPE rather than PVC.

There is a similar effect under fault conditions - if you look at Tables 54.2 and 54.3 you'll find different values of K for different insulation materials - basically this allows you to use smaller XLPE insulated cables than PVC cables for the same fault level - the final permitted temperature under fault being 160C for PVC and 250C for XLPE

Table 43.1 should show you a similar effect.

Just keep in mind that if you decide to go with higher operating temperatures, and the kit can cope with it, then Table 42.1 should show you (for cables) that the metallic glands of the armoured cable etc shouldn't exceed 80C unless shrouded - the sheath may be allowed to attain 90C - but that's pretty bloody hot

Any help

Regards

OMS

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 13 July 2013 10:23 PM
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I would have to argue slightly OMS. The point s that a cable at 90 degrees with close conductors does not have the conductors at 90 degrees if they are singles in fairly free air, probably less than 70. Our difficulty is that we do not have tabulated ratings for single cores inside DBs etc. On the whole I am fairly happy with XLPE conductors connecting to 70 C equipment, as I have yet to find a whole DB at 70 C, even if they are running at 90 degrees conductor temp for the cable, rather than singles inside the equipment. I know its difficult but this is the area where the inspector need the experience. I&T is not an area for the novice!!

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 15 July 2013 10:16 AM
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AJJewsbury

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The point s that a cable at 90 degrees with close conductors does not have the conductors at 90 degrees if they are singles in fairly free air, probably less than 70. Our difficulty is that we do not have tabulated ratings for single cores inside DBs etc.

Principle agreed with - although there will be some conduction of heat along the length of the conductors from the 90-degree zone to the 'free air' zone - didn't one of the wiring matters articles that suggested using 90-degree cable through thermal insulation suggest leaving at least half a metre of cable between the insulated area and 70-degree accessories? not quite the same situation, and presumably depends on c.s.a. but perhaps suggest that you might need a significant length of 'tails' inside the enclosure to be safe.
- Andy.
 15 July 2013 11:03 AM
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perspicacious

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"you might need a significant length of 'tails' inside the enclosure to be safe"

And that these need to be at sufficient distance away from adjacent cables rated at 70 deg C..... Not easily acheived in a populated enclosure Unless of course you oversleeve the 70 deg C cables with 90 deg C insulation

Regards

BOD
 15 July 2013 04:31 PM
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OMS

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I would have to argue slightly OMS. The point s that a cable at 90 degrees with close conductors does not have the conductors at 90 degrees if they are singles in fairly free air, probably less than 70.


OK - I agree in principle that there will be a temperature gradient - if foray with the clankies is anything to go by, you have the classic undergraduate problem of determining the 3 D heat transfers going on with an internally heated cylinder in proximity to other internally heated cylinders (and often close proximity to those cylinders - electricians just love ty wraps). So once escaped from the cable crutch you have conduction (often significant in copper in terms of distance D, convection - lots of numbers from german sounding characters like Grashof and Prandtl in enclosures that are often well sealed and convection is in a limited volume, and radiant heat transfer - fourth power of the temperatures and that nice chap Stefan-Boltzmann and his constant.

Not easy to determine, and your estimation of a 90C conductor dropping below 70C within the enclosure is a bit optimistic in practice.

Our difficulty is that we do not have tabulated ratings for single cores inside DBs etc.


Exactly, working it out isn't that easy, and in the absense of readily avialable data, risky.

On the whole I am fairly happy with XLPE conductors connecting to 70 C equipment, as I have yet to find a whole DB at 70 C, even if they are running at 90 degrees conductor temp for the cable, rather than singles inside the equipment


Depends on what you are looking at - for a particular client of mine, they adopt a 50C max conductor temperature on all installation design and re verification (for a number of valid reasons). The 50C is the trigger point for further investigation when viewed with thermography - I've seen plenty of installations where there is only a few degrees variation in 400mm+ of exposed core. On the whole, it makes me unhappy rather than happy, because some silly sod then has to go and verify what is essentially your assumption. Both within the field, and in a lab, I've measured this sort of thing with multiple thermocouples - there is a loss, but often not as much as perceived, and keep in mind, that on high form of seperation switchgear, cables don't actually have that much length between gland and termination (individual glanding chambers for example).

I know its difficult but this is the area where the inspector need the experience. I&T is not an area for the novice!!


Agreed - but nor is it an area where you can make unsubstantiated assumptions. Based on a simple ammeter reading, personally, I'd be raising the alarm if I came across conductors exceeding 70C by calculation in "normal" equipment. It tells me the design is potentially suspect and closer investigation is required - possibly by a simple IR scan or something a bit more detailed.

Perhaps we've over complicated things for the OP -

Regards

OMS

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 15 July 2013 07:59 PM
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MrOther

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OK - if I put 77A down a 1 x 4 x 16mm2 PVC/SWA/PVC submain clipped direct it will get to 70C when installed in an ambient temperature of 30C. That 70C will also exist at terminations etc


Ok, what happens if the Ca increases/decreases in this scenario? Haven't got BS7671 to hand, but is this cable running full load? If I ran 10A down this cable, would it eventually reach a core temp of 70C?


If I swap out the cable for 1 x 4 x 16mm2 XLPE/SWA/PVC submain clipped direct I could put 77A down it and keep the conductors at 70C or I could over run the cable to 90C and get 94A through it without exceeding a 90C limit.


OK - why do we get more current-capacity out of a cable running at hotter core temp - resistance and all that? And what do you mean by "without exceeding a 90C limit"?


The thermoplastic PVC is only good for 70C , the thermosetting XLPE is good for 90C - but, and it's a big but, the terminations are also at 90C and most equioment only has an equipment temperature rating of 70C.

Cool. Check equipment and terms are suitable.


There is a similar effect under fault conditions - if you look at Tables 54.2 and 54.3 you'll find different values of K for different insulation materials - basically this allows you to use smaller XLPE insulated cables than PVC cables for the same fault level - the final permitted temperature under fault being 160C for PVC and 250C for XLPE


Think this may be the next chapter of the book.

Just keep in mind that if you decide to go with higher operating temperatures, and the kit can cope with it, then Table 42.1 should show you (for cables) that the metallic glands of the armoured cable etc shouldn't exceed 80C unless shrouded - the sheath may be allowed to attain 90C - but that's pretty bloody hot


Is this the outter sheath you are referring to? If this is the case what happens to cores in panels? Surely this is hazard?
 15 July 2013 08:13 PM
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Reading some of your posts - some of them going a little over my head because I'm yet to pick up the understanding of this subject (reasons of the OP) but re-reading between the lines of some of your posts it seems that Length of Cable Run has a significant factor on cable core temps?
 15 July 2013 08:25 PM
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OMS

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Not really - heat gain (and therefore loss) is per unit length - as long as the installed conditions don't change, then for the same current, the distance is totally academic.

For your post above, laters

Regards

OMS

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 15 July 2013 09:55 PM
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MrOther

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Originally posted by: OMS

Not really - heat gain (and therefore loss) is per unit length - as long as the installed conditions don't change, then for the same current, the distance is totally academic.



For your post above, laters



Regards



OMS


Look forward to it
 16 July 2013 10:18 AM
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AJJewsbury

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Think of heat generation as the result of voltage drop (that's where the heat energy comes from after all) - related to amps per metre - so each metre of cable gets the same amount of heat generated in it regardless of how long the cable is. Length comes in to it a bit where you have different sections of cables that are running at different temperatures (depending on how easy it is for the heat to escape - i.e. the installation conditions) and how far along the lower temperature section the higher temperature will sneak along.
- Andy.
 22 July 2013 12:23 PM
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OMS

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Originally posted by: MrOther



OK - if I put 77A down a 1 x 4 x 16mm2 PVC/SWA/PVC submain clipped direct it will get to 70C when installed in an ambient temperature of 30C. That 70C will also exist at terminations etc



Ok, what happens if the Ca increases/decreases in this scenario? Haven't got BS7671 to hand, but is this cable running full load? If I ran 10A down this cable, would it eventually reach a core temp of 70C?



Well if the ambient temperature increases, then for that section of cable the current carrying capacity decreases - less headroom effectively. If you put 10A down the cable at an ambient of say 30C, where it;s curent carrying capacity was 77A it would get a bit above 30C bit nowhere near 70C. You could calculate it from:

Tactual = Tambient + Ib^2/It^2(Tpermitted - Treference)

Where Tpermitted = max permitted operating temp (in ths case 70C)
Treference = ambient temperature (usually 30C)


If I swap out the cable for 1 x 4 x 16mm2 XLPE/SWA/PVC submain clipped direct I could put 77A down it and keep the conductors at 70C or I could over run the cable to 90C and get 94A through it without exceeding a 90C limit.





OK - why do we get more current-capacity out of a cable running at hotter core temp - resistance and all that? And what do you mean by "without exceeding a 90C limit"?

We can put more current down it as the insualtion material can run hotter without damage.

The heating effect is effectively power absorbed by the cable.

We know Power = Volts x Amps

but we also know Volts = Amps x Resistance

So we can say that power absorbed in the cable is amps x amps x resistance = I x I x R = I^2R

It's the I2R that causes the heating effect.


For XLPE we aren't supposed to exceed a conductor temperature of 90C for prolonged periods - this causes insulation damage


The thermoplastic PVC is only good for 70C , the thermosetting XLPE is good for 90C - but, and it's a big but, the terminations are also at 90C and most equioment only has an equipment temperature rating of 70C.



Cool. Check equipment and terms are suitable.




There is a similar effect under fault conditions - if you look at Tables 54.2 and 54.3 you'll find different values of K for different insulation materials - basically this allows you to use smaller XLPE insulated cables than PVC cables for the same fault level - the final permitted temperature under fault being 160C for PVC and 250C for XLPE





Think this may be the next chapter of the book.



Just keep in mind that if you decide to go with higher operating temperatures, and the kit can cope with it, then Table 42.1 should show you (for cables) that the metallic glands of the armoured cable etc shouldn't exceed 80C unless shrouded - the sheath may be allowed to attain 90C - but that's pretty bloody hot





Is this the outter sheath you are referring to? If this is the case what happens to cores in panels? Surely this is hazard?


If the sheath gets to 90C then the armour, bedding, insulation and cores must be at a higher temperature - think of a temperature gradient with the copper being the hottest part - as that's the point of heating, effectively.

Regards

OMS

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