230v or 240v

[JohnW2]

Agreed. I've asked before but did not receive an answer but... can anyone tell me at what current would, say, a 2.5mm² conductor actually be rendered unusable and then finally melt ?

Indeed - and that's effectively what BAS has just asked, too. Let's hope someone has access to the Standard.

... but, of course, you could always 'do the experiment' Tell me ...the famous (was it TTC's?) video clip of some T&E dying at the hands of a welder - do we know what current was involved there?

As I've just written to BAS, I suspect we'd be 'surprised' by the answer. Having said that, I don't think I actually would be all that surprised to find that T&E can take at least 2 or 3 times it's tabulated CCC for at least a while before actually being seriously damaged - but I may be totally wrong.

Kind Regards, John.

 

[ban-all-sheds]

As some kind of guide it can't be that hard, if you know the formula or can find a calculator, to work out what a bare cylindrical conductor in free air at, say 30°C will get to for a given current. You know its resistivity, you know its specific heat, you know it's temperature coefficient, you know how much energy is going into it, you know its surface area and you know how it performs as a radiator...

 

[stillp]

I've got a copy of BS 6004: 1995, which isn't up-to-date but the laws of physics haven't changed.
What was the question exactly?
If it was about current carrying capacity, there is nothing in the standard dealing with that.
If about voltages, it states "The standard voltages recognized for the purposes of this standard shall be 300/500V and 450/750 V."

 

[JohnW2]

As some kind of guide it can't be that hard, if you know the formula or can find a calculator, to work out what a bare cylindrical conductor in free air at, say 30°C will get to for a given current. You know its resistivity, you know its specific heat, you know it's temperature coefficient, you know how much energy is going into it, you know its surface area and you know how it performs as a radiator...

By that process (the heat loss part would be the most difficult part) one could ascertain the current that would raise the conductor to its melting point - so would give an upper bound to the figure we want. However, the actual figure (current) we want would presumably be a lot lower than that, since the insulation would suffer thermal damage long before the conductor melted. Furthermore, when we move to T&E, with it's 'complex' structure, estimating head loss would become even more difficult.

... which is why I suspect that the empirical method is the way to go!

Kind REgards, John

 

[JohnW2]

I've got a copy of BS 6004: 1995, which isn't up-to-date but the laws of physics haven't changed. What was the question exactly? If it was about current carrying capacity, there is nothing in the standard dealing with that.

Yes, the question was about current carrying capacity.

We were hoping that the Standard would contain details of tests of current carrying capacity of T&E of various CSAs had to pass (i.e. what current they can 'take'). There surely must be such a requirement - if no test is specified in BS 6004, do you have any idea where it could be?

Kind Regards, John.

 

[stillp]

if no test is specified in BS 6004, do you have any idea where it could be?

The basic standard in my world is BS EN 60204-1 "Safety of machinery - electrotechnical spects", which derives its tables of current-carrying capacity and derating from IEC 60364-5-52, which refers to IEC 60502 and IEC 60702! Phew!
I'll try to check those later, but now I need a cup of tes and an afternoon nap.

 

[JohnW2]

As some kind of guide it can't be that hard, .....

A feel for the ceiling for a single bare conductor can be obtained from 'fusing current' tables. The following (funny CSA's 'cos it comes from an AWG table), is for solid bare copper conductors:

CSA(mm²) Fusing current
. 1.3.............117 A
. 1.6.............140 A
. 2.1.............166 A
. 2.6.............197 A
. 3.3.............235 A
. 4.2.............280 A
. 6.1.............333 A
. 6.6.............396 A
. 8.4.............472 A
. 10.5...........561 A
. 13.3...........668 A

As I recently wrote, the maximum usable currents will obviously be considerably lower than those figures (and insulation/sheathing will reduce the figures further) - but, as I said, it certainly represents a sort-of ceiling.

Kind Regards, John

 

[stillp]

No luck
IEC 60502 referred me to IEC 60228... I can find requirements for the resistance for each CSA, and how to correct for temerature when measuring the resistance, but I've failed to find the source of the current-carrying capacity tables.
I'll speak to some experts and report back.

 

[JohnW2]

No luck
IEC 60502 referred me to IEC 60228... I can find requirements for the resistance for each CSA, and how to correct for temerature when measuring the resistance, but I've failed to find the source of the current-carrying capacity tables. I'll speak to some experts and report back.

Many thanks for your efforts. We wait with bated breath!

Kindest Regards, John.

 

[ban-all-sheds]

'fusing current'

Are those the currents which will raise the temperature to about 1084.62°C?

 

[JohnW2]

'fusing current'

Are those the currents which will raise the temperature to about 1084.62°C?

Not quite - the table I used said 1083°C ... but I guess the answer is more-or-less 'yes'!

Kind Regards, John.

 

[stillp]

John, I'm not sure if this will let you unbate your breath, but I've been in touch with some people involved in the standards committee responsible for IEC 60364-5-52, which is the source of many of the tables in BS7671. Apparently 60364-5-52 is based on IEC 60287-1-1 and 60287-2-1. 60287-1-1 gives the equations for calculating the ac conductor resistance and the induced losses in other metal components such as screens and armour. These equations lead to determining the heat generated by the cable. 60287-2-1 gives the equations for calculating the thermal resistance of the insulation, sheath and other layers together with the external thermal resistance. .
Most of the current ratings in 60364-5-52 were calculated using the method in 60287 and nominal dimensions for the cable components.
The current ratings for cables in conduit and cables in insulated walls were derived from a review of test work that was available from several countries.

Does that help? I hope so, I'm losing the will to live here!

 

[JohnW2]

John, I'm not sure if this will let you unbate your breath, but ..... 60287-1-1 gives the equations for calculating the ac conductor resistance and the induced losses in other metal components such as screens and armour. These equations lead to determining the heat generated by the cable. 60287-2-1 gives the equations for calculating the thermal resistance of the insulation, sheath and other layers together with the external thermal resistance. .... Most of the current ratings in 60364-5-52 were calculated using the method in 60287 and nominal dimensions for the cable components. .... Does that help? I hope so, I'm losing the will to live here!

Well, it's interesting, but it certainly falls short of answering the question which was posed.

We're interested more in the required performance of manufactured cables, rather than anything calculated. Are you saying that there are no Standards which specify a current-carrying test which a T&E cable of a given CSA has to be able to pass?

Kind Regards, John.

 

[stillp]

Are you saying that there are no Standards which specify a current-carrying test which a T&E cable of a given CSA has to be able to pass?

Thanks for clarifying the question!
Yes, that seems to be the situation. As far as I can see the cable standards only specify dimensions, the resistivity, and the insulation withstand voltage. I guess if someone invented a new material for electrical insulation that was a much better thermal insulator, the conductor could get much hotter. However, if someone were to invent a new insulation material it would need a new standard.
Wherever they originate, the CCC tables, as well as the derating factors for different installation methods, seem to be "proven in use", so there is no incentive to change them.

 

[JohnW2]

Are you saying that there are no Standards which specify a current-carrying test which a T&E cable of a given CSA has to be able to pass?

Thanks for clarifying the question! Yes, that seems to be the situation.

Don't you share my view that such a situation is rather extraordinary? When a manufactured product has to comply with a Standard, doesn't one expect there to be specified tests which have to be satisfied in order to confirm that compliance?

As far as I can see the cable standards only specify dimensions, the resistivity, and the insulation withstand voltage.

Is that 'it', or have you left off an "etc."? If those criteria were all there were, a cable could be manufactured which satisfied all four of those requirements, yet with insulation which had thermal characteristics which considerably reduced (or increased) the safe current-carrying capacity of the cable.

Wherever they originate, the CCC tables, as well as the derating factors for different installation methods, seem to be "proven in use", so there is no incentive to change them.

The trouble with "proven in use" is that it's often a lot less 'proven' than one might think. There are, for example, countless examples in medicine of things (treatments or management policies) which were considered to be totally 'established' (and "proven in use") for very many years, with no-one daring to deviate from these 'established practices', until it was ultimately determined that those practices had been way OTT, in some cases to the detriment of patients. Similarly, it could be the case that, in reality, 2.5mm² or 4mm² shower circuits would be quite safe.

Kind Regards, John.