230v or 240v

[RF Lighting]

Do I need to get out the welder and a clamp meter again?

 

[JohnW2]

Do I need to get out the welder and a clamp meter again?

Seems like it - but, sadly for the fun, in a more controlled and less dramatic fashion. What we want to ascertain is the highest current which does not result in anything dramatic happening!

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.

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?

Yes, I do expect the standard to specify tests, and it does! Just not including the one(s) you were expecting.

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.

John, I did write "only". Remember the cable standards are specific for different kinds of insulation. However, in theory you could be right, and as I suggested someone might invent a new insulation material, in which case a new cable standard would be necessary.

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.

There are also some examples in 'our' field of things having been found to be unsatisfactory - VOELCBs come to mind, as well as lighting circuits with no protective conductor.

It might also be the case that 10mm2 or 16mm2 shower circuits are found to be too small in a few years. We used to think 4mm2 enough for MEBs a few years ago. Such is the nature of progress.

 

[stillp]

Do I need to get out the welder and a clamp meter again?

Not until John specifies the test method and conditions, and the criteria for determining the test results! In particular, how long he wants the current to flow before the conductor ruptures, or what temperature rise is acceptable.

 

[ban-all-sheds]

I think a rise to 70° would be reasonable.

That is actually pretty hot - if you were to touch something which was at that temperature you'd say a 4-letter word, such as "ouch", and un-touch it as fast as you could.

Copper conductors will instigate catastrophe in their surroundings long before they themselves suffer it.

RF - you're going to need a way to measure temperature too. I'm just curious - just how much current do you really have to put through a conductor to raise it to 70°C?

 

[stillp]

I'm just curious - just how much current do you really have to put through a conductor to raise it to 70°C?

BAS, you'll need to specify a time for which the current will flow, and what the surroundings of the conductor will be, and what it is connected to (i.e. will the termination act as a heat sink, and reduce the temperature, or will the termination add resistance and hence increase the temperature) as well as the method of temperature measurement. If you make some assumptions about the heat loss you could probably work out the theoretical answer to your own question, since I know you've done similar calculations to shhot down people who want a 3kW hot tub and so on.

 

[bernardgreen]

I'm just curious - just how much current do you really have to put through a conductor to raise it to 70°C?

A lot of things affect that.

Memories of heat experiments at school suggest a wire in free air required 3 times as much current as a wire encased in thermal insulation for the same temperature rise. But I recall that was a thin wire and probably the ratio is different for larger wires as the area of disipation only doubles if the wire is double the diameter while the heat generating mass increases by 4 times.

Send some burn dressings to RF lighting and await experimental results.

 

[ban-all-sheds]

BAS, you'll need to specify a time for which the current will flow,

"Indefinitely". i.e. when a steady state is reached, or almost steady state with negligible rate of increase.

and what the surroundings of the conductor will be,

Free air.

and what it is connected to (i.e. will the termination act as a heat sink, and reduce the temperature, or will the termination add resistance and hence increase the temperature)

Good question.

as well as the method of temperature measurement.

Ideally something with no observer effect, I guess, but I'm not expecting a seriously scientific experiment.

If you make some assumptions about the heat loss you could probably work out the theoretical answer to your own question, since I know you've done similar calculations to shhot down people who want a 3kW hot tub and so on.

It's the heat loss I have no idea about.

 

[stillp]

It's the heat loss I have no idea about.

Yes, that's the problem, and one of the reasons for the variability of fuses in the real world. For old-fashioned asbestos-enclosed rewireables you could probably ignore heat loss.
I've a textbook somewhere at home dealing with thermal performance of heat sinks (gosh, I'm sad aren't I) that might tell us how to account for heat loss to air, but I won't be able to get to it for a few days.

 

[JohnW2]

BAS, you'll need to specify a time for which the current will flow, and what the surroundings of the conductor will be, and what it is connected to (i.e. will the termination act as a heat sink, and reduce the temperature, or will the termination add resistance and hence increase the temperature) as well as the method of temperature measurement.

I've been away from my computer for a few hours, so I've been overtaken by a lot of responses - but my answers (at least, in the first intstance), much the same as others have given, would be .... indefinite time (i.e. until equilibrium reached), in free air, measured at the centre of a sufficiently long length of cable (probably a few metres,) such that the terminations would hopefully be having minimal effect, ideally measured with an IR 'point and measure' device. After that, we would probably want to move to other 'installation methods', particularly 'clipped direct' (to various surfaces).

If you make some assumptions about the heat loss you could probably work out the theoretical answer to your own question,

Unfortunately, that's the reason we need an experiment. Calculating the heat generation is obvioulsy trivial. All one needs to turn that into a temperature rise is knowledge of the thermal behaviour of the insulation/sleeving, but we really don't have that. Even knowledge of the thermal properties of the materials would not be enough, becase the shape/'layout' of T&E is complex.

Kind Regards, John.

 

[stillp]

All one needs to turn that into a temperature rise is knowledge of the thermal behaviour of the insulation/sleeving, but we really don't have that. Even knowledge of the thermal properties of the materials would not be enough, becase the shape/'layout' of T&E is complex.

Kind Regards, John.

Now you're complicating things. If you want RF to conduct this experiment with T & E, then the IR temperature measuring device won't measure the temperature of the conductor but of the insulation surface. The only real way to know the temperature of an insulated conductor will be to monitor the change of resistance, which will be affected by the terminations...
I'm beginning to see a possible reason why the standards don't specify such a test, because
a) there are too many variables, so it would be difficult to ensure repeatability between different test labs, and
b) there would be little point in making manufacturers repeat this test, since it has been done before for typical cables (apparently by ERA some decades ago), and the results won't change unless either the dimensions or the materials are changed.

 

[bernardgreen]

The only real way to know the temperature of an insulated conductor will be to monitor the change of resistance, which will be affected by the terminations...

Using "pin prick probes the voltage drop along the test sample can be measured, the micro amp current taken by the high impedance volt meter will not affect the current in the sample. The cooling of a few inches of conductor by the heat conducted out by the pin prick probes will not be significant if the sample length is long.

 

[bernardgreen]

pin p***k = pin p r i c k probes ( thank you censors )

 

[JohnW2]

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?

Yes, I do expect the standard to specify tests, and it does! Just not including the one(s) you were expecting.

True, but can you blame me for 'expecting' a test of one of the most important electrical properties? In electrical terms (I accept that there are other considerations) current-carrying and voltage-withstanding properties are really all there is!

John, I did write "only". Remember the cable standards are specific for different kinds of insulation. However, in theory you could be right, and as I suggested someone might invent a new insulation material, in which case a new cable standard would be necessary.

Yes, you did say that, but I remain surprised that the Standard allows assumptions to be made about the properties of the materials (allegedly) used - one normally 'expects' to see a test on the final product.

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 ....

There are also some examples in 'our' field of things having been found to be unsatisfactory - VOELCBs come to mind, as well as lighting circuits with no protective conductor.

There probably are some good examples in the electrical field but I would not say that either of those are! In the case of VOELCBs, in their day we had as much knowledge as we do now to enable us to decide that they were 'unsatisfactory' but, particularly in the absence of alternatives, we decided that they were worth having. The change in requirement for CPCs in lighting circuits arose because of changing attitudes/approaches to electrical safety, and perhaps (not sure about this one) an increase in light fittings with exposed metal parts (I hesitate because there was a fair bit of brass around 50-100 years ago!).

It might also be the case that 10mm2 or 16mm2 shower circuits are found to be too small in a few years. We used to think 4mm2 enough for MEBs a few years ago. Such is the nature of progress.

That is theoretically possible but, I would suggest, very unlikely. The laws of physics are not going to change and if current shower circuit cables were getting hot enough to represent a hazard, we presumably would already know. That, of course, doesn't mean that regulations might not 'progess' to have the requirements such as you suggest - but there would be no rational reason in terms of electrical safety. Unless the change were totally irrational, there would have to be some other reason (increased concern about 'energy wastage'?). I again presume that the required CSA for main bonding again relates to changing attitudes/ approaches to electrical safety - since, again, the lawys of physics (hence the current-carrying capacities of cables) has certainly not changed.

Kind Regards, John.

 

[JohnW2]

Now you're complicating things. If you want RF to conduct this experiment with T & E, then the IR temperature measuring device won't measure the temperature of the conductor but of the insulation surface.

'Complicating things' by mentioning T&E? Perhaps I misunderstood, but I thought that had been the intention all along. I also think you may be confusing two things - our interest in a very approximate empirical answer to the question 'how much current can T&E really safely take' and the matter of what precise tests might be specified in a Standard for such cable. ...but I accept that it would be much easier for us to work with a single insulated conductor, to get some preliminary answers.

The only real way to know the temperature of an insulated conductor will be to monitor the change of resistance, which will be affected by the terminations...

Yes, it would be very easy to measure conductor temperature in that way, and I don't think that it would be difficult to do it in such a way that the terminations were having minimal effect on the answer. However, conductor temperature itself would probably not fully answer the question of interest.

I'm beginning to see a possible reason why the standards don't specify such a test, because
a) there are too many variables, so it would be difficult to ensure repeatability between different test labs, and ....

That's true of very many tests, but it would not be difficult to specify conditions (e.g.as for temperature rise in BS1363 sockets) - the test conditions don't necessarily have to be all that 'realistic', so long as they are reproducible.

b) there would be little point in making manufacturers repeat this test, since it has been done before for typical cables (apparently by ERA some decades ago), and the results won't change unless either the dimensions or the materials are changed.

You keep talking as if the discussion was about 'type-testing'. I'm talking about requirements to confirm that manufactured products conform to a required specification (e.g. a Standard) - sampling/testing (or, in some cases, 100% testing) of manufactured products to verify that has been part of the QC of every sort of 'serious' manufactured product I've come across. Without that, one cannot rule out problems with the (actual, not theoretical) materials or manufacturing process.

Kind Regards, John.