appliance current rating

[Echo the husky]

I have repaired an industrial oven before where the element coils had been carelessly replaced in their metal tubes, and obviously not tested before being refitted. The ceramic cap at the end was missing meaning power was flowing from live through one half of the element then to earth, the current drawn was 4A instead of the correct 2A. There was a number of other glaring faults as well, all caused by shoddy repair work by a rather well known company...

 

[EFLImpudence]

i always assumed that the resistance of an element would reduce over time due to heat damage.

It's only a quite thin conductor.
For that to happen, the thickness of the element would have to increase with equally good material, not corrosion or burning, so it can only wear out.

You could say the same of ordinary wire.


I don't have any scientific credentials and am sure John's knowledge has overtaken mine, particularly in the variety of related subjects.

 

[bernardgreen]

What are the ovens cooking ? Could the atmosphere be causing oxidation on the terminals / contacts of the connector / isolators. If some food factories the electrical control gear has to be protected from the fumes given off by the food being processed.

Does the overheating affect all three phases equally or do one or two appear to be more overheated than the other one ?

Are the elements connected in Delta ( no Neutral ) or Star ( active Neutral ) ?

Are the three phase to Neutral voltage the same. ?

it might be that un-balanced loading of the three phase elsewhere in the local network is forcing phase to phase voltage or phase to neutral neutral voltage higher hence hifher than expected current on tthose phases.

Another remote possibility is that the current rating is the average over a period of time where the oven has reached operating temperature and heat control is by mark-space switching of the supply and when the oven is heating up the current is continuous ans above the quoted average. Hence while heating up from cold the connector / isolators may be overloaded

 

[Echo the husky]

How long are the ovens running for? Many industrial plugs and sockets are only rated to carry their full current for a limited time, some as short as 2 hours. The instruction leaflet with the last 16A single phase one said something like 16A allowed for a maximum of 6 hours, 10A allowed continuously.

It's a T5-1000, my day to day meter. The clamp meter is ok, but I would stop there. It fluctuates quite a lot even given a fixed load, and mine often reads 0.3A with no cable in the jaws, perhaps something to do with being open jawed. Accuracy according to fluke is +/-3% which is about 1A at 32A

My £100 fluke clamp meter is the same! it's really annoying. My cheap kewtech appears more accurate, and gives readings faster too.

 

[JohnW2]

Well till now, i always assumed that the resistance of an element would reduce over time due to heat damage.

As I said, quite apart from the effects of temperature on resistivity (which is a 'short-term' issue, only relevant whilst the element is heating up), if there were long term heat damage, it would presumably take the form of loss of some of the surface of the conductor (due to oxidation/burning/whatever). That would reduce the CSA of the conductor and therefore would increase the resistance.

you may not have the credentials, but the vast majority you do know, you not only explain well, clear and precise, you often illustrate with examples, like the light bulb scenario. May I ask, Do you have a tutoring background

Thanks for your kind comments. Although it's never been a primary part of my life, over the years I have done a certain amount of teaching, tutoring and 'mentoring' (e.g. of PhD students), although never in a field related to electricity. However, a lot of my life has involved having to explain things very clearly, either in writing or verbally, not uncommonly with intended audiences that needed 'spoon feeding' as regards some of the technical issues involved - so I guess I have probably developed some relevant skills as regards 'explaining clearly'

I certainly think, and have found, that illustrating things my examples and analogies (although someone will inevitably claim that it 's not a 'valid analogy'!) is an invaluable tool in helping people to understand. In the real world, rather than a forum, I would often try to be far more 'interactive', by trying to get 'the audience' to work out the answers for themselves, aided, if necessary by clues or questions. For example, in the case you cite, I might have asked you why you think that it is quite common for an ageing incandescent lamp/bulb to die at the moment of switch-on - and then you might have worked out what effect temperature has on the resistance of the filament, hence current through it.

Kind Regards, John