appliance current rating

[333rocky333]

we have been encountering overheating problems lately with 32 amp isolaters and sockets

The oven rating plates state 230/400 volt and maximum load 31.8 amps

however with a clamp meter it can read in excess of 35 amps for several minutes during the cycle.

Phases to neutral were aprox 235 volt, and I think im correct to say, for that sort of current increase, the supply voltage would need to be over 250 volt

They are commercial ovens on 3 phase 400 volt supplies.
The manufacturers say its not possible and blamed poor clampmeter,


I assume the units have 230 volt elements, so if after time could the elements deteriarate with the heat, causing the resistance to decrease and therefore cause the current to increase.

Does that sound feasible or is there another reason for the load current to increase

 

[EFLImpudence]

could the elements deteriarate with the heat, causing the resistance to decrease and therefore cause the current to increase.

Not really.
I would expect it to increase when they are hot.

Can you measure the resistance of the elements?

is there another reason for the load current to increase

Can't think of anything.

 

[JohnW2]

we have been encountering overheating problems lately with 32 amp isolaters and sockets ... The oven rating plates state 230/400 volt and maximum load 31.8 amps ... however with a clamp meter it can read in excess of 35 amps for several minutes during the cycle.

Whatever the truth/answers about the current being drawn, I would be pretty surprised if there would be a significant difference in heating of isolators/sockets between 31.8A and 35A.

Are the currents identical in all three phases?

I assume the units have 230 volt elements, so if after time could the elements deteriarate with the heat, causing the resistance to decrease and therefore cause the current to increase.

I would very much doubt that. In the short-term increasing element temperature would obviously increase the resistance and hence decrease the current. Long-term deterioration of an element would be most likely to be manifest by loss of some of the surface (e.g. due to oxidation etc.), which would again increase the resistance.

The manufacturers say its not possible and blamed poor clampmeter,...

A tpical clamp meter will have an accuracy of about 3% + 10 digits on a 40-50A range, so 31.8A could read as high as about 33A, so that might be part of the answer. When was the meter last calibrated? Just a percent or two error could extend that reading up to 35A.

Does that sound feasible or is there another reason for the load current to increase

Are these fan ovens, or do they otherwise have motors etc. (e.g. for rotisseries etc.)? If so, do you know what the PF is?

Kind Regards, John

 

[333rocky333]

I will have to get more details, from the makers, possibly meet them on site, maybe get them to check element resistance.
apparently its happened at at least 15 sites now.
The tester was a fluke, I think they are called T1000, so not really a True clampmeter, so as you suggest using a calibrated unit may be the way forward

Personally I dont think a 32 amp isolater or socket should be affected by 35 amps through them, should it, the isolaters I would think could possibly take maybe up to 40 anps with no problem, so im a bit baffled.
Though I have noted manufacturing quality of most rotary isolaters and red 5 pin sockets/plugs has reduced somewhat lately

Interestingly I note you mention loads per phase, can, different phase loads have an affect.
The ones I witnessed was our old favourite of the Neutral burnt up,

 

[333rocky333]

In the short-term increasing element temperature would obviously increase the resistance

Would you mind elaborating please John

 

[flameport]

Are the isolators actually used, or just left in the on position all the time?
If used, are they rated for on-load switching?

 

[Iggifer]

The tester was a fluke, I think they are called T1000, so not really a True clampmeter, so as you suggest using a calibrated unit may be the way forward

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

 

[333rocky333]

Are the isolators actually used, or just left in the on position all the time?
If used, are they rated for on-load switching?

left on in most cases as the ovens have a digital display touch screen, touching puts them on standby when not in use

 

[JohnW2]

The tester was a fluke, I think they are called T1000, so not really a True clampmeter, so as you suggest using a calibrated unit may be the way forward

Although, as Iggifer and I have both said, the manufacturers usually claim an accuracy of around ±3% for clamp meters, I have to say that surprises me. My experience is like Iggifer's, that in practice they can seem to be be much more iffy than that suggests - at the sort of current you're talking about, I can often get "an amp or three" of variation in displayed current just by moving the conductor around within the clamp's 'jaw'.

Personally I dont think a 32 amp isolater or socket should be affected by 35 amps through them, should it, the isolaters I would think could possibly take maybe up to 40 anps with no problem, so im a bit baffled.

That was, of course, the first point I made in my previous post, so I obviously feel the same as you.

Interestingly I note you mention loads per phase, can, different phase loads have an affect.

I can but presume that you are using your clamp meter to measure the current in just one phase (if you clamped all three phase conductors, you should get a zero reading for a balanced load). I was therefore asking whether you were getting the 35A reading for each of the three phases - if not, things would clearly be a bit more complicated!

The ones I witnessed was our old favourite of the Neutral burnt up,

In single-phase situations, that, of course, usually results from the neutral terminal not being tightened up adequately. If, much to my surprise, the current, per se, was overheating isolators or plugs/sockets etc., one would expect the phase bits to be affected as much as the neutal ones. However, if we're talking about a balanced (or nearly balanced) 3-phase, the neutral current should be zero, or near zero, so I don't really understand how it could get 'burnt up'!

You didn't comment on my question about fans, motors and PF.

Kind Regards, John

 

[JohnW2]

Would you mind elaborating please John

That's how it is. Nearly all materials (certainly including copper) have a 'positive temperature coefficient of resistance', such that the resistance of a conductor made from that material will increase as temperature rises. That's why there is a high 'inrush' current (which sometimes kills the lamp/bulb) when switching on an incandescent lamp/bulb - since the resistance/impedance of the filament is much lower when it is cold.

Just a very small number of materials have negative temp coefficients (resistance decreases with rising temp), and they are specifically used to exploit that property (e.g. in 'negative temp coefficient resistors').

Having said that, heating elements are, I think, usually made of a material which has a very low temperature coefficient. The rise in resistance (hence reduction in current) as temperature rises in therefore fairly small, but it is still 'positive' (resistance increases with increasing temp), not 'negative'.

Kind Regards, John

 

[333rocky333]

You didn't comment on my question about fans, motors and PF.

Kind Regards, John

sorry , thats what i meant by meeting the makers, I was only recently informed of this so its secondhand news to me, so Im not sure whats in the oven, the ones I witnessed could well have been just poor Neutral connections.
Apparently the clients now getting a bit concerned, so another engineer asked my thoughts, so i in turn asked here for any thoughts.
I wiil ask the engineer for a pic of the rating plate, maybe theres more info on that

 

[JohnW2]

sorry , thats what i meant by meeting the makers, I was only recently informed of this so its secondhand news to me, so Im not sure whats in the oven

Fair enough. If something in them is shifting the PF appreciably away from 1, that could obviously affect measured currents, but I would have thought that the 'rating plate' really ought to take that into account.

... the ones I witnessed could well have been just poor Neutral connections.

Presumably with single-phase ones. As I said, it would seem unlikley in a relatively well-balanced 3-phase situation.

Apparently the clients now getting a bit concerned, so another engineer asked my thoughts, so i in turn asked here for any thoughts.

Again, fair enough. Although I am far from being any authority or 'expert', as you know I essentially agree with you - namely that even if there really were 35A in each phase, I wouldn't really expect that to result in thermal damage of 32A accessories. It will be interesting to hear how this story unfolds - keep us posted!

Kind Regards, John

 

[333rocky333]

John I apreciate your replies, I like to think I have some knowledge, but its nice to have a second opinion, from someone like you and Efli and others obviously, before i report back, rather than humiliate myself.

 

[JohnW2]

John I apreciate your replies, I like to think I have some knowledge, but its nice to have a second opinion, from someone like you and Efli and others obviously, before i report back, rather than humiliate myself.

You're welcome - but don't forget that, unlike EFLI, I have absolutely no (electrical) 'credentials'!

Kind Regards, John

 

[333rocky333]

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