Voltage drop question

On 21/11/2011 17:25, Andy Burns wrote:

Checking if there is a way of accessing an independent earth in the workshop. If there is, then whatever it is (pipes etc) need to be included in the main equipotential bonding. Hence that will also need to be carried on the SWA CPC.

There is a (rare) failure mode with PME supplies, that a loss of suppliers Neutral (the so called Protective and Neutral conductor that comes into the building), then your installations neutral and hence earth will be left floating, and connected to live via all your appliances.
This could result in live casework on the kiln. This becomes dangerous if you can touch something else that is still connected to a real earth. If all the stuff around you is only earthed via the PME earth, then it should all be sat at the same elevated voltage, and hence not pose a direct shock risk.

Yup, especially with slow start - that could push it well into the cycle.
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John.

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On 21/11/2011 02:47, Andy Burns wrote:

Could you simply measure the voltage at each end of the cable, under load...? That would tell you what the actual volt drop is. Then you'd know if it was worth worrying about...
I've heard that kiln elements go higher resistance with age - if it's really old then it might be worth replacing the element(s) - though that could result in an increased voltage drop.
What is it about the installation that is worrying your Dad ? Adrian
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that method is asking for wrong results. Run a wire temporarily so you can measure the Vdrop directly with a multimeter.

NT
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You can meaure it entirely at the kiln end, by switching the kiln on and off and looking at the difference.
You really want professional (fused) test leads for this though, or if you have a socket too, use a plug-in power meter on its voltage setting.
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Andrew Gabriel
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Andrew Gabriel wrote:

Not sure if his controller insists in ramping up over several hours, I realise the same measurement still applies, just less convenient to take readings,

I've conveniently ignored the lighting circuit (lower tolerance on voltage drop, probably scrapes in even with existing configuration) and sockets (I think he avoids running the pugger on its 1ph->3ph convertor while the kiln is running and I doubt the wheel uses much power).
Anyway, yes there is a socket circuit, so I could leave a meter (or UPS) plugged in there rather than poking about with unfused leads.
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You could use another known load, such as a kettle and a 2kW heater, to work out what the supply impedance is at the kiln, or at least to cross-check the results if you think the kiln might not have been running at full blast.

If you're using CFLs or fluorescents with electronic control gear, that won't notice. Filament lamps OTOH will amplify the effect.

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Andrew Gabriel
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Adrian Brentnall wrote:

He thinks it will be struggling to sustain top temperature, and shortening the life of elements more quickly in the process of trying.
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On 21/11/2011 10:38, Andy Burns wrote:

I'd say it was unlikely that a few volts drop would make any significant difference..
I'm guessing it's a pottery kiln ? Does it have a temperature controller, or does it rely on cones to control the firing ?
My glass kiln (computer-controlled - and smaller at only 14kw) uses a surprisingly-small amount of electricity per firing - as the heating elements are turned on and off to achieve the required ramp and hold times.
I would think that he's probably worrying about nothing... <g>
Adrian
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Adrian Brentnall wrote:

It is.

Has a controller, he uses it to ramp up over several hours so that peak load should be on E7 rates, he's talked about the thermocouple for it, and calibrating using cones.

larger, his is 11kW

Yes, it is quite well insulated, as I mentioned I gave him an energy meter which can clip on the tails from the Henley blocks, but he's never mentioned how much it costs per firing ... though it did show him the cost of leaving the pond pump/UV treatment on 24x7

Maybe, I was more concerned that he doesn't do anything daft with the wiring, that what results that achieves <g>
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On 21/11/2011 10:38, Andy Burns wrote:

forget that about the power on my kiln - it's 3kw or thereabouts - duty cycle comments still apply! Adrian
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Adrian Brentnall wrote:

Yeah, no probs.
When he upgraded the controller a couple of years back to one that uses external solid state relays (DIN rail mounted TRIACs or similar I suppose) they recommended special fast acting fuses to protect them, turned out the fuses were more expensive than replacing the 'relays'
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On 21/11/2011 12:58, Andy Burns wrote:

Hmm - the standing joke in the power electronics I was involved with (fork trucks) - was that the semiconductors were there to protect the fuses! - it always seemed to work out that way <g>
Without actually running some sort of data logger that shows you 'time on' and 'time off' for the elements it's hard to be certain, but my gut feel is that the odd 5v subtracted from 240v (say 2% difference) isn't going to make that much difference to the power from the elements. I guess w=vi.... but then, the element's R will vary with temperature....
Adrian
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I've use FF fuses to protect mains switching power MOSFETs in one of my designs. Not yet had the race to see which blows first though. Heating elements don't normally have any switch-on surge, so you can fuse the current quite closely, but pay attention to the fuse's own power dissipation when close fusing.
MCB's are pretty damn fast too if you run into the fault protection zone (magnetic trip area). Where I had an expensive X10 DIN rail dimmer, I put a 3A Type B MCB next to it. However, it's no longer dimming mains filament lamps (they're now all low voltage), so that shouldn't be an issue anymore.
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On 21/11/2011 02:47, Andy Burns wrote:

Is there any reason why you can't use an RCD to provide protection and use a local earth?
I guess I would "weigh" up the cost of cables csa vs cost of lost power over a number of years, given expected use of the kiln, to pick the optimum size of cable in terms of cost.
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Fredxx wrote:

That would be quite straightforward, if it there was any benefit.

Replacing the existing cable is not really on the cards, its been buried for many years, the workshop and garden have grown up around it. Besides buying a replacement with a total CSA greater than 4x16mm^2 wouldn't be cheap.
The question was really around whether it was allowable to do the doubling up, or just to stick to the status quo, rather than anything else.
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