Electric Kiln Power Supply Problems

[Tim T]

When I wired in my electric kiln I put in an isolator switch as it is wired in direct to the consumer unit, which is tucked away in a corner and not easily accessible.

The kiln is rated at 30A, so I put in an electric cooker twin pole isolator rated at 45A. This was fine for about 3 years, firing every week or two,  though it was a bit warmer than room temperature but not worryingly so. Then the isolator switch packed up, and when I opened it up the back had gone into pretty severe meltdown.

I then replaced it with another, bought from a high street DIY chain rather than through a proper electrical distributor, and this lasted a total of 3 firings! When replacing it, the copper in the wires had built up an oxide layer, so I cut back the wire until it looked fine, and then scraped the wire to ensure no oxide would increase resistance at the contacts.

It hadn't fully melted so I opened it up, and there are plastic pins behind the contacts that press on them, and these had got hot and melted, so shortened a bit and breaking the connection.

I've now replaced it with a proper industrial isolator switch, which is much better as there is a lot more space so the wires aren't strained, and the wire clamps have a much bigger area, and the general build quality is sturdier.

Thinking through why the switch may have failed, it wasn't from using the isolator switch when the kiln was on as I didn't do that, so it was never switched under high current. Also the loads are resistive, not inductive, so that shouldn't be a problem anyway.

What I did discover, though, is that as with most metals the resistance of Kanthal increases with temperature, so when it starts up at room temperature the resistance is much lower than when heated, so the current is much higher - a multiplier of 12 applies for a Tungsten light bulb, for example. With Kanthal the resistance doesn't really start increasing until above 500C. So my guess is that what was happening was that the startup current was above the capacity of the isolator switch, causing it to overheat until the kiln elements had heated up sufficiently, when the current dropped down. However I'm not an electrical engineer by training, so any other thoughts on this would be interesting.

 

[glazenerd]

The term you are looking for is "start up amperage". < google

You will find a calculator on the first page.

 

EX, A refrigerator compressor can draw 12 amps to start up, and run at 1.5- 2 amp running there after. Not the same situation in demand, but the calculator will give you some numbers.

 

What concerns me is why you are drawing such a load anyway? I understand that you are estimating peak amperage at peak load. If the kiln is NEMA rated at 30 amp peak: they have already calculated peak demand, and added room. So it makes me wonder if an element or a relay is malfunctioning at some point, drawing this kind of demand?  From experience, after building a thousand houses or so: I have had to replace many breakers because they failed, or where faulty.

 

Nerd

 

Edit- next time you fire and start hitting peak demand- go touch the breaker and check it for heat. if it is hot- pull it!!! replace.

[Dick White]

You have a 30 amp kiln and you installed a 45 amp breaker (you call it an isolator, in North America we call it a circuit breaker, same thing).  The ampacity of the circuit for a kiln needs to be 125% of the nominal rating of the kiln to allow for heat dissipation in the wire. Thus, the breaker should be adequate. However, what size wire is between the circuit breaker panel and the kiln outlet? A 30 amp circuit normally has a wire size of 10 AWG (American standard) which corresponds to approximately 5 mm2 wire EU standards. When you raised the breaker to 45 amps, the wire should also be changed to at least 8 AWG, which is approximately 8-10 mm2 EU. If you did not install new larger wire, then it is the wire that is burning, not the breaker.

[Tim T]

Hi Nerd, Dick,

Thanks for your comments.

The circuit breaker in the distribution box is fine, no raised temperature. It is just the isolator switch. FYI the CB is rated at 40A from memory, below the isolator switch rating, and hasn't tripped out.

In English English (real English?) an isolator switch and a circuit breaker are not the same - the breaker will trip if overloaded, whereas the switch is just a switch so that the power to the kiln and controller can be switched off at one place, as the kiln controller box doesn't have an on/off switch itself..

The wire is 6mm2 which is enough for the CB. The isolator switch is intentionally rated higher than the CB as if there is a problem I want the CB to trip, rather than the switch to fail (even if this theory hasn't been achieved in practice!) 

The kiln rating is 30A max current, and measuring the element resistance this is definitely for once the elements have heated up, not when cold. This is England, so NEMA doesn't exist here!

Tim

[Dick White]

Tim, I figured from some of your original post that you were on the other side of the pond, but not sure exactly where (though the high street comment strongly suggested the mother country). So yes, we need some translating between our two different English languages. Who was it that quipped we are two great countries divided by a common language? I see now what you mean by isolator switch - here in the colonies, we call it a disconnect, and yes, it is best to have one of those in the circuit, though unfortunately many do not bother with one. I'm not familiar with the electric code over there, but here in the colonies a nominal 30A kiln requires a circuit of at least 40A, including the wire. It is the wire that is most vulnerable. Most devices are in use only intermittently, even heavy draw household items like toasters and hair dryers. That allows the cable and associated circuit breaker to cool off between uses. Consequently, the engineers who wrote the electric code designed the wire specifications with the expectation that short intermittent use and cooling cycle. For heavy continuous draw appliances, like our kilns, we are required to treat them as if they need 125% of the nominal rating so that the continuous flow of the full amperage does not overheat the heavier wire. In our code, a 30A ordinary circuit uses 10AWG (American Wire Gauge) wire, which corresponds in EU standards to 5.26mm2. Bumping this up by the required 125% to prevent overheating the wire and breaker puts us at a 40A breaker and 8AWG cable. If the kiln is distant from the main, we bump the cable one more step to 6AWG, but this is for voltage drop, not heat dissipation. 8AWG converts to 8.36mm2 (and 6=13.29), so by US standards, your wire at 6mm2 is too thin and likely will overheat when the kiln is continuously drawing full power in the last few hours of the firing. Overheated wiring typically burns out where it is connected to another device like a switch or outlet because the resistance between the screw terminal and the wire is greater than within the wire itself, causing even greater heat concentrated at that point. That's why I think the failures of your isolator were not switch faults, but insufficient wire size.

[Mark C.]

Hey now that England bailed out of the EU are the EU standers also a moot point?

All kidding aside

(Overheated wiring typically burns out where it is connected to another device like a switch or outlet because the resistance between the screw terminal and the wire is greater than within the wire itself, causing even greater heat concentrated at that point. That's why I think the failures of your isolator were not switch faults, but insufficient wire size.)

​This can also be attributed too loose connections but the result is as Dick said above. cooked wire connections.

[Tim T]

Dick, Mark,

Thanks for the fuller explanation, that makes sense. The wire run is short - perhaps 10' - so voltage drop is irrelevant, and the wire insulation doesn't get warm  (it is just running alongside a wall, so no heat buildup, but I hadn't thought about the termination. I'll bump up the wire to the next gauge - the original switch wouldn't have coped with this, but the new one will.

We are (thankfully) still in the EU and I anticipate all wiring products will continue to be made to EU specs even after we leave, as if we want to sell into the EU the products will have to comply with their standards. Hopefully we won't start rewriting wiring regs etc just to show we can do things differently than when in the EU!

We had our moment of madness in voting to leave the EU - especially as the biggest leave votes came from regions that benefited most from EU funding - but perhaps you guys will follow when Trump becomes president?

Tim