Mains contactor expected lifetime?

It's a heating coil. I bet it's not pure resistive load. Surely, any inductance would tend to affect the life of the switch? So, when you find the spec sheet and it says the contactor will survive 200,000 cycles (or whatever) in ideal circumstances, what fudge factor will you apply?

How bad? Melt the heating coil and cause you annoyance, or burn your workshop down and shoot molten glass over the survivors - which end of the scale?

Top quality British manufacturer there.

Can you put in a hysteresis circuit so the contactor makes fewer operations?

Contactum stuff still seems to be fairly well made

Owain

Not seen any data sheets for that...

Most of the contactors I have looked at are rated at well in excess of

Perhaps in a commercial use every day scenario it would be worth replacing it every five years or so.

If you search for contactors on RS they normally have the data sheets for them as well.

What's the control system like that drives the contactor? Something that does a bit of failure sensing and could open a series wired "normally closed" lockout contactor might be worth having. (or possibly just crowbar the supply and let the MCB do it!)

Probably not _purely_ resistive.... but much more so than (say) a big motor. Being a cautious sort, I'd probably derate by 50% from the manufacturer's quoted switching lifetime - simply because I'd rather replace a £25 contactor every 5 years (or whatever) than sort out the mess if it decided to stick 'on'.

Either of those scenarios are bad enough.. The most likely failure (I guess) would be the melting of the heating element at or around 1500c (which would shut off the heating soon enough). The fibre insulation in the kiln is rated to 1000c - so it could be damaged & need replacing if the kiln spent any time above that temperature. If the insulation should break down then I'd have 1000c's worth of heat let loose in a timber-framed workshop - with thousands of euro's worth of equipment and raw materials...

So that's why I'm wondering how frequently I should swap out the £20 contactor....

Yes indeed!

Currently the control is via a dedicated kiln controller

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The long-term aim is to have the Raspberry Pi (that's currently only monitoring temperature / switching in the kiln) to actually do the controlling, at which point I'd be able to fine-tune the hysteresis.

Could be. Can't seem to find any quoted 'operational lifetime' on the web though...

Thanks Adrian

No - me neither..

OK.... Or about 8 years, fired 3 times a week... Kiln's been built for 4 years, and, some weeks, doesn't get fired at all... other weeks probably 5 times per week, still - allowing a 50% de-rating (on the grounds of paranoia! ) - we're not that far away from 'swap-out' time..?

RS have a Legrand 40A DP mains coil contactor - but they want ?107 + vat for it - and quote a mechanical life of 10^6 cycles The Chint unit costs ?25..... draw your own conclusions!

I _think_ I'd rather swap the Chint unit out every 4 years, than shell out for the Legrand unit - but it'd be interesting to know what life Chint claim..

Currently the contactor is driven by a commercial kiln controller. The power switching box is wired with two 40A contactors in series - original plan & still long-term aim was to have one as the main 'switching' contactor, and the other as a 'safety' contactor. I've a simple circuit using a separate thermocouple and dedicated ic to kill the safety contactor if (either) the safety thermocouple malfunctions or the measured temperature goes above a preset limit (probably 875 - 900c). By a relay latching circuit requiring a manual 'go' button to be pressed, the kiln would fail-safe.

It'd be a lot more effective if it was actually implemented - rather than just being a circuit-diagram - so I need to get round to that.. possibly sooner rather than later.

Thanks Adrian

NT

It's quite unusual for them to "fail on." The coil is most likely to fail. Which is fail off. If the contact points are visible they can be chaecked for burning.

Most kilns have a fusible link (bit of lead/solder) to guard against overheating. These are virtually fool proof. You might find one in an old storage heater.

Two in series, you would need to be very unlucky to get both failed on? Brian

or many times that on some of the more "industrial" rated ones!

(worth checking if the rating is for mechanical cycles or contact life though. You may find some that are good for far more cycles than the contacts will survive when run at near maximum load. (although you are not running yours that hard by the sounds of it - depending on how much reactive element there is to the load)

You could swap the contactors - retire the high switch count one into the always on role... that will get you an extra few years out of it ;-)

Yup, although its easier and cheaper just implemented on paper ;-)

If the coil is the cause of failure rather than the contacts welding together...

At 50Hz it is going to be resistive for all practical purposes.

Given that, I think you absolutely need something in the circuit that melts before the insulation gives way, but obviously not at normal operating temperatures.

Harry says they are commonplace in kilns, and, given what you said above, we can well understand why.

True, but its not the steady state powered state that matters. Its the transient switching where you will see any inductive effects (i.e. a large back emf from the heater)

Except the inductance of a heater is approximately zero for anything at

What is everybody drinking?

Isn't a heater lots of coiled wire?

John Rumm is right to point out that the operating frequency is irrelevant to the stored energy and back EMF on switching. However, the sort of inductance we are talking about is not going to store a significant amount of energy even at the maximum instaneous voltage of about 350V. If anyone has any figures for practical inductance we could work it out.

or add an RC snubber. Though as said there's very little to snub.

NT

What's the frequency content of a switching transient?

Much depends on the heater wire topology. Classic tubular heating elements with helical wound element wire (like that of a bar style electric fire) will give you quite a significant back voltage on switching.

Innis & Gunn Oak Aged Original or the Rum Finish ;-)