Mains contactor expected lifetime?

HI Folks. I'm using a 40A mains contactor to switch a 7kw resistive load (heating elements in a glass-fusing kiln) The particular contactor is made by Chint - part no IC40/2. Can't seem to find a spec online for the expected life (no. of on/off cycles) of the contactor - and wondered if anybody out there had any experience of how long these things last, or a copy of the data sheet.
A recent 'play' with a RaspberryPi to monitor the heating / cooling / switching on the kiln indicates that the contactor is switched some 450 times per 8-hour fusing schedule - which is why I was wondering...
As the kiln doesn't yet have the 'over-temperature' safety circuit (that some of you helped to design - thanks) installed, a fail-on of this contactor would be a Bad Thing - so wondering about swapping out the contactor _before_ it's expected end-of-life...
Thoughts / experiences / actual specs? I'd thought about using a solid-state relay instead - but I'm not convinced that they'd be any more reliable, and (apparently) they can also fail 'on'....
Thanks Adrian
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On 06/04/2017 08:40, Adrian Brentnall wrote:

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?

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On 06/04/2017 11:19, GB wrote:

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....

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On 06/04/2017 13:16, Adrian Brentnall wrote:

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.
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Two in series, you would need to be very unlucky to get both failed on? Brian
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At 50Hz it is going to be resistive for all practical purposes.
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On 06/04/2017 18:57, Roger Hayter wrote:

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)
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On 07/04/17 03:07, John Rumm wrote:

Except the inductance of a heater is approximately zero for anything at 50Hz.
What is everybody drinking?
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On 07/04/2017 06:46, The Natural Philosopher wrote:

Isn't a heater lots of coiled wire?
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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.
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On Friday, 7 April 2017 10:48:03 UTC+1, Roger Hayter wrote:

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

It is not the frequency of the transients which is relevant, just the very low energy.

Are you thinking of Lenin (1919), "On the ultra-left rejection of coiled furnace element."?
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On 07/04/17 13:00, Roger Hayter wrote:

No.
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On Fri, 07 Apr 2017 10:48:00 +0100, Roger Hayter wrote:
====snip===

It's not the maximum instantaneous voltage so much as the maximum instantaneous current. In this case, a mostly resistive load with a small element of inductance, the two are almost the same thing.
It's the interruption of ca 29 amps of current that creates an inductive back EMF to trigger an arc. However, the use of an AC supply allows for a much smaller contact separation distance than that needed when using a 240v DC supply. The 100 reversals per second of a 50Hz supply ensures that any arc will self extinguish within 10ms of the contacts separating.
In fact the 'auto extinguish' feature of AC supplies is so effective that a much cheaper 'toggle effect' can be used in light and socket switches, one where the end (ab)user can, with some dedication to the task, defeat the toggling action to admire the SFX and, in some cases, the 'light show' from behind the plastic switch plate.
The old fashioned "Tumbler" light switches used on DC supplies had a more foolproof toggle action, along with a contact separation measurable in centimetres rather than the millimetres of a modern light switch.
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On 09/04/17 01:35, Johnny B Good wrote:

Well no, there is no 'inductive EMF' in the case of a resistor. However mains voltages are enough to strike one anyway. fact any voltage is, if the gap is small eniough.

Yup. Which is why many relays are derated for DC load operation.

Yup. Never said you wont get arcing, just that 'inductance' was a red herring.
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On 07/04/2017 06:46, The Natural Philosopher wrote:

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 ;-)
http://innisandgunn.com/our-beers/favourites/original/
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On 07/04/17 11:07, John Rumm wrote:

As much as a millivolt?
Dont be silly.
You simply have no clue do you?
Coils like that are what we used to use to resonate at 10-20Mhz, in conjunction with less capacitance than you would see between the leadout wires going to such an element.

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On 07/04/2017 17:16, The Natural Philosopher wrote:

Oh good, I can join your club.
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On 07/04/17 18:20, John Rumm wrote:

No John, sorry, you cant.

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On Friday, 7 April 2017 11:07:02 UTC+1, John Rumm wrote:

You are talking total drivel. TurNiP is right.
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