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Donuts > Fried Eggs
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Fused solid state relays can have wicked high short-circuit current ratings. Many manufacturers publish coordination info that show what combination of fuses are needed to get to a specific SCCR. You can get well up to 100kA with this method, but you'll still be paying to replace fuses.

Out of curiosity, what kind of contactors are you using? Upsizing a NEMA contactor would get you a lot more life than an exactly sized IEC contactor.

Why are these failing this frequently? You might consider putting some sort of ground-fault protection on them, and maybe you'd catch an impending failure before it turned into a hard short.
 

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Discussion Starter · #12 ·
Big John said:
Fused solid state relays can have wicked high short-circuit current ratings. Many manufacturers publish coordination info that show what combination of fuses are needed to get to a specific SCCR. You can get well up to 100kA with this method, but you'll still be paying to replace fuses. Out of curiosity, what kind of contactors are you using? Upsizing a NEMA contactor would get you a lot more life than an exactly sized IEC contactor. Why are these failing this frequently? You might consider putting some sort of ground-fault protection on them, and maybe you'd catch an impending failure before it turned into a hard short.
I've looked at solid state relays and in answer to your question I've already up sized relays two sizes. The ovens have staged heating so the relays are opening and closing 100 times per cycle. 3 cycles a day. I meg out these heaters as a matter of course every few months (to cut out bad ones) but there's no way to know.
 

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I assume that a contactor closing into a fault will become extinct very quickly.
Think about a motor starter, they close iwith inrush current every time it closes,

Figure maximum load, figure maximum instantaneous pickup current of the OCP and size accordingly.
 

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Donuts > Fried Eggs
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So what about fusing SSRs? Why wouldn't that work?

Alternately, are you going through enough contactors and man-power to make it worth it to drop in a 10kVA isolation transformer, and leave the secondary ungrounded with ground-fault detectors? It would show you when an element failed without destroying anything or tripping the breaker. But that will probably set you about about $1,300.
 

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I've looked at solid state relays and in answer to your question I've already up sized relays two sizes. The ovens have staged heating so the relays are opening and closing 100 times per cycle. 3 cycles a day. I meg out these heaters as a matter of course every few months (to cut out bad ones) but there's no way to know.
Well, we still don't know where you started from, so saying that you upsized twice is meaningless. You also don't describe your circuit very well, ie is this single phase, 3 phase? Hopefully it is 3 phase, because 9kW @ 480V would be 18.75A, and you said it was a 15A breaker. So if it is an even 3 phase distribution to the elements, then 10.83A per phase, what size contactor were you using? If you started with a 10A relay and went to 12A then 15A, that will not hold up to anything but running current, not fault capability at all. But if you started with 20A, then went 40A, then 90 A, that's a different story.

Also what kind of "relay"? Electro-mechanical, mercury? All we know is that it isn't solid state. 300 operations per day means getting to the 1 million mark, a typical lifespan of a NEMA contactor, will take you roughly 9 years. But if you use cheap DP contactors, you might get to the end of life in 32 months.

But back to your exact question, a properly selected and protected contactor is supposed to be able to handle carrying fault current until an up stream protective device clears the fault. But NO contactor is designed to close into an existing fault, that condition must be specifically avoided. If you have a ground fault (the typical failure mode on a heater element), the potential fault current is whatever the available fault current is in the system. On a 480V system with no thought put into the design, this could easily be 10s of thousands of amps. Nothing (within reason) will survive that.

This is, by the way, why ovens that can have high failure rates like this, are often designed where the heater elements are using very low voltage / high current, and a transformer is put between the 480V control device and the heater elements. The transformer limits the fault current so that the up stream protective device can clear the fault in time to protect the devices.
 

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No normal contactor will survive closing on a fault. This is not a good application to use a contactor for temperature control. Use a contactor as a high temperature shut down and a solid state relay to cycle the heater. Install fusing in each heater lead - It may take a little work, but I would probably same from replacing contactors all the time. You could also install a ground fault relay on each bank of heaters and wire this relay to drop out / prevent energizing the contactor.
 

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Discussion Starter · #19 · (Edited)
An update if you're all interested. After having looked at this problem for the better part of 12 hours, I took a deep breath and went back to basics. Here was the problem. The terminal strip was rated for the heat. Stupid me didn't check voltage rating. Thanks for all your help.
 

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Sparks fly from my finger
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Good catch. Surprising there isn't more carbon blasted around being 480. My guess is though that it is arcing there from the inrush when the heater goes bad.

Does the unit have current limiting fuses?
 
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