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I'm working with some 230 VAC resistive heating elements which are switched by a solid state relay (https://uk.rs-online.com/web/p/solid-state-relays/9045728) and driven by a 4-20 mA signal from an external PID controller. There are 4 x resistive elements and all of them are in parallel. The resistance of the elements (before the initial test) were as follows:

R1 = 86 ohm R2 = 85.5 ohm R3 = 85.7 ohm R4 = 86.9 ohm

The test was then conducted with all 4 x resistive elements inside a stainless steel cylinder bonded with thermal compound and the temperature raised to 600°C and controller via the external PID controller.

I'm having some problems with the resistive elements themselves. For some reason, after around 10-12 hours of testing one of the elements either short circuits or goes open circuit (I've run this test multiple times and I continue to get failures). My questions therefore are as follows:

  1. Is the reason the element is going open / short circuit because the SSR is too harsh on the elements themselves? Should I instead be using a Silicon Controlled Rectifier that will allow a gradual voltage change across the resistive element? Something similar to this: https://uk.rs-online.com/web/p/signal-conditioning/8513563

  2. When one of the elements goes open or short circuits, the resistance of the other elements jumps from around 80 ohm to >200 ohm (Note that I'm not measuring in parallel, I take each element away and measure the resistance). Is the reason for this similar to the thermal runaway effect found in diodes and transistors in the respect that if one element breaks, the other elements will try to compensate by maintaining the same level of current?

For information, the resistive elements are 230 VAC devices and are capable of handling up to 700°C. The chamber in which the elements are installed only reaches 600°C and the maximum temperature the elements themselves reach is around 610°C so the elements themselves aren't overheating.

The elements after their 600°C test and after returning to room temperature changed to the following values:

R1 = 85.7 ohm R2 = 84.8 ohm R3 = 85 ohm R4 = 85.9 ohm

The jacket of the chamber is water cooled by a booster pump that provides more than enough cooling capability to keep the entire chamber at ambient. I would suspect that we're overdriving the elements rather than overheating them.

Thanks in advance for your help. Happy to provide any additional details as required.

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  • \$\begingroup\$ At these temperatures I'd consider rebuilding to be more like a kiln, using resistive heating wire directly. Then it could handle 1200°C; so operating at half of that should last practically forever. \$\endgroup\$
    – rdtsc
    Apr 9, 2021 at 12:11
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    \$\begingroup\$ If the chamber reaches 600C you are almost certainly exceeding the rated 700C inside the elements causing their breakdown. Datasheet on the elements may be useful. \$\endgroup\$
    – user16324
    Apr 9, 2021 at 12:49
  • \$\begingroup\$ As far as I'm aware, there's nothing wrong with this method of controlling resistive heaters. I would look at the heaters themselves. Verify that the correct voltage is being applied to the heaters. Verify the internal temperature is only 610 degrees like you think it is. Verify the heaters are designed to operate for over 12 hours at that temperature (perhaps they can handle the temperature, but not for very long?). \$\endgroup\$
    – user253751
    Apr 9, 2021 at 12:50
  • \$\begingroup\$ I will triple check the results, make sure that the correct voltage is on the elements, make sure the internal temperature is not exceeding the maximum temperature of the device again. Is there a possibility that when one element breaks that it could have an adverse effect on the resistance of the other elements? Or does it make more sense that the elements are derating because the temperature is just too high? The jacket of the chamber they're inside is water cooled so I wouldn't have thought it would be the elements being too hot. I'll add this information to the original question. \$\endgroup\$
    – CT123
    Apr 9, 2021 at 13:08
  • \$\begingroup\$ The heat must dissipate fairly evenly from the resistance elements or you will create hot spots. Sections of the element that rise in temperature too fast (relative to other sections), will have increased resistance and dissipate a disproportionate amount of heat. This will cause the temperature of the hot sections to rise even more. This is a form of thermal runaway. Example: when a hot spot forms on an oven heating element, the element will quickly fail. \$\endgroup\$
    – Mattman944
    Apr 9, 2021 at 13:48

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It sounds strange a solid state relay (on-off) controlled by a 4-20 mA signal. Anyhow the supply voltage should not damage itself the heating resistors, it looks like the internal temperature of these elements is over 700ºC. The resistance about 80 ohm, is it at cold temperature? Probably at 600ºC the resistance is much more and could explain 200 ohm. Try to check before the open/short issue comes up, the resistance of elements. If you can measure voltage and current, it is possible to calculate parallel value.

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