Short Circuit Protection Against SSR

"If the controller output has hit bottom limit and the temperature is still rising, then alarm."

"If the furnace temperature hits 2nd high limit, then alarm and trip." - they are called trip amplifiers ["trip-amps"] for a reason!

I've also seen circuits that monitor the current (ammeter) going into the SCR used for this.

Thank you for your informative reply. Since the oven I am designing is only 15 kW rating, will the regulation of providing an extra electro mechanical relay controlled by an over-temperature-controller be mandatory ?

I'm in the US. Your CR4 ID says Calcutta. I have no idea what your national regulatory requirements are, but since India was a British colony for centuries, I would suspect that India 'inherited' the bureaucratic nature of regulatory agencies and that there are regulations requiring such.

All that aside, thermal engineering good practice demands a safety high limit because a run-away 15Kw heat source can burn down the building.

I call it a high limit controller, they call it a safety alarm unit. Same functionality, same concept of separate process controller from safety controller so there's no common failure mode.

Thanks for your valuable advice

Iris: In your opinion (most respected and highly valued) for smaller heat control circuits of only up to a few kW, is it enough to control with a small relay and a definite purpose contactor as to meet safety requirements as a fail safe? Or since if the relay is SSR, then it might or will fail closed, then the safety will not go open circuit? Thus even on smaller circuits a trip needs to wired into a hard metal contactor to unlatch it?

I honestly don't know the regulatory aspects at lower heater capacities because I work with higher capacity industrial burners that are covered under NFPA 86.

To my knowledge, all kitchen appliances with a heating element intended for a circuit with 15A fuse in a 120Vac line (in the US) have either a thermal fuse or thermal cut-out: electric toasters, my Panini (hot sandwich 'press') maker, the coffee maker. A thermal fuse or cut-out performs the same safety function as a limit controller, interlocking the electrical power.

The SCRs in my microwave oven and photo copier heater both failed 'closed'.

I suppose there are some heaters that by their nature are low powered enough and cannot heat up enough (even when misused and covered with a stack of newspaper to trap all the heat) to create a combustion source to not warrant a thermal fuse, but I'm guessing on the order of tens of watts, not hundreds. I'm sure regulators look at misuse potential as much as use potential.

We use a 65W control panel heater. It's line fused for protection against a heater element short to ground and it has a very large aluminum surface area designed so that a person working in the panel can't be burned by the heater. I don't know whether it has a thermal fuse or not, but given that a thermal fuse is probably $0.50 or thereabouts in production quantities, I suspect that even that heater has one for liability/tort reasons, if not code.

All that's anecdotal, but the nature of SSR failure is neither anecdotal nor a secret; those involved in the chain typically state the limitations. Omron sells SSRs and has an excellent 9 page engineering applications guide, and states the short-circuit output failure warning upfront on page one:

"SSRs, however, use semiconductors, and semiconductors may commonly malfunction or fail. Short-circuit failures represent the main failure mode and can result in an inability to shut OFF the load. Therefore, for fail-safe operation of control circuits that use SSRs, do not use circuits that shut OFF the load power supply only with an SSR, but rather also use circuits with a contactor or breaker that shuts off the load when the SSR fails."

http://www.omron.com/ecb/products/pdf/precautions_ssr.pdf

Maybe someone else who has direct experience in low heat capacity heaters can chime in and can render a qualified answer.

Thank you! That should provide insights as to how to develop small projects. I used a definite purpose contactor for heating circuit (500-1500 W) on a bench-scale test reactor (chemical reactor) as a subtitute for steam heating that was being used in production. It modeled well, performed well, and not problems with electrical failure, etc. This may have been before these SSR's were all the rage. I seem to recall I used the output relay from a 1/4 DIN temperature controller, and that may well have been a small SSR, but I also remember checking rated output current, and the latch current for the definite purpose contactor, that they were indeed well inside the envelope. Of course there was also a circuit breaker on the mains.

If you are using Eurotherm SSR's as well, then they offer feedback of current measured throught the SSR, via the control cable between the temperature controller and the SSR. So if you have no output from the controller and the process is still drawing current, then you know you have a problem and, you could drop a contactor out feeding the supply to the process.

Our you could have purchased equipment that will do this self checking process periodically automatically for you. Like PSG http://www.psg-online.de/en/home/

I worked in a lab where before my tenure they were replacing I^2t fuses left and right until I set things up correctly.

We were using phase angle fired into a variable transformer and operated our own wound low voltage tantalum heaters in a vacuum chamber thus shorts were common place.

I made sure the current feedback was working and the SCR unit was oversized, BUT I added a 3AG fuse in series with the load. So instead of blowing a $30.00 fuse, the $0.25 fuse popped.

Eurotherm makes controllers that are for resistive loads and tungsten loads. Their end controllers use like 0 to 100% to be proportional to V^2 which gives you finer control. Eurotherm uses terminology like SCT (slow Cycle Triac), PAF (Phase Angle Fired) and ZCT (Zero Cross Turn-on) which are methods with different abilities of control.

One system had to have a separate limit controller as suggested here. Eurotherm called them "policemans". Some of my simple controllers could be configure to drop out on power fail and those had a contactor that removed power completely. We had to because in loading the Vacuum reactor, the wiring would be potentially exposed. Never thought of incorporating a GFCI.

In one system I designed in the 80's i used a concept that I called "Energy Limit" in watt-minutes.

The heaters were DC and thus I, V and T were always measureable. The heat-up energy was used as a limit. If a thermocouple was out of position, the heat-up energy became a good parameter to look at. If the thermocouple shorted or was out of position on HEAT UP only the controller would shut down. It had an overtemperature limit too.

What was really cool, is this was was basically an 7 loop PID controller with a 5 sec repeat rate that did voltage, current, power, temperature control all simultaaneously. When you selected temperature, the other parameters were used as limits. dp/dt was controlled too. The PC hadn't been invented yet. It also incorporated recipies. A Fortran program created the files necessary to tweek the system. This was research and not production.

FWIW, it controlled up to 3 evaporative sources and a substrate heater.