An oven meltdown can be a very ugly thing, especially if the oven is full of your work when it happens. Here is an inexpensive way to minimize the chance of this happening to you.

If a temperature controller with an alarm output relay is controlling an oven, a high temperature limit switch can be added very easily. This way if the power control SSR (or relay) malfunctions and locks on for any reason, the alarm circuit will cut the power and keep the temperature at a level which will protect the oven - if not the work inside it - from damage. There is a drawback to using this system though. If there is a malfunction in the temperature controller itself, or in the thermocouple connected to it, then this setup might NOT protect the oven from a meltdown. The safest way of protecting an oven is with a totally independent high temperature limit controller and thermocouple, and in some areas that may be the only system that will meet code requirements. But, where permitted, this is a decent alternative to spending the extra money for a separate limit controller. A good quality temperature controller is the most reliable link in a control system, so the chances of one failing and causing a meltdown are slim. And even the cheapest digital controllers available today offer thermocouple break detection, which - when programmed properly - will turn off the control circuit if the thermocouple breaks. Not including human error, 99 times out of 100 an oven meltdown is caused by a failure of the oven control relay (SSR or mechanical relay). According to Murphy's Law, the chances of a relay failing in the "on" position are directly proportional to the value of what is in your oven...

The drawing below is obviously very basic. It assumes a 240V single phase oven being controlled by a Solid State Relay, driven by a temperature controller. There are many different controllers and control hookups possible, so this may not be the same as your particular setup. The same general principles should apply, though.

In the drawing below the alarm output circuit in the controller is connected to the control coil in the contactor. When this circuit is energized, the coil turns the contactor on, power flows through the contactor, and the SSR can turn the oven elements on or off depending on the control signals received from the temperature controller. If the alarm circuit shuts the contactor off, it doesn't matter what the SSR is doing, no power will reach the elements and the oven will stay off. In this setup the alarm circuit in the controller is energized when NOT in an alarm condition an de-energized when an alarm is tripped. Tripping an alarm removes power from the contactor coil and shuts off power to the elements.

With the circuit wired this way and the temperature controller alarm programmed correctly, the alarm circuit will shut down the power if the temperature goes above a pre-set limit. Depending on the features available in the particular model of controller used, the alarm could be latching, which will simply kill the power to the elements and keep it off until the controller is manually reset, or non-latching, which will kill the power whenever the temperature reaches the limit, but allow it back on once it falls below a preset temperature again. One needs to decide which is appropriate for his/her process, depending on what the controller being used offers (some controllers offer additional options, too).

This is about the simplest way to set up this system. However, with just a little extra effort and wiring, a few more desirable features can be added. One I recommend is a door switch, which shuts off power to the elements whenever the oven door is opened. This is an especially good idea when using SSR's, because SSR's (and SCR's) leak electricity even when they are turned off. They don't leak much, but under the right circumstances it can be enough to do some serious damage to someone. It is very easy to wire a door switch into the alarm circuit controlling the contactor (which is what I have done on my glass furnace). Another possible addition is an alarm buzzer or other warning device that will alert you in case the alarm is tripped. If you are worried about something happening when you are away from the studio you could even wire up an automatic telephone dialer (available at Radio Shack and elsewhere) to let someone know there's a problem.