Safety Considerations PID Temperature Controllers

When designing a temperature control system we recommend considering the below factors. This is intended for general information purposes and can not be applied to every installation. The notes are most applicable to a heating system as found widely in ovens, kilns and industrial process.

Temperature Thresholds - all components need to be able to withstand the maximum working temperature of the system with a margin of safety for over-temperature. Any cables routed near to high temperature areas should be suitable for the environment.

Electrical Ratings - calculate the maximum current i.e. based upon element KW. Ensure suitable cable size and circuit breaker/fuse protection

Safe Isolation - consider the method of carrying out any maintenance on the system. By incorporating an electrical isolator it is possible to remove all power from the system and implement a Lock Off Tag Out procedure

Solid State Relay SSR - allow for heat dissipation within any enclosure or cabinet. SSRs do generate heat and our recommendation if in doubt would be to use heat sinks and a ventilated panel. Performing temperature measurements under normal load conditions can verify the SSR is within normal limits for operation. When sizing solid state relays it is better to over specify to ensure the device is operating at 50-60% of it's maximum current

Controller Parameters - the parameters in the PID temperature controller can help to avoid an over-temperature condition. Maximum setpoints can be added to prevent operators from accidentally increasing the target temperature beyond safe limits. It is also possible to add alarm conditions for Minimum and Maximum temperature

Failsafe - in the event of an unsafe condition what would prevent a dangerous occurrence? Consider the impact of possible component failure. For example, a Solid State Relay which has failed closed would maintain power to the elements

Safety Limiters - a dedicated safety limiter may be applicable for protection in an industrial environment. This can provide redundancy and isolation in the event of a malfunction. It is also possible to use two controllers in series to offer some form of protection in the event of a single failure, the over-temperature controller can often be a basic digital thermostat set to operate a latching relay on a high temperature condition 

Basic Protection - mechanical over-temperature thermostats are found widely in ovens and kilns. They can operate at an Ultimate High level and remove power from the elements. Wired in series with the element and SSR they can offer some form of redundancy in the event of SSR or Temperature controller failure

Manual Reset - it is recommended that in the event of an overtemperature condition the process will shutdown, alarm and prevent automatic resetting. By adding a latching relay and reset button this can ensure any operators are alerted to the condition before the system is restarted

Alarms - audible and visual alarms can be used to alert persons if the controller/process is operating outside of normal limits

Door Switches - micro and limit switches are often incorporated into heating controls to ensure the power to the elements is interrupted when any doors, panels or other access points are opened

Signage - there are various standards relating to signage and warning notices which differ from country to country. Correctly positioned signage can be useful to people who are unfamiliar with the process 

Initial Setup - please remember that during initial configuration and Auto Tuning it is possible for the temperature controller to run outside normal expected tolerances. Always test and verify the controller has been setup correctly for stable operation

It is always recommended to seek professional assistance if you are unsure of how to correctly install equipment.