Contactors are used to physically disconnect power to maintain user safety. The contactors in our equipment, referred to as Definite Purpose Contactors (DPC), disconnect the power when the doors are opened, or when the temperature passes a high limit.
These relays are used to monitor and trigger several actions in the equipment. Most commonly they will cause the DPC to open when the door switch or high limit are tripped.
These are very similar to what you find in your service panel at home. Circuit breakers cut the power when a high current is detected, protecting the user and the equipment. Circuit breakers are most often found on large equipment with several heating circuits or zones.
Fuses and Fuse Holders
Fuses are used to protect certain components inside the control panel. Small fuses are used to protect the control circuit (Watlow controller, safety relay, etc) and large fuses may be used on the power circuit to protect large and expensive power controllers, such as Din-a-mites and ePower’s. They also protect equipment and the user in the same way as circuit breakers, but are often faster reacting and more sensitive.
The door switch senses when the door has been opened and causes the DPC to trigger and cut power while you are gathering (Electric Furnace) or entering an oven to prevent electrical shock.
High Limit, Door Switch, and Interlocks
The interlocks is a safety feature present on all our equipment. Depending on the equipment, the exact components it involves changes. Simply put, the interlocks is a series of sensors and switches. If any of these sensors has tripped its switch, the equipment will not turn on, or if on will turn off. On an electric oven this consists of a high temperature switch, and a door switch. On combustion equipment this consists of high temperature and gas pressure switches.
On electric ovens these switches are tied to a safety relay. A safety relay is like a light with two switches, for the light to come on, both switches must be on, if either is switched off, the light bulb turns off. On an oven the component the safety relay switches is the Contactor. This is a mechanical disconnect that physically cuts all power to the elements. If the high temperature is triggered or if the door is opened the safety relay causes the contacot to open, cutting power. When the door is closed, the contactor closes, allowing power through again.
The interlocks loop is often where other safeties are added in. This could be an air switch that proves the ventilation is running, and fire alarm, or a CO monitor. If any of the sensors in the loop are tripped, the power to the elements are cut off.
SELV Safety on Electric Furnaces
Electric furnaces can pose dangerous situations. You’re reaching a metal punty into a space with electrical elements. While we always use door switches to cut power to the elements, some users in production settings find that they are opening the door to often for the elements to keep up with the heating needs if they are off half the time. This is where the SELV systems comes in.
Safety Extra Low Voltage (SELV) is a combination of very low voltage and an isloated floating system. Voltage is what causes an electrical shock to be dangerous. The amperage or current is what will hurt and kill you, but the amount of current you experience when shocked is decided by the voltage. The higher the voltage, the more current you will experience flowing through you. This is what makes high voltage dangerous.
While low voltages will still shock you (have you ever touched a 9v battery to your tongue) they do not cause enough current flow to endanger you. In our electric furnaces, the electricity passes through a transformer that drops the voltage significantly. By keeping the voltage underneath 48v, we stay under the threshold that usually causes dangerous conditions.