Thermal protection, thermoregulators and thermostats are thermodependent relays that switch specific heating or cooling devices when the temperature they regulate reaches a certain preset limit. Thermal protection ensures normal thermal operation mode in various machinery and equipment, working in conditions of emitting significant heat flow from their surface. Thermal protection can be adjustable and non-adjustable, restorable and non- restorable – which in turn is divided into manually or automatically restorable. The automatically restorable thermal protection gets back to its initial position automatically after cooling enough and the manually restorable one needs to be manually returned to its original position after troubleshooting.
Normally, its active component is a bi-metal plate (but it may as well be fluid or gas) built into the device that controls the temperature, which deforms when heated and activates or deactivates certain contacts. For example, thermostats are used in refrigeration systems as well as in boilers or cars.
In thermodynamics a thermostat is referred to as a system with such a large heat capacity that the temperature which it can adopt does not alter its own temperature. Depending on the range of their operating temperature, thermostats are divided into:
- thermostats for high temperatures (300-1200 ° C);
- thermostats for medium temperatures (60 – 500 ° C);
- thermostats for low temperatures, also called cryostats (lower than -60 ° C).
According to their area of usage, thermostats are divided into:
- industrial thermostats;
- air thermostats;
- immersion thermostats.
Depending on the coolant (the operating element) thermostats are divided into:
- air thermostats;
- liquid thermostats;
- solid state thermostats (with a Peltier element or bimetal).
Thermostats can also be classified based on their accuracy in maintaining a certain temperature:
- 5 -10 ° C and more – it is achieved without additional measures, only by air convection;
- 1-2 ° C is a good thermal stability for air thermostats, but rather weak for liquid thermostats;
- 0,1 ° C – very good thermal stability for air thermostats and medium thermal stability for liquid thermostats;
- 0.01 ° C – it is reached only by liquid thermostats.
With regard to thermal protection, most common are capillary thermostats with two normally closed contacts and manual restorability.
When selecting such protection the main parameters that need to be addressed are, of course, its operation temperature, but also the length of the probe, and the length of the contact part of the probe. It is important to coordinate the electrical load and the capabilities of the protection to take it. For example, if we have a hot water boiler with capacity of 3500W, at nominal voltage of around 220VAC, we must seek protection that is designed for at least 15A rated current when the load is resistive.
Bimetallic protections with ceramic and plastic housing are widely used in hot water
boilers. Unlike thermoregulators or thermostats, they can be non-restorable or if they are manually restorable, the number of activation cycles is much smaller, because their function is to deactivate the heater if the thermoregulator won’t work due to failure.
Bimetallic thermal protection represents a simple bimetallic relay which can quickly and easily be replaced, in case it burns out.
Thermal fuses are a kind of non-restorable temperate-dependent fuses which usually contain a small metal sphere (ball) made of fusible material and a spring.
Upon reaching the temperature limit, the sphere melts, thus releasing the spring, which opens the normally closed contacts and disconnects the electrical circuit. The principle of operation of thermal fuses is based on the thermal component of the electric current. The size of the nominal current of the protected device, i.e. its power, also needs to be taken into account.
Thermal fuses are used in household appliances which operate at higher than the ambient temperature – hair-dryers, irons, coffee makers, hair curlers, etc.
Unlike overcurrent protection, thermal fuses are passive elements which do not respond to short duration peak currents, they are activated by heat. Many of the network transformers (with primary winding at 220VAC) have a thermal fuse which is integrated into the housing or is placed in a special socket.
Thermal protection is an important component of electrical circuits. It protects the devices and appliances from overheating, melting, explosion or fire. Therefore thermal components must be carefully selected and if they are non-restorable and need to be replaced, like thermal fuses, we mustn’t use fuses with higher burn out temperature but instead find and eliminate the failure.