Functional parts of rp and a devices and conventional symbols used in them

Fig 3.5. bimetallic strip

If one line is overloaded, the bimetallic strip is overheated and thus it bends, breaking this way the contact. shown in below figure

Fig 3.6. bimetallic strip is overheated

It should also be mentioned that there is another kind of overload protection relays called “electromagnetic relay”. This operates exactly the same as the overload protection relay, but has inside also another electromagnet.If this electromagnet is powered, then the relay will be forced to break connection, as if it was overheated. This functionality allow to check for faults and stop a motor to avoid any other problem, even if the motor itself is not overheated.
Temperature relays
These relays operate similarly to the overload protection relays above. The major difference is that the bi-metallic strip is not heated by the current that flows within the strip, but from an external factor. This factor could be the ambient air, water temperature, another fluid refrigerator temperature etc. You may know these relays with another name… thermostats, used extensively in heating applications.

Fig 3.7. DTZ-48 | Digital Temperature Control Relay with Built-in Timer - Tense Electronics

Another difference from the protection relays is that the temperature relays usually do not need an external mechanical movement to restore it’s state. The process is done automatically according to the temperature of the bimetallic strip.
Category 2. Coil Actuation
Another type of relay categorization is the coil. In this category i separate the relays according to the way that their coil is powered to actuate the armature. So we have:
AC/DC relays

• 
  The coil can operate with either AC or DC voltage.
  

Neutral relays

• 
  These relays have the most common coil. The armature is actuated when current goes through the coil, regardless the polarity.
  

Biased Relays
This is a variation of the neutral relays. These kind of relays have exactly the same coil as the neutral relays, but they carry a permanent magnet on the armature. The polarization of the magnetic field of the coil depends on the polarity of the supply. Therefore, the armature is actuated only if the polarity of the coils’ magnetic field is opposite to the polarity of the permanent magnet’s magnetic field. This way, the relay is actuated only if the coil is correctly biased.