A circuit breaker as a switching and current interrupting consist of fixed and moving contacts, which are touching each other and carry the current under normal condition. When the circuit breaker is closed, the current carrying contacts, called the electrodes, engage each other under the pressure of a spring.   

Whenever a fault occurs on any part of the power system, the trip coils of the breaker get energized and the moving contacts are pulled 

The theory of operation 
All objects above absolute zero (-273.15°C) give ou t infrared energy. This radiated energy is emitted in all directions at the speed of light.
The instrument’s lens picks up this energy and focuses it on an infrared detector. This detector gives a voltage signal proportional to the quantity of energy received and hence proportional to the temperature of the object.
English

What is the  emissivity  that mean ??
Some objects not only give out infrared energy, but also reflect it as well. Unlike matt surfaces, brilliant or highly polished surfaces tend to reflect energy. This possible reflection is represented by a factor known as the emissivity, and can vary between 0.1 for a highly reflective surface, and 1 for a black body.


((TARGET DISTANCE / MEASUREMENT FIELD DIAMETER))  RATIO
This ratio, also known as the field of vision, indicates the diameter of the probes’ measurement field at a given distance from the target (see figure below).
In the case of a measurement point of small dimensions, it is important to bring the probe to a short enough distance from the target, so as to avoid including other points into the field of measurement.