Compressor

They are two separate pieces that should have no relationship with each other. The mechanical safety is there to prevent bursting the tank from over-pressure without regard to the source of pressure. The unloader is there to make sure that the compressor is not trying to start against the cylinder/tank pressure, it does not relieve the pressure in the tank, just in the cylinder or at the output port of the compressor. That "whooshing" sound you hear at the end or beginning of the compressor cycle is the unloader operating.

You have merged three questions in one and confusion is natural.

Working of mechanical safety system in un-loaded condition of Compressor

Electrical pressure switch

Mechanical safety valve

Mechanical safety in unloaded condition of compressor, you have to define, what type of mechanical safety system you want.

Electrical pressure switch functions in two way. first, as soon as designated pressure is achieved, it will unload the compressure and secondly, after un-loading it may trip the compressor motor.

Mechanical safety valve is to take care of over pressure of the system due to failure of electrical pressure switch or unloading device or compressure trip system.

when required pressure reaches how does electrical pressure switch start unloading operation in reciprocating compressor?

It doesn't, unloading is usually a mechanical function, not electrical. The unloader is a three port valve; 1. input from compressor, 2. output to atmosphere, 3. output to tank, plus a bellows that operates switch and unloading lever. When setpoint pressure is reached, bellows have expanded to operate a snap-action switch opening it and also causing an overcenter lever to lift check valve on atmosphere port thereby relieving pressure in compressor while check valve on output to tank prevents backflow out of the tank. Designs vary, sometimes the two check valves are replaced with a single double-acting shuttle valve that directs flow to either the atmosphere or the tank but not both simultaneously.

When pressure reaches the desired set value, Pressure switch sends signal to relay.

The relay either trips the compressor or sends signal to Electro-Pneumatic convertor, which converts electric signal to pneumatic signals. Unloader forks are provided in the compressor suction valves connected through Pneumatic actuators. As soon as the signal is recieved Actuator pushes the forks in suction valve unlaoding the plates. Suction valve stops working and thus compressor is unlaoded.