what are earth leakage circuit breakers (ELCB)? what are they used for?

Earth Leakage Circuit Breaker is used for the protection against electrical leakage in the circuit of 50Hz or 60Hz,
rated voltage single-phase 230V, 3 phase 400V. When somebody gets an
electric shock or the residual current of the circuit exceeds the fixed value, the ELCB can cut off the power automatically to protect the personal safety and prevent
the equipment from the fault resulted by the residual current. Check points for selecting ELCB:
1. Sensitivity current
2. Earth fault current breaking
capacity
3. Short-circuit breaking capacity
4. Operating time
5. Selective protective coordination
6. Rated voltage and frequency
7. Rated current
8. Detecting device

ELCB is earth leakage circuit breaker. Earth leakage is the flow of current to earth through extraneous conductive parts of any installation which is in normal condition electrically sound that is either isolated or non-current carrying. It occurs in LV circuits.

Leakage is possible either due to direct contact or indirect contact.
Direct contact can result due to mistake, an absence of attention of the person, absence of or bad isolation…This risk is very important when installation are old or not correctly maintained or used by unskilled persons. Magnitude of current flowing in this case would be V/Rc here V = LV voltage 230V, Rc = body path resistance in the range of 2000 to 3000Ohm so current is = 230/2500 = 92mAmps .You can consider 100mAmp
Indirect contact can result In case of insulation failure in an appliance, accessible parts may be live, and a contact with a person may be fatal. Working out similar to above methodology, magnitude of this current will be lesser than that of direct contact. ELCB is either voltage operated or current operated. Current operated are also called RCD (residual current devise)