Why ELCB are measured in terms of milliampere. how will we measure the capacity required for any application. how do we calculate it?

Answer 1

ELCB's are "earth leakage circuit breakers". They are used in situations where high impedance grounding is used, meaning a phase to ground fault has very low current levels. This results in standard overcurrent/breaker protection not necessarily "seeing" the fault.

And I do not believe ELCBs are usually rated in milliamperes. Their interrupting rating, and load is usually similar to MCCBs. They include a leakage current rating, which is in mA (leakage current is current to ground).

You calculate how much ground current you will have from a fault study. If you are intentially high impedance grounding (such as for a generator), then you should know the value of impedance you are using, and this value is usually chosen to limit ground current to a specific current (such as 5 amps).

If you are high impedance grounded for some other reason, you need to determine the impedance to ground (the best method to do so will depend on your situation); once you know this, you also know your normal line to ground voltage, and expected current flow is a simple calculation.