Residual Current

How do you measure the leakage current in an equipment?.

Why measure <rhetorical question>? Just protect the user with an appropriately-rated residual current circuit breaker in accordance with British Standard 7671.

If leakage current is not given in the nameplate of a machine or home/office appliance we might overload {the tripping current(mA)}a circuit. Consider a 30mA 2P RCD used to protect 20nos PCs in a school connected in a ring circuit. If leakage current of a PC exceed 1mA the leakage current will be 20mA for which RCD will not trip but it exceeds the 50% rated tripping current of RCD,which the circuit should withstand. That's why I have said leakage current,no load/LR current of motors,THD,PF etc to be included in the nameplate.

Leakage/residual current indicates a fault. The trip settings are there to prevent electrocution in the first instance, and to operate the circuit protective device(s).

What is the nature of the fault in each of 20 PCs that merits the measurement of it? Why not just rectify the fault in each one and be done with it?

Without a fault there could be a leakage current in components used in a machine if not manufactured to high standards.

When the current going out does not equal the current coming back the difference between the two is the residual current. Googling provides a decent answer and some animations that illustrate the concept. http://en.wikipedia.org/wiki/Residual-current_device.