For proper functioning of a system we need to maintain its parameters within specific minimum and maximum limits. As per IEE regulations the minimum winding insulation resistance is 100Mega ohm. Is there any technique to online monitor this insulation resistance? One idea is to use a core balance or zero sequence current transformer. Suppose we are in need online monitoring the insulation resistance of a 11KV 3 phase 50HZ motor, and the minimum limit is 100M ohm, then phase to ground potential is 11KV/1.73=6300VA IR is 100x1000x1000 ohms. The leakage current should not be drop than I-V/R 6300/(100x1000x1000)=0.000063 or 63 micro AC amps. We can online monitor this leakage current with sensitive amps meter and ZCT. I will appreciate if someone guides to calculate the exact design of a ZCT.