Leakage current or resistance measurement method, and monitoring apparatus and monitoring system of the same

A leakage current measurement apparatus comprising:

• an amplifier unit for amplifying the output signals of a zero-phase current transformer which is installed on a target measurement circuit and detects any leakage current;
• an A/D converter unit for converting analog signals, which are output signals of the amplifier unit, into digital signals;
• a zero-cross detecting circuit for detecting a voltage zero-crossing point of said target measurement circuit and outputting a sampling start signal;
• a memory unit for storing digital data of a sine wave form whose effective value is 1;
• and
• a CPU mainly intended for arithmetic processing, wherein:
• the output signal of said zero-phase current transformer is sampled with said sampling start signal, products of multiplication are averaged by synchronizing the sampled output signal value and said stored digital data of a sine wave form whose effective value is 1, and an active component current of leakage currents is obtained.