How may I be able to measure low current leakage about micro amps, using usual lab equipment. This is a current leakage of a heating cable from the live wire to the ground. I will be measuring the current passing through he ground wire. I have problems measuring with a amp clamp. thank you

 

If you need to measure microamp leakage at 110 volts, then you need equipment with an input impedance of more than 110 Megohms. This eliminates most average test equipment. However, if the leakage path is resistive, you can scale the voltage back to 9 volts and measure nanoamp leakage.

This is actually not very hard to do. Get a MSP102 JFET and hook it up as a source follower. +9 volts to the drain, and a 4.7k resistor on the source to ground. Now connect a polycarbonate .1 uF cap from the gate to ground. The leakage resistance of the cap and the gate of the JFET is more than 1000 Megohms. Use free air wiring so that this high impedance is not degraded by leakage resistance of a circuit board. Now connect your circuit (unpowered, of course) to the gate. Charge the cap up to 9 volts with a clip lead and then remove the clip lead. Have a voltmeter on the source and monitor the rate of decrease of the source voltage as the cap discharges through the leakage resistance of your circuit. A leakage resistance of 110 Megohms and a .1 uF cap would give a time constant of 11 seconds. You can measure the time constant with a stopwatch. If it is 11 seconds, then you would have 1 microamp leakage at 110 volts, assuming the leakage is resistive.

However, if the leakage is voltage dependent in a non-linear way, then you will have to measure it at the operating voltage. Then maybe you need to buy a commercial hypot tester.

 

Isolate the ground wire and connect a 1 k ohm series resistor to the ground wire and supply ground. Measure the voltage across the resistor with a digital miltimeter and you can calculate the leakage current though the ground wire. But be careful not to touch the ground wire during any power up as the potential on the ground wire (no more ground wire!) will be lifted with any leakage.

Further to my remarks please note that apart from the ground wire the whole equipment will also be live as the potential on the equipment body is now lifted. So do not touch the equipment or heaters when the device is powered.

regards,
sekar