insulation resistance

Remove the load and measure the current flow, use Ohms law to calculate resistance. There are plenty of Amperage panel meters available, some are mechanical and use a meter movement, others are digital. If the current to be measured is substantial, they will utilize a shunt.

Could you please throw more light on "Remove the load and measure the current flow". What current are we talking about here, load current or what? Would there be any current flow when the load is removed?

Sola,

Hesham wants to know how she can measure insulation resistance for live circuit at a given mains voltage. I am aware that this is not standard practice, but I do not presume to know the reason she is asking the question. The odds are great that she will not be able to measure any current flow and will therefore calculate the insulation resistance to be infinite. However if the insulation resistance is low enough there will be some current flow, which when measured may be divided into the voltage to determine the resistance of the insulation. (R=E/I)

I suppose the same thing could be done without removing the load, but then one would need to know the exact current draw of the load in order to subtract it from the current measured at the circuit feed. I saw no need to explain this previously, as Hesham did not specify that the load could not be disconnected.

This is usually done using a megger or a high pot tester depending on what you are looking for. Both tools demand that the power be switched off and sensitive loads disconnected.

The megger will give you the insulation resistance usually with respect to ground.

The high pot tester will tell you how much voltage the insulation can take before arcing over. This must be done a very low current setting (uA to mA) as it can damage the insulation.

Both tools demand caution and should be used by trained professionals as the voltages generated can easily reach thousands of volts.

1. Is the source delta or wye (star)?
2. Are you seeking an exact resistance value, or a pass/fail test to determine if a ground exists?
3. Are the load currents balanced between phases, or is there significant imbalance?

If the load is relatively balanced, either of the following will work:

A simple pass/fail test for a delta system uses 3 instrument voltage transformers with lamps, and a pushbutton.

Close switch S and compare the brightness of all 3 lamps. If they are about the same, the ground resistance on all phases is roughly equal. If there is a ground on one of the phases, the primary of the associated instrument transformer is shunted by the ground and the switch, causing that lamp to go dark.

For a wye system, connect a panel volt meter from each phase to ground. Each phase should read nominal phase-neutral voltage, around 220V. What you are actually reading is the voltage drop across the insulation. Reduced insulation resistance will lower the voltage drop across the insulation, lowering the meter reading. (This system may also work on delta systems, but the "float point" of a delta system can change with load current, leading to false readings.)

For higher-tech solutions, General Electric makes a pulsing ground detection system. A description is provided here.

Multi-Tech Industries also makes high-frequency portable and permanent energized ground detection systems described here.