From fig 1 we are going to measure the resistor value of a cable with outer metallic sheath available. The galvanometer measures the current between the outer sheath and the conductor current. The galvanometer is very much sensitive to the current because the current value is in the range of nanoampere. If the applied voltage is of 1KV the resistance value may vary from 10MΩ to 1012MΩ. In the given Fig 1 any leakage current IL is absorbed by the guard wire. With the known value of voltage V and measures value of current IR, the insulation resistance IR can be calculated by the formula R=V/IR.
Cable without metal sheath can be tested in the same manner shown in figure 2. In this metal a long cable is being dipped into the breaker full of water for testing purpose. The conductor of the cable and the other circuitry arrangement (supply voltage, voltmeter, ammeter) will remain outside the breaker. The cable is dipped into water for 24 hrs. So that the cable will absorb water if any leakage is present. During this time ammeter is short circuited and a full high voltage supply is given to the circuit. After some time short circuit path of ammeter is opened and the same supply voltage is given. If any leakage current arises it will be absorbed by the ground wire. The insulation resistance of the cable R can be calculated by the formula supply voltage/current in ammeter (IR).
Alos Read- Voltmeter-Ammeter method