loss of charge method
In loss of charge method unknown resistance is connected in parallel with capacitor and voltmeter. The capacitor is initially charged to some suitable voltage by means of a battery of voltage V and then allowed to discharge through the resistance. The terminal voltage is observed during discharge and it is given by,
V=v exp (-t/CR)
OR
V/v = exp (-t/CR)
Or insulation resistance is given by,
R= t/(C ln V/v)
= 0.4343 t / (C log V/v)
The variation of voltage v with time is shown in fig,
loss of charge method plot
From above equation it follows that if V, v, C and t are known the value of R can be computed.
Loss of charge method is applicable to some high resistances, but it requires a capacitor of very high leakage resistance as high as resistance being measured. Let R1 be the leakage resistance of the capacitor. Also R’ be the equivalent resistance of the parallel resistances R and R1.
- loss of charge method with leakage resistance of capacitor
Then discharge equation of capacitor gives,
R’=0.4343 t / (C log V/v)
The test is then repeated with the unknown resistance R disconnected and the capacitor discharging through R1. The value of R1 obtained from this second test and substituted into the expression,
R’=(R R1) / (R+R1)
In order to get value of R.
