MEASURING CIRCUITS (high voltage engineering )

 A simple measuring circuit for the measurement of resistance is shown in Fig 1. The galvanometer is first calibrated using a standard resistance of 1 to 10 MΩ (±0.5% or ±1%) and a standard universal shunt is used with the galvanometer. The deflection in cm per microampere of current is noted.

The specimen (Rp) is inserted in the circuit and maintaining the same supply voltage, the galvanometer current is observed by adjusting the universal shunt.

E - dc stabilized power supply 500 to2000v 

V- voltmeter 

G- galvanometer 10^-8 to 10^-9A/cm deflection

Rsh- Universal shunt

Rp-specimen

 

The resistance of the specimen is given by 

𝑅𝑃=𝑉(𝐷×𝐺)

Where

 D – deflection in cm (with specimen)

G – galvanometer sensitivity. 𝐺=(𝑉𝑅𝑠)×(1𝑛)×(1𝐷𝑠)

n = the universal shunt ratio,

Ds = deflection in cm with the standard resistance in position,

Rs = standard resistance used for calibration, and

V = the supply voltage

Deflection with the specimen in the circuit, will change with time. The initial high deflections indicate the high charging current required by the dielectric specimen.

➢ For calculations, the steady current obtained after a considerably long changing time should only be taken.

➢ Switch B is used to discharge the specimen after the measurements are complete and as well as for the initial discharging before the measurements are undertaken

In the deflection method described, a limit is reached when the insulation resistance is so high around 1012 that galvanometer deflection is not at all adequate

Under these conditions, the loss of charge method is convenient.