High Ohmic Series Resistance with Micro ammeter

High D.C. voltages are usually measured by connecting a very high resistance (few hundreds of mega ohms) in series with a micro ammeter. Only the current flowing through the large calibrated resistance R is measured by the moving coil micro ammeter. The voltage of the source is given by

Series resistance micrometer

The voltage drop in the meter is negligible, as the impedance of the meter is only few ohms com pared to few hundred mega-ohms of the series resistance R. A protective device like a paper gap, a neon glow tube, or a zener diode with a suitable series resistance is connected across the meter as a protection against high voltages in case the series resistance R fails or flashes over. The ohmic value of the series resistance R is chosen such that a current of one to ten microamperes is allowed for full-scale deflection. The resistance is constructed from a large number of wire wound resistors in series. The voltage drop in each resistor element is chosen to avoid surface flashovers and discharges. A value of less than 5 kv /cm in air or less than 20 kV/cm in good oil is permissible. The resistor chain is provided with corona free terminations. The material for resistive elements is usually a carbon-alloy with temperature coefficient less than 1T Carbon and other metallic film resistors are also used. A resistance chain built with ±1% carbon resistors located in airtight transformer oil filled P.V.C. tube, for 100 kV operations had very good temperature stability.

 The limitations in the series resistance design are:

•    Power dissipation and source loading

•    Temperature effects and long time stability

•    Voltage dependence of resistive elements, and

•    Sensitivity to mechanical stresses

Series resistance meters are built for 500kV D.C. with accuracy better than 0.2%.