Calculation Procedure

1. Select Voltmeter

Self-contained ac voltmeters with scales ranging from 150 to 750 V are available. Where higher voltages are to be measured, a potential transformer is required to produce a voltage suitable for indication on a meter with 150-V full-scale indication. A 150-V meter is therefore selected.

2 . Select Potential Transformer

By dividing the line-to-line voltage by the voltmeter full-scale voltage, one obtains an approximate value of transformer ratio: 4160 V/150 V=27.7. Select the next higher standard value, 40:1. To check the selection, calculate the secondary voltage with the potential transformer: 4160 V/40=104 V.

3. Connect Transformer and Voltmeter to the Line

The potential transformer and voltmeter are connected to the three-phase line, as shown in Fig. 2.1.

Figure 2.1: Voltmeter connections to a three-phase power line.

Precautions

Any leakage in the insulation between windings in potential transformer can cause high voltage to appear on low voltage side.

Related Calculations. Instruments used for measuring electrical quantities in utility or industrial service, such as voltmeters, ammeters, and wattmeters, are referred to as switchboard instruments. Instruments used for measurement of nonelectrical quantities, such as pressure, temperature, and flow rate, involve more complex techniques that include a sensor, transmission line, and receiver or indicator.