Basic Electrical Measuring Instruments

1. In the field of electricity, as well as in all other physical sciences, accurate quantitative measurements are essential. This involves two important items — numbers and units. Simple arithmetic is used in most cases, and the units are well defined and easily understood. The standard units are those of current, voltage and resistance.

2.  The operator commonly works with ammeters, voltmeters and elect ron-tube analyzers; but he may also have many occasions to use watt meters, watt-hour meters, power-factor meters, synchroscopes, frequency meters and capacitance-resistance-inductance bridges.

3. Electrical equipment is designed to operate at certain efficiency levels. A good understanding of the functional design and operation of electrical instruments is important. In electrical service work one or more of the following methods are commonly used to determine if the circuits of an equipment are operating properly.

a) Use an ammeter to measure the amount of current flowing in a circuit.
b) Use a voltmeter to determine the voltage existing between two points.
c) Use an ohmmeter or megger to measure circuit continuity and total or partial circuit resistance.

4. It may also be necessary to employ a wattmeter to determine the to tal power being consumed by certain equipment. If we wish to measure the energy consumed by certain equipment or certain circuits, a watt-hour or kilowatt-hour meter is used.

5. For measuring other quantities such as power factor and frequency it is necessary to employ the appropriate instruments. In each case the instrument indicates the value of the quantity measured, and in this information helps to understand the way the circuit is operating.

6. Occasionally the operator will need to determine the value of a capacitor or an inductor. Inductance or capacitance bridges may be employed for this purpose.

7. A thorough understanding of the construction, operation and limitati ons of the basic types of electrical measuring instruments, coupled with the theory of circuit operation is most essential in servicing and maintaining electrical equipment.

Measuring Instruments and Alarms

Given below is a minimum list of measuring instruments that are to be provided in order to ensure permanent and direct indication of the system parameters: 1) an ammeter in the main current circuit; 2) a voltmeter in the main current circuit; 3) an ammeter in the field circuit for adjustable excitation system; 4) a voltmeter in the field circuit for adjustable excitation system; 5) a tachometer for propulsion motors or propeller shafts; 6) a rudder angle indicator.

For a-c systems the following is to be provided additionally: 7) a frequency indicator; 8) a synchronizer for generator paralleling.

The electric propulsion plant system is to be equipped with an insu lation resistance measuring instrument. Continuous insulation resistance measuring is to be provided in the main current circuits, with visual and audible alarms to operate in the event of inadmissible decrease of the insulation resistance.

Where a control desk or a control panel incorporates measuring in struments with oil, steam or water supply to them, suitable measures are to be taken to prevent the possibility of any of the above agents gaining access to live parts if the instruments or piping connections are damaged.