Module 8. Electrical measuring instruments

Lesson 28

CLASSIFICATION OF INSTRUMENTS

28.1  Introduction

The instrument principally used to measure electrical quantities like current, voltage, energy, power etc are known as electrical instruments.  Tests and measurement are important in design, performance evaluation, repair and maintenance of electrical equipments systems and circuits. To measure electrical quantities like current, voltage, resistance etc it is necessary to transform these quantities into a visible indication.

28.2  Analogue and digital instruments

There are basically two types of instruments:

a)      Analogue: Magnitude of the electric quantity is indicated by the position of a pointer moving over a graduated scale.

Fig. 28.1 Analogue measuring device

b)     Digital: When the electrical quantity is indicated in form of decimal number.

Fig. 28.2 Digital measuring device

28.3  Classification of Measuring Instruments

Electrical instruments can be classified on the basis of

28.3.1  Function

a)      Indicating instruments: Such instrument directly display the value of electrical quantity immediately at the time when measurement is taken. E.g. Voltmeter, ammeter.

Fig. 28.3 Indicating device

b)     Recording instruments: Such instruments are used to record electrical quantities during a period of time. A pen plots the data on a paper. Digital recording instruments or data loggers are also available.

Fig. 28.4 Recording device (Chart type)

c)      Integrating instruments: These instruments measure the total quantities of electricity in a given time. The best example is energy meter (Fig. 28.5). The data measured and recorded by such instrument is cumulative.

Fig. 28.5 Integrating instruments (energy meter)

28.3.2  Type mounting

a)         Switch board mounted: Such instruments are mounted permanently on the switch board. These are not completely covered and terminals are placed on the back of the instruments for connections.

Fig. 28.6 Switchboard or panel mounted instruments

b)        Portable devices: These are small in size and handy to be carried around easily. Portable devices are useful for onsite/multi location testing and measurement. Few portable are designed in a manner that it can also be mounted on the switch board.

Fig. 28.7 Portable device for current and voltage measurement

28.3.3  Principle of operations

a)      Moving iron instruments

b)      Moving coil instruments

c)      Induction instruments

d)     Hot wire instruments

e)      Electrostatic instruments

f)       Electrolytic instruments

28.3.4  Current type

a)      A.C. instruments

b)      D.C. instruments

28.3.5  Electrical quantity to be measured

a)      Voltmeter-Voltage

b)      Ammeter-Current

c)      Wattmeter-Power

d)     Watt Hour meter-Energy measurements