What are secondary electrical instruments?

Secondary electrical instruments are devices that receive signals from primary instruments like transformers or sensors and provide measurement or indication, often used for monitoring, control, or protection purposes.

What are the common types of secondary electrical instruments?

Common types include panel meters, recorders, relays, indicators, and measuring instruments such as voltmeters, ammeters, and wattmeters connected through instrument transformers.

What is the role of secondary instruments in power systems?

Secondary instruments play a vital role in power systems for measurement, monitoring, protection, and control by providing accurate readings derived from primary instrument transformers.

Secondary Electrical Instruments: Types, Principles, and FAQs | SSC JE Electrical - Measurement

The secondary instruments are those which are so designed that the value of current, voltage or any other quantity to be measured can be determined from its deflection only if the instrument has been compared with an absolute. In other words, secondary instruments need calibration.

The secondary instrument is usually used at large absolute instruments are only used in standard laboratories. examples are PMMC, Moving iron instruments etc.

Indicating Instruments
Recording Instruments
Integrating Instruments

1. Indicating Instrument:

The indicating instruments indicate the R.M.S. value of the electrical quantity. which exists at the moment when the measurement is being done. The indication is by means of the deflection of the pointer on a calibrated scale. for example, Ammeter, Voltmeter, Wattmeter etc. as shown in the following diagram.

2. Recording instrument:

The Recording instruments are those which not only indicate the existing value of an electrical quantity being measured. but also provides a record of variations in it over a predetermined period of time. for example Cardiogram, chromatogram, X-Y plotter etc.

3. Integrating Instrument:

The integrating instrument measures, registers, and integrates the electrical quantity being measured over a pre-defined period. The integrated added value is displayed with the help of a set of scales and pointers. for example odometer, energy meter etc. as shown in fig below.

Principles of operation of Electrical Instruments:

Electrical measuring instruments work on the principle of operation of different effects such as Magnetic effect, Electro-dynamic effect, Electromagnetic Induction, thermal Effect, Chemical Effect, Electrostatics Effect, etc. The principle of operation of the different electrical instruments is given in the table below-

Type	Effect	Suitable for	Work as
Moving Iron	Magnetic Effect	DC, AC	Ammeter, Voltmeter
PMMC	Electro-dynamic Effect	DC	Ammeter, Voltmeter
EMMC (Dynamometer)	Electro-dynamic Effect	DC, AC	Ammeter, Voltmeter, Wattmeter
Induction Type	Electro-Magnetic Induction	AC	Ammeter, Voltmeter, Wattmeter, Energy Meter
Hot Wire	Thermal Effect	DC, AC	Ammeter, Voltmeter
Electrolyte Meter	Chemical Effect	DC	Amp-Hr Meter
Electrostatics Meter	Electrostatics Effect	DC, AC	Voltmeter

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Difference between Analog and Secondary instruments:

Characteristics	Analog	Secondary
Definition	Those instruments which give the value of the quantity to be measured in terms of the constant of the instrument and its deflection are called absolute instruments.	Those instruments in which the electrical quantity being measured is given directly by the deflection of the instrument are called secondary instruments.
Environmental conditions	Used in any type of environmental conditions.	Digital instrument involves electronic devices which required proper environmental conditions.
Effect of Electronic noise	More susceptible to electronic noise.	Less affected by electronic noise.
Accuracy	Less accurate	More accurate
Moving parts	Present	Not present
Cost	Less	More than analog instrument
Sensitivity	More sensitivity	Less sensitivity
Size	The size of analog instruments is larger than a Secondary instrument.	Digital instruments are comparatively smaller than analog instruments.
Extra power supply required	Not required an Extra power supply	Required extra power supply
Example	PMMC, EMMC, MI etc	digital multimeter, digital ammeter and voltmeter, clamp-meter, etc.

Frequently Asked Questions (FAQs)

Q1. What is the main difference between an absolute and a secondary instrument?

Ans: Absolute instruments determine the value of the measured quantity directly based on physical constants and deflection, without needing prior calibration. In contrast, secondary instruments require calibration against absolute instruments to display accurate measurements.

Q2. Why are secondary instruments preferred in daily use?

Ans: Secondary instruments are easier to use, quicker in operation, and provide direct readings. They are compact, more accurate for practical applications, and suitable for field use, unlike absolute instruments, which are mainly confined to laboratories.

Q3. What are the examples of indicating instruments?

Ans: Common examples include Ammeter, Voltmeter, and Wattmeter. These instruments show the instantaneous value of the quantity being measured using a pointer and scale.

Q4. How does a recording instrument differ from an indicating instrument?

Ans: An indicating instrument only shows the present value of an electrical quantity, while a recording instrument not only shows but also records the variation of that quantity over time for future reference.

Q5. What are integrating instruments used for?

Ans: Integrating instruments measure the total quantity over a certain time period. For instance, energy meters measure total energy consumption, and odometers measure total distance traveled.

Q6. Which effect is used in PMMC instruments?

Ans: PMMC (Permanent Magnet Moving Coil) instruments work on the electro-dynamic effect and are suitable for DC measurements.

Q7. Which instrument works on the principle of electromagnetic induction?

Ans: Induction-type instruments, like AC energy meters and certain AC voltmeters and ammeters, operate based on the principle of electromagnetic induction.

Q8. Why is calibration necessary for secondary instruments?

Ans: Calibration ensures the accuracy of secondary instruments by comparing them against a standard (absolute) instrument. This compensates for mechanical imperfections, scale errors, and environmental influences.

Q9. What is a digital secondary instrument?

Ans: A digital secondary instrument displays readings in numeric form using digital electronics. Examples include digital voltmeters, digital multimeters, and digital clamp meters.

Q10. Can secondary instruments measure both AC and DC quantities?

Ans: Yes, depending on the type and construction. For example, Moving Iron instruments can measure both AC and DC, while PMMC instruments are suitable only for DC measurements.