Page 321 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

METER ACCURACY
METER TYPICAL ACCURACY

Moving coil 0.1 to 2%
Moving iron 5%
Rectifier type moving coil 5%
Thermocouple 1 to 3%

Loading effect of voltmeter and voltage drop effect of
ammeter in circuits

Objectives: At the end of this lesson you shall be able to:
• define the term ‘multiplier’
• analyse the loading effect of the voltmeter
• analyse the effect of voltage drop across the ammeter in the resistance measurement.

Multipiler
In the case of P.M.M.C. instruments, we have seen that the moving coil consists of fine gauge copper wire. This
copper wire can carry very low current in the order of milli or micro amperes only.

The acceptable current which enables the instrument to read full scale is called full scale deflection current or
F.S.D. current. When such a P.M.M.C. instrument is to be converted as a voltmeter, the moving coil has to be
connected with a high resistance in series so that the current could be restricted within the F.S.D. current value.
This series resistance is called multiplier resistance.
Let us study how the voltmeter sensitivity causes loading effect in the circuit by the voltmeter.
Loading effect of a voltmeter: The sensitivity of a voltmeter is an important factor when selecting a meter for
a certain voltage measurement. A low sensitivity voltmeter may give an almost correct reading when measuring
voltages in low-resistance circuits, but it is certain to produce very high errors in high resistance circuits. It is due
to the fact that the voltmeter, when connected across a high resistance circuit, acts as a shunt for that portion of
the circuit, and, thereby, reduces the equivalent resistance in that portion of the circuit.

As such, the meter will then give a lower indication of the voltage drop than what actually existed before the
meter was connected. This effect is called the loading effect of a voltmeter and it is caused principally by the low
sensitivity of the voltmeter.
The meter with the higher sensitivity of ohms/volt rating gives the most reliable result. It is important to realize
the factor of sensitivity, particularly when voltage measurements are made in high-resistance circuits. Hence the
following points are required to be followed while using a voltmeter.
• When using a multi-range voltmeter, always use the highest voltage range, and then decrease the range until
a good up-scale (above mid-scale) reading is obtained.
• Always be aware of the loading effect. This effect can be minimised by using a voltmeter of high sensitivity and
highest range in voltmeter.
• Before reading the meter, try to select a range in the multi-scale instrument such that the reading obtained is
above mid-scale. The precision of the measurement decreases if the indication is at the low end of the scale.
Effect of voltage drop across the ammeter in resistance measurement: The ammeter/voltmeter method of
measuring resistance is very popular since the instrument required for this is usually available in the laboratory.
In this method, two types of connections of meters are possible (Figs 1a and b).

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CITS : Power - Electrician & Wireman - Lesson 50-53