In this article we will discuss about the connection of an ammeter for measuring heavy current.
When the line current is high, total current of the supply line cannot be sent through the coil of the ammeter. For carrying high current, the cross-section of the coil must be large, and it becomes very difficult to make coil of a conductor having large cross-section.
The range of an ammeter can be extended by connecting a low resistance in parallel with the coil of the instrument. This parallel resistance is called a shunt. Shunt is usually made of a material like manganic which has a very small temperature co-efficient of resistance.
ADVERTISEMENTS:
When a shunt is used, most of the line current is by-passed through it and the coil of the ammeter carries only a fraction of the total current depending on its resistance compared with that of the shunt. But the scale of the meter is graduated in such a way that the pointer of the instrument indicates total current of the circuit directly on the scale. The shunt and the ammeter are enclosed in the same cover forming a complete instrument.
Although a shunt can be used with an ammeter for both d.c. as well as a.c. supply systems, in many cases (specially when the current is very high) an iron-cored current transformer is used with an a.c. ammeter. Such an arrangement is shown in fig. 57 (b). The primary coil of the current transformer is connected in series with the live line. This coil has a few turns and large cross-sectional area. Sometimes the live line itself is used as the primary of the transformer.
The secondary coil of the transformer has large number of turns and comparatively small cross-sectional area. The ammeter is connected across this secondary. The ratio of primary to secondary turns are so adjusted that when full load current flows through the primary, the secondary carries only five amperes. But the scale of the instrument is graduated in such a way that the pointer reads directly the line current on the scale.
ADVERTISEMENTS:
Besides reducing the magnitude of current flowing through the instrument, a current transformer isolates the ammeter from the supply system. This ensures safety to the operator specially in case of a high voltage circuit.
Precaution:
When the ammeter remains connected across the secondary of a current transformer, the flux density in the iron-core is extremely low and the e.m.f. induced in the secondary coil is almost zero. If the secondary circuit of the transformer were opened, the primary would act like a choke coil and the flux density would now be high.
At the same time a very high e.m.f. would be induced in the secondary winding. This would damage the insulation of the secondary coil, and due to high flux density the accuracy of the instrument would be destroyed. Hence, before removing ammeter from the circuit, the secondary winding of the current transformer must always be short-circuited by a wire of very low resistance.