multirange ammeter

The range of the basic d.c. ammeter can be extended by using number of shunts and a selector switch. Such ammeter is called multirange ammeter. and is shown in the Fig. 1.
 Fig. 1 Multirange ammeter

R1 ,RR3 and R4 are four shunts. When connected in parallel with the meter, they can give four different ranges I1 ,II3 and I4. The selector switch S is multiposition switch, having low contact resistance and high current carrying capacity. The make before break type switch is used for the range changing.
If the ordinary switch is used, while range changing the switch remains open and full current passes through the meter. The meter may get damaged due to such high current. So make before break switch is used. The design of such switch is so that it makes contact with next terminal before completely breaking the contact with the previous terminal.
The multirange ammeters are used for the ranges upto 50 A. While using the multirange ammeter, highest range should be used first and the current range should be decreased till good upscale reading is obtained. All the shunts are very precise resistance and hence cost of such multirange ammeter is high.
The mathematical analysis of basic d.c. ammeter is equally applicable to such multirange ammeter. Thus,
R1 Rm /m1  -1
R1 Rm /m2  -1  and so on,
where m1  ,m2  ,m3   .... are the shunt multiplying powers for the currents I1 ,II3 ......