FL-2 DC shunt 
The fixed custom shunt is an external shunt that is suitable for measuring DC current directly acting on the analog display instrument below 10 KA and used together to expand the DC measurement range. It can be seen as a standard resistor for current sampling in a sequential circuit, and its sampling voltage can be regarded as an analog signal of the primary current for measurement. 
 
Working principle 
The shunt is actually a resistance with a small resistance value. When a DC current passes, a voltage drop is generated for the DC ammeter to display. 
2.DC ammeter is actually a voltmeter, full value of 75V, DC ammeter and shunt are used together ; 
for example, the shunt resistance of 100A ammeter is 0.00075 ohm, that is, 100A * 0.00075 ohm = 75mV. 
The shunt resistance of 50A ammeter is 0.0015 ohm, 50A * 0.0015 ohm = 75mV.