The current ratio of the ammeter is the same as that of the current transformer. The primary cable passes through the transformer P1. And secondary current is formed, usually 5A, which is connected to the secondary device, such as an ammeter, through the S1 and S2 of the transformer.

The screws behind ammeter and transformer S1, S2 are connected randomly. The current transformer cannot open the secondary circuit, otherwise will cause high voltage risk. So S2 must be reliably grounded.

As shown in the figure, 50/5A transformer, the ammeter must be selected with ratio 50/5. Primary current 50A, secondary current output is 5A. The ammeter reading panel 5A, we are able to calculate the value of the primary current by the ratio of the ammeter as function 5*50/5A=50A.