The transformer is frequently used to isolate one portion of an electrical system from another. Isolation implies the absence of any direct physical connection. As a first example of its use as an isolation device, consider the measurement of line voltages on the order of 40,000 V (Fig. 1).
Fig. 1: Isolating a high-voltage line from the point of measurement.
To apply a voltmeter across 40,000 V would obviously be a dangerous task due to the possibility of physical contact with the lines when making the necessary connections. By including a transformer in the transmission system as original equipment, one can bring the potential down to a safe level for measurement purposes and can determine the line voltage using the turns ratio. Therefore, the transformer will serve both to isolate and to step down the voltage.
As a second example
, consider the application of the voltage $v_x$ to
the vertical input of the oscilloscope (a measuring instrument) in Fig. 2.
Fig. 2: Demonstrating the shorting effect introduced
by the grounded side of the vertical channel of
If the connections are made as shown, and if the generator and
oscilloscope have a common ground, the impedance $Z_2$ has been effectively shorted out of the circuit by the ground connection of the oscilloscope. The input voltage to the oscilloscope will therefore be meaningless as far as the voltage $v_x$ is concerned.
Fig. 3: Correcting the situation of Fig. 2 using an isolation transformer.
In addition, if $Z_2$ is the current-limiting impedance in the circuit, the current in the circuit may rise to a level that will cause severe damage to the circuit. If a transformer is used as shown in Fig. 3, this problem will be eliminated, and the input voltage to the oscilloscope will be $v_x$.