Page 854 - Basic Electrical Engineering

Figure 11.29 Use of CT and PT for measurement of high current and high voltage

Advantages of CT and PT: The advantages of using CT and PT for
measurement of high value of current and voltage in preference to directly

using ammeters and voltmeters of higher ratings (through shunts and
multipliers) are mentioned below.

i. If we had to use ammeters and voltmeters directly on the high-voltage lines, the instruments
would have had to be insulated heavily, and hence their physical size would be very big.
Morever, it would be dangerous to bring high-voltage lines inside the control station. In case
of any leakage of currents, the operator using these instruments may get a fatal electrical
shock. Thus, the safety of the operator would be at stake.
ii. With the help of standard instruments of low range, large current and voltage can be
measured. Instrument transformers can either be of single range or of multirange.
iii. From the output of the CT and PT, connections can also be taken to the current coil and
pressure coil of a wattmeter for the measurement of power. Similarly, connections can be
taken for the frequency meter also. Thus, several instruments can be connected to a single set
of instrument transformers.
iv. Instruments of moderate size and standards can be used for measurement.

It may be noted from Fig. 11.29 that the secondaries of CT and PT have

been earthed. This is because, in case of open circuiting of the secondary by
the operator, or by other reason the operator may get a severe shock if there is

some leakage. That is why it is a warning to the operator as to not to open the