Current Transformers

Author: E.E. Kimberly

Current transformers are used in a-c circuits, either when large currents are involved or when the line voltage is dangerously high and it is advisable to insulate the meter from the power circuit. Current transformers usually "step down" the current to 5 amperes or less, and for this reason meters and other devices connected in the secondary circuit should be suitable for only 5 amperes. All instruments or other devices, such as protective relay trip coils, should be connected in series when used in current-transformer secondary circuits.

If the secondary circuit of a current transformer be open when the primary circuit is energized, all of the primary current becomes magnetizing current, and a high voltage dangerous to life may appear at the secondary terminals. The high flux density may also overheat the iron core seriously enough to damage the insulation. To prevent this condition, the secondary winding of a current transformer should be short-circuited when its circuit is not otherwise closed. The secondary circuit of a potential transformer must never be short-circuited.

The theory of current transformers and potential transformers is the same as that of all other transformers described in Chapter 17, With the few precautions already described these transformers may be used advantageously, even though the theory of their operation is not fully understood.