Which type of current is typically read by a current transformer (CT)?

A current transformer (CT) is specifically designed to measure alternating current (AC). The principle of operation of a CT is based on the electromagnetic induction which occurs when a conductive wire carrying AC creates a varying magnetic field. This varying magnetic field induces a proportional current in the secondary winding of the transformer, allowing for measurement and monitoring of the primary current without direct connection to high voltage systems.

Alternating current is characterized by its periodic change in direction and magnitude, making it ideal for current transformers that rely on this variability for accurate readings.

In contrast, direct current (DC) flows in one direction and remains constant over time, which does not induce the required magnetic field changes for a CT to function. Pulsating current, while it does change, is typically not a smooth sinusoidal waveform and may not provide stable or reliable readings for the purpose that a CT is designed for. Static current implies no movement or change, which is also unsuitable for current transformer applications. Thus, alternating current is the correct choice as it is what current transformers are specifically built to measure.