Why CT Secondary Should Not Be Kept Open?

Why CT Secondary Should Not Be Kept Open?

The electrical power system load current always flows through current transformer primary; irrespective of whether the current transformer is open circuited or connected to burden at its secondary. If CT secondary is open circuited, all the primary current will behave as excitation current, which ultimately produce huge voltage.

    Every current transformer has its own non-linear magnetizing curve, because of which secondary open circuit voltage should be limited by saturation of the core.

    If one can measure the rms voltage across the secondary terminals, he or she will get the value which may not appear to be dangerous.

    As the CT primary current is sinusoidal in nature, it zero 100 times per second (As frequency of the current is 50 Hz). The rate of change of flux at every current zero is not limited by saturation and is high indeed.

    This develops extremely high peaks or pulses of voltage. This high peak of voltage may not be measured by conventional voltmeter.

    But these high peaks of induced voltage may breakdown the CT insulation and may case accident to personnel. The actual open-circuit voltage peak is difficult to measure accurately because of its very short peaks.

That is why CT secondary should not be kept open.