Current Transformer

I've never seen a CT with a secondary. Perhaps you are talking about a PT?

Joe, A CT is a PT with a closed loop (shorted) secondary. The design is such that the 6 volts (or so) developed in the secondary circuit will circulate a current through the resistance of the secondary windings in a specific proportion to the current in the primary. In the case of a 'bar' type CT the primary winding consists of 1 turn (the bar passing through the coil of the secondary windings). A 'window' CT primary consists of the wire or wires passing through the hole in the donut. For both types, the secondary is connected to the 2 small terminals or the two small wires on the body of the CT.

Ohm's law dictates that when resistance is constant and the current is reduced for the circuit, voltage increases proportionally. That being the case, if the secondary circuit of a CT is opened while load is present in the primary, and current values drop to zero thereby, the voltage values approach infinity (theoretically). Newer CT's have a limited amount of iron core available for transformation and are thus a bit safer, but older CT's have been known to VAPORIZE some things when opened under significant load.

This reality dictates that a fuse would have a potentially disastrous effect if it opened. You will never find a fuse in the secondary of a correctly installed CT. Properly trained technitions always shunt these circuits to keep them closed when testing etc. to avoid the possible damages to man and equipment.

For the same reason that most individual components do not have fuses included. A single fuse can protect many components but if we include them in each component it becomes a waste. The CT is just that - A CT. It does not need to be fused unless your circuit needs it to be fused.

The most common usage of a CT is to monitor a large primary current by stepping it down by a very large factor then driving it into a resistor to generate a voltage that is scalable back to the primary current. The current scale can be 25, 50, 100 or more. The primary may have 100A and the secondary may carry only 10mA if the termination resistance is 50 ohms then the output voltage is 0.5V for 100A primary current. I use CTs often and have never put a fuse in the secondary circuit.

The CT is the Secondary, technically speaking. The Primary is the conductor on which the current is being measured.

And as previously said, as you step down the current you will step up the Voltage by exactly the reverse ratio.

V1/V2 = N1/N2 = I2/I1

An as such you Never want the CT to be Open Circuit which is why they a generally supplied with a "Shorting Bar".

Regards,
Sapper