CT secondary terminals


Thread Starter



why is it recommended to short the secondary terminals of CT if they are not connected to ammeter? they are step up transformers and so the voltage between the terminals will be high. so if the transformer is live, it can't be short. if the point is to short the terminals before current input is given, what's the point? why shouldn't they be PVC taped and let them be disconnected, as we generally do?

John Gardner

Voltage will be high<b> if left open-circuit.</b> A CT has a very low burden, and when it supplies a load such as a meter or relay the voltage is negligible. So you short unused CTs to prevent the terminals from developing high voltage. Some CTs are capable of developing enough voltage to arc over if not intentionally shorted, and any CT will start to break down it's own turn-to-turn insulation as the voltage rises under open-circuit conditions. This will, at a minimum, render the ratio void, and at worst it could burn down your CT or the oil-filled apparatus to which it is mounted.

Curt Wuollet

CTs _are_ step up transformers for voltage, which is implied by the current ratio. They are normally used with a current measuring device with very low resistance, essentially a short. They may not be insulated for high voltages because they are not used in the conventional sense as voltage transformers. Two things will happen when the secondary is left open. It's voltage may go higher than it is insulated for, and the high secondary impedance will be reflected back in the primary so that the burden voltage across the primary terminals will increase. This will drop the delivered voltage below its normal. Another way to look at it is that in normal use it is a small impedance in series with the circuit being measured. That's a good thing for current measurements as you usually don't want to change conditions much to measure current. Open circuited, it becomes an just an inductor in series with your circuit which can cause significant change in the operation depending on the load. And someone who is reconnecting the current metering shunt may get shocked. Short it, it's how they were meant to be used.