current transformers

[trainee]

why/how are high voltages generated across the secondary of a CT when the 2ndary is open circuited?

is that the reason why the secondary of CT's are never fused?

 

[Dark Knight]

^^^ I will get back to you with a detailed explanation about this. But for now think of the turn ratio of the CTs. It takes a current of a certain value and "tranforms" it to another value.

Lets say: A 800:5 CT will take give you 5 Amps on the seconday when 800 Amps are flowing on the primary. Well, apply the same concept to the voltage but the other way.

Turnsa ratio is known as "a".

a=Vp/Vs

1/a = Ip/Is

If you play with these relationships you will see how high or how much the voltage will increase on an open circuit transformer.

The reason you do not fuse the secondary is exactly to avoid an open circuit. When a fuse is blown the circuit breaks or is open.

This is the way I see it. I am sure the EE Corps will probably come with a better explanation.

 

[King Seso]

why/how are high voltages generated across the secondary of a CT when the 2ndary is open circuited?
is that the reason why the secondary of CT's are never fused?

Just like BringitOn said, the transformer ratio shows the following:

sqrt(Z1/Z2) = a

V1/V2 = a

where V1 = primary voltage, V2 = secondary voltage, Z1 = primary impedance and Z2 = secondary impedance

V2 = sqrt(Z2/Z1)*V1

If Z2 = open circuit or high impedance, V2 will be very high.

If you add a MOV or TVS across a CT, then you can use a fuse to disconnect the secondary from the rest of the circuit without creating a high voltage. In switchgear, electricans would work on the gear live, if a CT is open circuited the TVS will hold the voltage to the TVS rating (use a bidirectional TVS). If the current going through the primary is fault current, then the TVS will typically be damaged failing in a permenant shorted condition causing the CT output secondary current unless the TVS is removed.