How do I select CT ratios?

[Ingenieur]

SF=Service factor?

I am getting some conflicting info outside this forum, but from what I am being told is that a 1200:5 CT should never exceed 1200amps?

Also as a general rule a C800 CT is good to limit the wiring to 1.2 ohms? (Yes/no/maybe? )

Thanks to everyone who has answered thus far, its been a wealth of info :happyyes:

sf safety factor

for normal operating range, yes, for a fault no

ct800 = linear to 800 v
800/1.2 = 666 A or 133 times the rated 5 A
since even an agressive bolted fault is 30-50 pu x i rated not an issue nor should saturation be an issue

so the 1.2 is conservative (but with modern relays not an issue unless crazy long leads with small wire but this is bad design)

 

[mbrooke]

sf safety factor

for normal operating range, yes, for a fault no

ct800 = linear to 800 v
800/1.2 = 666 A or 133 times the rated 5 A
since even an agressive bolted fault is 30-50 pu x i rated not an issue nor should saturation be an issue

so the 1.2 is conservative (but with modern relays not an issue unless crazy long leads with small wire but this is bad design)

In cases like this conservative is a worthwhile investment :thumbsup:


Side question. Why arent CT circuits protected from open circuiting? Won't an open cause several thousand volts in the CT - causing it to burn up?

 

[Bugman1400]

In cases like this conservative is a worthwhile investment :thumbsup:


Side question. Why arent CT circuits protected from open circuiting? Won't an open cause several thousand volts in the CT - causing it to burn up?

I'm not sure there is a way to protect a CT from an open circuit other than using typical shorting blocks or test switches. Sounds like a $M idea.

 

[mbrooke]

I'm not sure there is a way to protect a CT from an open circuit other than using typical shorting blocks or test switches. Sounds like a $M idea.

In your experience, how often do open CTs pop up in the real world?