Current Transformer Limits 1

[mutimuti]

Can a 150:5 CT survive a 25-30kA fault current?

 

[davidbeach]

Why not? It will probably saturate but that causes no harm unless the saturation causes misoperation of something else.

 

[apowerengr]

Agree with davidbeach - the CT can survive however the devices connected to the CT secondary will most likely see saturated current and/or current in excess of their rating. 200 amps secondary current (if the CT could drive it) is at least twice what many secondary devices are rated for (100 amps for relays, 6 amps for meters, transducers, wiring, etc.). By chance are the CT's multi-ratio such that you can change taps?

 

[davidbeach]

What I should have included in my original response, but forgot was that it was based on the assumption that the fault current would be cleared quickly. The CT and any device connected to the secondary will have a withstand rating that will be surprisingly high for a few cycles. Unless you have a very good CT (C400 or C800) you are likely to be well into saturation so the secondary current will be well less than the CT ratio might suggest.

 

[scottf]

What voltage class?

Window-type or wound primary?

If medium voltage wound type, this answer is most probably no. Normal thermal rating is 100 x Inom, which would be 15 kA.

I assume most of the above comments are assuming a window-type CT or bushing CT. Remember even those have limits on how much current the secondary windings can take without damage...even if a short-time current. Considering your case above, for a 25 kA primary current, the secondary peak current could reach 833A (25 kA / 150/5). While the core would saturate well below that level more than likely, the saturation point is a function of the connected burden. If the burden was electronic and the leads relatively short, then the secondary current could get pretty high and could indeed cause damage.

 

[mutimuti]

I appreciate your responses. Looks like we need to get to the bottom. Would it be possible to get a manufacturer's declaration of what the absolute maximum ratings are.

mm

 

[scottf]

Mutimuti-

Yes...the manufacturer should be able to tell you.

Also, there should be a thermal and dynamic current rating marked on the nameplate.

 

[cuky2000]

“CT’s are typically able to carry fault current up to 20 times their rating for short period of time. Their ability to do this depends on their loading being within their specified burden rating.”

 

[scottf]

Cuky-

I think you're talking about the accuracy of a CT, i.e. C800 or 5P20 rating and it is related to burden.

The ability of a CT to physically handle (handle meaning without damage) is not related to the applied burden and is really only a function of the cross-sectional area of the windings used in the primary and secondary and the mechanical design of such.