current transformer's, overload 1

[streeta]

What danger's can arise from putting to small of C.T in circuit. We have a motor rated at over 300A full load but only have 250/5 amp current transformers. will these do until we can get the right size.
thanks
greg

 

[DanDel]

A CT used for metering or protective relays should be sized for the FLA of the circuit. That being said, the difference you mention should not result in any significant damage from overheating. You didn't say if the CT was for metering or protection. If the CT is used for protection, relays commonly have tap settings above the rated 5A secondary current. If for analog metering, the meter is typically sized for the CT(250:5A CT, 250A max scale meter), and the meter may be damaged from over-scale readings.

 

[RRaghunath]

Greg,

If th motor rated current happens to be 300A, it is quite likely that the motor would be drawing less than that (typically 80%) during normal running conditions at full load.

The CTs have thermal rating that is typically 120% (Refer IEC 60044-1) of rated current. Hence, your 250/5A rated CT should not pose any problem if loaded to 300A for some time.

Trust you find the above useful.

 

[busbar]

Some CTs have a rating factor of 1.33 at 30°C. In a pinch, you may be able to add turns to the secondary, if the usual 10kVBIL insulation does not need preservation. The ammeter would have to be rescaled accordingly. Depending on duty, the meter may very well get repeatedly pegged during startup.