Current Transformers

Current transformers normally have a single conductor as a primary winding connected in the run of the line conductors. The voltage which appears across the primary terminals is very small and therefore the voltage at the secondary terminals is small.

The primary conductor however does carry the full system current and because current transformation is the inverse of the turns ratio the secondary current is reduced to a manageable value for measurement purposes.

With a single conductor primary and 500 turns on the secondary winding the turns ratio primary/secondary is 1/500 and for 500 amps in the primary there would be 1 amp in the secondary winding this enables the currents in the system conductors to be accurately measured. The secondary current although small is driven by the capacity of the primary system. 

For this reason, the CT link is provided to always provide a path for the secondary current in the event that the meter or protection is removed from the circuit.

Any attempt to disconnect the secondary circuit (open circuit) can damage the CT winding and poses a significant risk to whoever is removing the wire as the current will continue to be forced through the increasing resistance causing heavy arcing.

Current transformers are designed to cope with the system fault levels. The fault current flowing through a current transformer again gives the two parallel wire situation, with current flowing in opposite directions and a repulsion or bursting force, proportional to I2, within a porcelain bushing full of oil.

Care must be taken to monitor for any increase in system fault levels and if it appears that an increase will exceed the rating of any current transformers then they must be changed.


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