Current transformer

A current transformer is an instrument transformer that produces a smaller, proportional current from the alternating current in a conductor, enabling measurement and protection circuits to sense it safely.

Ampère's circuital law: A law relating the magnetic field around a closed path to the electric current passing through that path. It explains how primary current creates the magnetic field that drives the transformer's secondary current.
Ammeter: An instrument that measures electric current. A current transformer lets an ammeter measure currents too large for direct connection.
Voltage transformer: An instrument transformer that produces a reduced, proportional voltage for measurement or protection. It scales voltage rather than current and is normally designed to operate with a secondary burden in parallel.
Electromagnetic induction: The production of electromotive force by a changing magnetic flux. It is the physical principle that induces voltage in a current transformer's secondary winding.
Magnetic flux: A measure of the magnetic field passing through a specified surface. Changing flux in the core induces voltage in the secondary winding.
Protective relay: A device that detects abnormal electrical conditions and initiates protective action. Current transformers supply relays with scaled signals for detecting faults.
Power transformer: A transformer designed to transfer electrical power between circuits, often at different voltage levels. Unlike a current transformer, it is designed for useful power transfer rather than current sensing.
Alternating current: Electric current that periodically reverses direction. A changing current produces the changing magnetic flux on which conventional current transformers depend.
Current ratio: The ratio between a transformer's primary and secondary currents under specified operating conditions. The winding ratio sets the smaller current delivered for measurement.
Differential protection: A protection method that detects faults by comparing currents entering and leaving a protected zone. Matched current transformers provide the current comparisons this scheme requires.

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