Current transformers measure the amount of electric current flowing through a particular wire on the electrical grid by making a secondary current that is proportional (but much less than) the amount of current flowing in that wire.
A current transformer is made of a transformer surrounding a current-carrying conductor. The conductor serves as the primary for the transformer. The current passing through the conductor generates a magnetic field which cuts through to the secondary winding generating a secondary current proportional to the current through the primary (conductor). This device is useful as a current measuring device because it allows ratio to be applied where the full current doesn't matter. For example a 600:1 current transformer would generate 1 A of secondary current for every 600 A of primary current. This is extremely useful to have for the electrical grid because it allows for the measurement of high-current values without having to interrupt operating circuits.
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