The Basics of Current-Sensing Relays

Current transformers

Current transformers (CTs) are a special transformer that change current from one level to another for monitoring or measuring purposes. They typically have a one-turn primary. The cable whose current is to be measured is placed through the window, which is the primary turn. The secondary connection is made at the screw terminals.

CT current step-down ratios are represented as follows — primary current: secondary current. An example is a CT having a 1,200A primary and 5A secondary. Its ratio will be 1,200:5. A 5A secondary is a common secondary current. Some secondary currents are 1A, but those are not common.

The secondary for a CT should never be open-circuited when current is flowing through the primary. High voltages can be generated that can cause insulation breakdown and consequent destruction of the CT or shock/injury to the user. The secondary should be short circuited instead. Special shorting blocks are often employed on which the CT’s leads, and the leads for the load — relay or meter — are terminated. Shorting bars short circuit the leads so the CT’s load may be safely removed from the circuit for repair or calibration.

Current transformers are available with different window diameters. When specifying, make sure the window opening is adequate for the cables, especially if paralleled cables are used.             

Bredhold is an application engineer with Eaton Corp., Louisville, Ky. He can be reached at DavidBredhold@eaton.com.