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Don’t Open-Circuit a CT’s Secondary Winding

When measuring or monitoring the amount of current passing through a transmission line, an ammeter—a device that measures electric currents—is connected to the primary conductor (transmission line) via a current transformer (CT). A CT reduces the amount of current flowing through the primary conductor to protect the measuring instrument. The CT, in turn, is connected to the measuring instrument via a secondary winding.

This supposedly safe setup can become troublesome, however, if the secondary winding is open-circuited while the CT is still receiving load from the primary conductor. Once the connection is cut, electricity is stopped at the tip of the open circuit, causing a high electric voltage to saturate the CT. The current building up in the CT will keep pushing toward the tip until it releases an electrical charge that arcs toward the other point of breakage.

The arch formed by the saturation of the CT then fires off at a very high temperature, causing the insulation to break down and triggering a fire. Such a scenario can be dangerous both for the instruments connected to the setup and any person working on it. As such, it is imperative for anyone installing or repairing a CT setup to never open-circuit the secondary winding.

Given the risks involved in installing and repairing CTs, it only makes sense to purchase high-quality CT products—be they solid donut-like devices, open-core varieties that aid installation, or flexible CTs that allow smoother use.