Every seasoned commissioning engineer has heard the stories of the high-voltage hiss or the distinct smell of burning resin that accompanies an open-circuited current transformer. When current flows through the primary winding of an open-circuited CT, the secondary attempts to drive current through an infinite impedance, creating extremely high voltage peaks that can instantly destroy insulation and cause lethal arc flash events. Before you connect your secondary injection test set, your first line of defense is a manual, physical verification of the circuit loop.
The Danger of Uncapped Inductive Energy
In a standard protection scheme, the CT secondary circuit must always remain closed through a low-impedance path like a relay coil, a meter, or a shorting link. When we disconnect a relay for testing or calibration, that low-impedance path is broken. If the primary circuit is energized or if test currents are applied incorrectly, thousands of volts can develop across the open secondary terminals. This voltage spike will break down the CT winding insulation, ruining a multi-thousand-dollar piece of equipment and exposing anyone nearby to dangerous electrical shock.
Step-by-Step Shorting Link Verification
Never assume the wiring schematic in front of you matches the reality of the terminal block. Begin by visually verifying that the heavy-duty orange shorting links are physically screwed down tight across the current transformer terminal strip. Once visually confirmed, use a digital multimeter on the lowest resistance scale to measure the loop resistance across the shorted terminal pairs. A healthy shorted loop should read under one ohm; any reading higher than this indicates a loose screw, a dirty contact, or an incorrect connection that must be addressed immediately.
The Final Verification Sequence
When you are finally ready to apply secondary injection, do not immediately crank the current to the nominal five amps. Begin by slowly ramping up the test current to ten percent of the target value while monitoring the voltage drop across your test leads. If you see the voltage climbing rapidly while the current struggles to rise, stop immediately and check your connections. This simple safety ramp-up sequence has saved countless CTs and relay cubicles from catastrophic failure on site.