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Don’t Forget to Short a Current Sensor

It’s something that even experienced electrical engineers forget at times: never operate a current sensor with the secondary winding in an open circuit. Doing so results into catastrophic overvoltage failure of the transformer, which usually manifests as arcs bridging the gap between the terminals. Aside from overvoltage, this error can also cause insulation failure, which increases the chances of a fire breaking out.

Usually, this takes place when current transformers are disconnected from monitoring equipment and technicians forget to short circuit the current sensor. As such, protocols should be laid out and enforced for CT terminals to be shorted before they are disconnected from the circuit.

The Phenomenon Explained

The reason that overvoltage occurs when the secondary is open-circuited is due to a loss of the demagnetizing effect provided by current continually moving through the coils. When the secondary is opened, flux density quickly increases in the core. This instability drives the production of an electrical charge proportionate to its density. With core saturation the only limit, this electrical charge can build up well into the thousand-volt range.

Moreover, core saturation also produces the unfortunate effect of waveform change. The usual sine waves seen in normal operation turn into peaked waves. As is the nature of peaked waves, these produce a high instantaneous rate of voltage change over time (dV/dT), which is often the primary cause of dangerous heat generation that can even lead to the ignition of insulation.