The rule that a CT secondary must never be left open while primary current is flowing is one of the most fundamental safety principles in electrical work. Understanding the physics behind this requirement explains why it is so critical.
A current transformer operates as a special type of transformer in which the primary winding is connected in series with the load. Under normal operation, the secondary winding is connected to a low-impedance burden, such as a meter or relay. The secondary current is determined by the primary current divided by the turns ratio. The secondary voltage is relatively low, typically a few volts or less, because the burden impedance is low.
If the secondary circuit is opened while primary current is flowing, the secondary current drops to zero. However, the primary current remains unchanged (it is determined by the load, not by the CT). The CT then behaves as a step-up transformer with no load on the secondary. The core flux increases dramatically, driving the core into saturation. This causes a very high voltage to be induced across the secondary terminals-potentially thousands of volts. For 1 A and 5 A secondary CTs, an open secondary can develop hazardous voltage and may damage equipment or create a shock hazard.
The consequences of an open CT secondary are severe. Current transformers operated while open on the secondary side can result in severe bodily injury or death due to the high voltage peaks that pose a hazard when touched. The high voltage can also damage the CT itself by breaking down the insulation between turns or between the winding and core. In addition, the core may overheat due to the excessive flux, potentially causing thermal damage to the CT.
The danger is not limited to personnel safety. Equipment connected to the CT secondary, such as meters and relays, can be damaged by the high voltage. There is a risk of electric shock if a current transformer is operated when open. If the secondary circuit is open and the primary circuit is live, a high voltage will be induced in the CT which is connected to the open connection cables, creating a risk of serious or fatal injuries. It may also result in material damage to current transformers and energy meters.
To prevent this hazard, several protective measures should be in place. Before wiring, de-energize the CT secondary by shorting it via a shorting block. If a CT must be installed without the secondary wiring connections made, a shorting link should be installed between the secondary connections, and this link should only be removed when final secondary connections are made. Test switches that automatically short-circuit the secondary lines of current transformers should be switched to the "Test" status before interrupting the current supply. All external current transformer secondary windings must be grounded to the ground bus to prevent capacitive coupling from allowing the secondary voltage to float up to the voltage of the mains. These measures collectively ensure that the secondary circuit never presents an open-circuit condition while primary current is flowing.