Grounding the CT secondary circuit is a mandatory safety requirement that protects personnel and equipment from hazardous voltages. The purpose of grounding is to ensure that if the insulation between the primary and secondary windings fails, the secondary circuit remains at or near ground potential rather than rising to the primary voltage level.
The fundamental rule is that the CT secondary circuit must be grounded at exactly one point. All external current transformer secondary windings must be grounded to the ground bus. If the secondary windings are not grounded, capacitive coupling can allow the secondary voltage to float up to the voltage of the mains, posing a serious safety hazard.
The standard practice is to ground the S2 terminal of the CT secondary. The secondary circuit of a current transformer must have only one grounding point, generally grounded through the earthing bar in the terminal box-. For three-phase systems, the S2 terminals of all three CTs are typically connected together and then grounded at a single point. This common connection is sometimes referred to as the CT common terminal.
The grounding point is usually located at the meter or relay end of the secondary circuit, rather than at the CT itself. This ensures that the entire secondary circuit is at ground potential and that any fault current has a direct path to ground. Where applicable, the shorting link between the filter ground and safety ground must be removed prior to dielectric strength testing to protect the transient suppression circuitry of the power supply.
Grounding is not optional-it is a fundamental safety requirement. One end of the secondary side must be grounded to prevent the high voltage of the primary side from entering the low voltage side of the secondary side once the insulation is damaged, causing an accident. The secondary side of the current transformer is typically equipped with a short-circuit switch to prevent the secondary side from being open-.
When installing or maintaining CT secondary circuits, always verify that the grounding connection is intact and properly tightened. A loose or missing ground connection defeats the purpose of the grounding system and creates a potential shock hazard. The grounding conductor must be sized appropriately for the maximum fault current that could flow through the secondary circuit under fault conditions.
In addition to grounding, shorting blocks and fuses are recommended for voltage leads and power supply to prevent hazardous voltage conditions or damage to current transformers. Before wiring, de-energize the CT secondary by shorting it via a shorting block. This ensures that the secondary circuit is never accidentally opened during work.