How to Safely Operate & Maintain Current Transformers?

I. Permissible Operating Conditions for Current Transformers

• Rated Output Capacity: Current transformers (CTs) must operate within the rated output capacity specified on their nameplate. Operation beyond this rating reduces accuracy, increases measurement errors, and causes inaccurate meter readings, similar to voltage transformers.

• Primary Side Current: The primary current may continuously operate up to 1.1 times the rated current. Prolonged overload operation increases measurement errors and may overheat or damage the windings. The secondary current of a CT is typically 5 A or 1 A (commonly 5 A). Under normal operation, the secondary circuit operates close to a short-circuit condition.

• The Secondary Circuit Must Never Be Open-Circuited During Operation: An open secondary circuit while the CT is energized will induce dangerously high voltages, endangering equipment and personnel. If the secondary circuit must be interrupted (e.g., for meter removal), the secondary terminals must first be securely short-circuited using a shorting link.

• The Secondary Winding and Core Must Be Solidly Grounded: This prevents high-voltage transfer from the primary to the secondary side in case of insulation failure between windings.

• Secondary Load Impedance Must Not Exceed the Rated Value: To ensure measurement accuracy, the connected burden must be within the rated impedance.

• Observe Terminal Polarity When Wiring: Correct polarity must be maintained during installation and connection.

• Never Interconnect CT and VT Secondary Circuits: Connecting a CT secondary to a VT secondary may leave the CT effectively open-circuited, creating a hazardous high-voltage condition.

• Safety During Work: When performing work, a qualified supervisor must be present. Insulated tools must be used, and personnel must stand on an insulating mat.

II. Routine Inspection of Current Transformers in Service

• Inspect porcelain insulators for cleanliness, absence of damage, cracks, or discharge marks. 

• Check oil level is normal, oil color is clear and not darkened, with no signs of leakage or seepage. 

• Listen for abnormal sounds or detect any burnt odor from the CT. 

• Inspect primary lead connections for firmness, ensuring no loose bolts or signs of overheating. 

• Verify the secondary winding grounding conductor is intact, securely connected, and free from looseness or breakage. 

• Check that the terminal box is clean, dry, and free from moisture; ensure secondary terminals have good contact with no open circuits, arcing, or sparking.