Current Transformer(CT)
25 April, 2026
The vital link between high-voltage networks and sensitive relays. Engineered for exceptional accuracy (up to 0.2s) in revenue metering and unwavering reliability (up to 10P20) in fault protection.
A Current Transformer (CT) is a type of instrument transformer designed to proportionally step down extremely high primary currents (e.g., 1000A, 2000A) to manageable, standard secondary values (typically 1A or 5A). These secondary currents safely feed measurement instruments, energy meters, and protective relays, isolating them from the dangerous high-voltage network.
Never open-circuit the secondary winding of an energized CT. Because a CT acts as a constant current source, an open secondary will cause the magnetic flux in the core to reach extreme saturation, inducing a lethal, destructive high voltage across the secondary terminals that will destroy the insulation and endanger personnel.
FOR SWITCHFEAR
Vacuum-cast in cycloaliphatic epoxy resin using APG technology. Designed for installation inside KYN metal-clad switchgear panels. Highly flame-retardant and maintenance-free.
FOR SUBSTATIONS
Engineered with high-creepage porcelain or silicone rubber insulators. Weatherproof and UV-resistant, ideal for open-air high-voltage substations and metering stations.
CABLE FAULT DETECTION
Toroidal design where the primary cable acts as the single primary turn. Specifically designed to detect minute earth-fault currents (Zero-Sequence) for instant relay tripping.
Utilizes advanced nanocrystalline materials to ensure extreme accuracy, even at 1% of the rated current. Designed to intentionally saturate quickly during a short circuit to protect delicate measuring meters from thermal damage.
Built with high-grade silicon steel. The "20" represents the Accuracy Limit Factor (ALF), meaning the core will maintain its linear transformation ratio (within 5% or 10% error) even when the fault current reaches 20 times the rated normal current, ensuring relays trip accurately.
| Feature | Current Transformer (CT) | Voltage Transformer (VT/PT) |
|---|---|---|
| Primary Function | Steps down High Current | Steps down High Voltage |
| Circuit Connection | Connected in Series | Connected in Parallel |
| Standard Secondary Output | 5A or 1A | 110V or 100V |
| Primary Winding | Thick wire, very few turns (or busbar itself) | Thin wire, thousands of turns |
| Safety Hazard | Never Open the Secondary | Never Short the Secondary |
| Parameter | Unit | 12kV Indoor CT | 24kV Indoor CT | 36kV / 40.5kV CT |
|---|---|---|---|---|
| Highest System Voltage (Um) | kV | 12 | 24 | 40.5 |
| Primary Current Options | A | 50A, 100A, 400A, 630A, 1000A, 1250A, 2000A, up to 4000A | ||
| Secondary Current | A | 5A or 1A | ||
| Metering Accuracy Class | - | 0.2s / 0.2 / 0.5s / 0.5 | ||
| Protection Accuracy Class | - | 5P10 / 5P20 / 10P10 / 10P20 | ||
| Rated Output Burden | VA | 10VA / 15VA / 20VA / 30VA | ||
| Short-Time Thermal Current (Ith) | kA/1s | Typically 100 x Rated Primary Current (Specific to design) | ||
In the designation 5P20, "5P" indicates it is a Protection class core with a composite error limit of 5%. The "20" is the Accuracy Limit Factor (ALF). This means the CT will maintain its 5% accuracy even when the primary current reaches 20 times its rated value during a short circuit, ensuring protective relays trip correctly without magnetic core saturation.
Both are metering classes, but the "s" (special) designation means the CT maintains high accuracy even at very low currents (down to 1% or 5% of the rated current). Class 0.2s is typically used for precision tariff metering (revenue billing) at utility connection points, while Class 0.5s is sufficient for standard industrial metering and monitoring.
Yes. The rated burden (e.g., 10VA, 15VA, 30VA) depends on the impedance of the connected meters, relays, and the length of the secondary wiring. If your switchgear design requires long cable runs to the control room, we can manufacture CT cores with a higher VA capacity to ensure accurate signal transmission without voltage drop.
Submit your requirements on a business day, and our engineers will provide an initial OEM proposal within 24 hours.
We supply complete medium & high voltage solutions, including:
• Air Insulation Series
• Gas Insulation Series
• Switching Devices
• Instrument Transformers
