A bushing current transformer is a window-type instrument transformer that slips over a primary power conductor—usually integrated into the insulating bushing of a transformer or switchgear—to step down high primary currents into standardized, measurable secondary currents for metering and protection relays. By physically separating the high-voltage primary circuit from the low-voltage secondary wiring, it changes a lethal, unmeasurable busbar current into a safe 5A or 1A signal that microprocessor relays can process. Beginners often confuse bushing CTs with wound-type CTs (which have physical primary windings) or split-core CTs (which are hinged for temporary retrofits), but a true bushing CT is a solid, continuous toroidal core designed to be permanently slipped over a conductor during apparatus assembly.
Core Operating Principle and Circuit Impact
At its core, a bushing CT operates on the principle of magnetic induction. The primary 'winding' is simply the straight busbar or cable passing through the center of the toroidal core. This single-turn primary generates a magnetic flux in the core, which induces a proportional current in the multi-turn secondary winding wrapped around the outside of the toroid. Think of it like a mechanical gearbox: the primary busbar is the high-torque, low-speed input shaft, and the secondary winding is the low-torque, high-speed output shaft, perfectly scaling the energy down without altering the fundamental frequency or phase relationship.
What this changes in a real installation is profound. Without a bushing CT, measuring a 1200A circuit at 13.8kV would require running massive, heavily insulated cables directly into a delicate microprocessor relay—an impossible and highly dangerous task. The CT provides galvanic isolation, ensuring that if a fault occurs on the primary side, the secondary metering circuits and the personnel interacting with them remain completely isolated from the primary voltage.
The core material dictates the CT's behavior. Metering bushing CTs use highly permeable grain-oriented silicon steel to remain accurate at normal load currents, but they intentionally saturate during faults to protect connected meters from thermal damage. Protection bushing CTs, governed by standards like IEEE C57.13, use specialized alloys or air gaps to resist saturation during massive fault currents, ensuring the protection relay sees the true fault magnitude to trip the breaker.
Worked Numeric Example: Ratio, Saturation, and Burden
Let's look at a real-world scenario involving a 1200:5A, C200 accuracy class bushing CT installed on a 13.8kV switchgear feeder. The 'C200' designation means the CT can deliver 20 times its rated secondary current (100A) into a standard burden without exceeding a 10% ratio error, provided the secondary voltage does not exceed 200V.
The Scenario: A phase-to-ground fault occurs downstream, pushing 18,000A of primary fault current through the bushing. The connected microprocessor relay and the copper wiring back to the switchgear terminal block present a total secondary burden of 0.5 ohms.
- Calculate Secondary Current (I_s): Using the turns ratio (1200/5 = 240), the secondary current is 18,000A / 240 = 75A.
- Calculate Developed Voltage (V_s): Using Ohm's law (V = I × R), the voltage the CT must push across the burden is 75A × 0.5 ohms = 37.5V.
- Check for Saturation: The calculated 37.5V is well below the C200 saturation threshold of 200V. Therefore, the CT will not saturate. The relay will see an accurate 75A signal and trip the breaker in milliseconds.
Where You Meet This in Practice
You will rarely find a true bushing CT in low-voltage (120V/480V) commercial panels. They are the domain of medium and high-voltage infrastructure. Here is where they live in the wild:
- Medium-Voltage Metal-Clad Switchgear: Inside compartments like ABB UniGear or Siemens GM-SG, bushing CTs are slipped over the epoxy-encapsulated bus spouts before the circuit breaker is racked in.
- Pad-Mounted Distribution Transformers: Inside the locked compartment of a 12.47kV/480V padmount, you will find them slipped over the primary bushings to feed the transformer's internal fault protection or utility revenue metering.
- Generator Step-Up (GSU) Neutrals: In power plants, bushing CTs are mounted on the neutral end of the generator leads to detect restricted earth faults.
| Feature | Bushing CT (Window) | Standard Panel CT (Wound/Bar) | Split-Core CT (Retrofit) |
|---|---|---|---|
| Primary Connection | Pass-through busbar or cable | Physical primary terminals (H1/H2) | Hinged, clamps over existing cable |
| Installation Phase | Factory assembly or major overhaul | Panel wiring phase | Post-installation retrofit |
| Accuracy Class | High (Metering or Protection) | High (Metering or Protection) | Low to Moderate (Mostly metering) |
| Typical Voltage Class | 5kV to 765kV | 600V to 15kV | 600V nominal |
Polarity, Shorting Blocks, and Safety
When wiring the secondary leads (X1 and X2) to a relay, polarity is non-negotiable. The H1 mark on the primary bushing must face the source, and the X1 mark on the CT secondary must wire to the polarity-sensitive terminal on the relay. Reversing this will cause directional overcurrent relays to see a reverse fault and fail to trip, or cause differential relays to trip instantly on normal load current.
For deeper insights into testing and maintaining these devices in the field, the instrument transformer theory guides and NETA maintenance standards provide excellent baseline procedures for ratio and polarity verification.
Bushing Current Transformer FAQ
Can I retrofit a bushing current transformer onto an existing busbar?
No. Because a true bushing CT is a solid, continuous toroid without a hinge or split, it cannot be installed over an existing, terminated busbar or cable. It must be slipped over the conductor before the primary connections are made. If you need to add metering to an existing, energized or already-terminated circuit, you must use a split-core CT, which clamps around the conductor in two halves.
What happens if the secondary circuit of a bushing CT opens under load?
If the secondary opens while primary current is flowing, the CT core goes into deep magnetic saturation. The rapid change in flux induces extreme voltage spikes (often several kilovolts) across the open secondary terminals. This can cause dielectric breakdown of the CT insulation, arcing at the terminal block, permanent damage to the core's magnetic properties, and a severe electrocution hazard for anyone nearby. This is why CT secondary circuits are always equipped with shorting blocks.
How do I choose between a metering class and a protection class bushing CT?
Choose a metering class (e.g., 0.3 or 0.6 accuracy) when the CT feeds revenue meters or ammeters; these cores are designed to saturate at low overcurrents to protect delicate instruments from fault-level energy. Choose a protection class (e.g., C200, C400) when the CT feeds protective relays; these cores are designed to resist saturation during massive fault currents so the relay receives an accurate waveform to make a trip decision. Many modern switchgear installations use dual-core bushing CTs, providing one core for metering and a separate core for protection on the same physical unit.
Why do bushing CTs have multiple tap ratios?
Many bushing CTs are manufactured with multiple secondary taps (e.g., 600/800/1000/1200 to 5A) to provide flexibility for future load growth. If a feeder is initially loaded to 400A, you can wire the relay to the 600A tap for optimal measurement resolution. If the facility expands and load increases to 900A five years later, you can simply rewire the secondary to the 1200A tap without replacing the physical CT or the switchgear bushing.
