A bar type current transformer is an instrument transformer where the primary winding consists of a single straight conductor—often the actual busbar or heavy cable of the circuit—passing directly through the window of a toroidal secondary core. It changes a high, potentially dangerous primary AC current into a standardized, low secondary current (typically 5A or 1A) for safe metering and protection relay operation, while providing critical galvanic isolation between the high-voltage power circuit and the low-voltage control wiring. People commonly confuse it with a wound-type CT (which has a multi-turn primary coil physically wound inside the casing) or a Rogowski coil (a flexible, air-core sensor that outputs a voltage proportional to the derivative of current rather than a direct current replica).
| CT Type | Primary Winding | Typical Current Range | Core Material | Best Application |
|---|---|---|---|---|
| Bar Type | 1 turn (the busbar/cable itself) | 100A to 5,000A+ | Grain-oriented silicon steel, Nanocrystalline | Main switchboards, bus ducts, MCCs |
| Wound Type | Multi-turn physical coil | 1A to 100A | Silicon steel, Nickel-iron (Permalloy) | Feeder cabinets, low-current metering |
| Split-Core Solid | 1 turn (existing cable) | 50A to 800A | Split laminated steel | Retrofit metering, energy audits |
| Rogowski Coil | Air-core flexible loop | 10A to 100,000A+ | None (Air) | Power quality analysis, massive busbars |
How the Magnetic Circuit Actually Works (and the Math)
In a bar type current transformer, the primary "winding" is just the straight busbar passing through the core window. Because it passes through only once, the primary turn count ($N_p$) is exactly 1. The secondary winding ($N_s$) consists of many turns of fine enameled copper wire wrapped around the toroidal core. The transformation ratio is dictated by the ampere-turn balance equation:
$$I_p \times N_p = I_s \times N_s$$
Since $N_p = 1$, the ratio is simply the number of secondary turns. If you have a 1000:5 CT, the secondary has 200 turns ($1000 / 5 = 200$).
Worked Numeric Example: Sizing and Burden Calculation
Let's say you are metering a 1000A main feeder using a 1000:5 bar type CT. The actual load on the busbar is 850A. You need to verify if your chosen 10VA CT can handle the connected burden without saturating and losing accuracy.
- Calculate Secondary Current:
$I_s = I_p \times (5 / 1000)$
$I_s = 850A \times 0.005 = \mathbf{4.25A}$ - Calculate Total Secondary Burden (Impedance):
Your digital power meter has an input impedance of 0.15 Ω.
You are using 30 meters of 2.5 mm² (approx 14 AWG) copper control wire. The loop resistance (out and back) is roughly 0.35 Ω.
Total Burden ($Z_b$) = $0.15 Ω + 0.35 Ω = \mathbf{0.50 Ω}$ - Calculate Required VA:
$VA = I_s^2 \times Z_b$
$VA = (4.25)^2 \times 0.50 = 18.06 \times 0.50 = \mathbf{9.03 VA}$
Because your calculated burden is 9.03 VA, a standard 10 VA Class 0.5 metering CT is sufficient. If you had an older electromechanical relay in the loop adding another 4 VA, your total would exceed 13 VA, and you would need to step up to a 15 VA or 20 VA CT to prevent core saturation and metering errors.
Where You Meet This In Practice
You will rarely see a bar type CT inside a small residential panel or a low-power control enclosure. They are the heavy lifters of commercial and industrial power distribution.
- Main Distribution Boards (MDBs): At the main incoming breaker of a commercial building (e.g., 1600A to 4000A), the primary conductors are thick copper or aluminum busbars. The CTs are literally slipped over the busbars before the bolted joints are torqued down.
- Bus Ducts and Busways: In systems like Schneider Electric Canalis or Siemens SENTRON busways, the CT chambers are integrated directly into the plug-in units or feed boxes. The busway's internal aluminum bar acts as the primary.
- Motor Control Centers (MCCs): Large vertical busbars running up the back of an MCC will have arrays of bar type CTs stacked on them to feed individual overload relays and multifunction protection relays (like the SEL-487E or ABB RELion series).
Modern digital relays have drastically changed how we specify these. Older electromechanical relays (like the classic GE IAC) drew heavy burden, requiring massive, expensive 30VA or 50VA bar type CTs. Today's microprocessor relays draw less than 0.1 VA, allowing engineers to specify much smaller, cheaper 2.5 VA or 5 VA cores, provided the wire run isn't excessively long.
Sizing, Accuracy Classes, and the Open-Circuit Hazard
When specifying a bar type CT, you must choose the correct accuracy class based on its job. According to industry standard practices outlined by IEC and IEEE, metering and protection require fundamentally different core behaviors.
- Metering Classes (0.2, 0.5, 1.0): These cores are designed to be highly accurate at normal operating currents (80% to 120% of nominal) but intentionally saturate during a short circuit. This saturation protects the delicate wiring and electronics of your power meter from being destroyed by a 20,000A fault current.
- Protection Classes (5P, 10P, Class X/PX): These cores are designed to resist saturation during faults. A 5P20 CT guarantees 5% composite accuracy up to 20 times the rated current. If a fault hits, the protection relay needs an accurate replica of that massive current to trip the breaker in milliseconds.
Unlike a voltage transformer, a current transformer acts as a constant current source. If the secondary circuit is opened while primary current is flowing, the entire primary current acts as magnetizing current. The core drives into extreme saturation, and the collapsing magnetic field induces a massive voltage spike across the open secondary terminals—often exceeding 2,000V to 5,000V. This will instantly destroy insulation, shatter the CT casing, and poses a lethal shock hazard. Always use a shorting test switch (like a Mak-plus or FT-1 test block) before disconnecting a meter, and ensure the secondary is shorted if the CT is installed but not immediately wired.
Jobsite FAQ: Bar Type CT Quirks and Edge Cases
Can I pass the busbar through the CT window more than once?
Yes, and this is a common jobsite trick to increase the effective ratio of a CT for better low-current resolution. If you have a 1000:5 CT but your actual maximum load is only 300A, your secondary current will only be 1.5A, leaving you in the lower, less accurate part of the metering curve. If you loop the primary cable through the window twice, the primary turn count becomes 2. The effective ratio halves to 500:5. Now, a 300A load yields 3.0A on the secondary, pushing you into the sweet spot of the CT's accuracy class. Just ensure your physical cable bending radius allows for the loop without stressing the lugs.
Does the physical position of the bar inside the window matter?
For basic 5P protection class CTs, slight off-centering won't trip the breaker. However, for high-precision Class 0.2 revenue metering, the bar should be centered in the window. An off-center busbar creates an asymmetrical magnetic flux distribution in the toroidal core, which can introduce localized saturation and ratio errors, especially if the primary current contains high harmonics from VFDs (Variable Frequency Drives). Fundamental transformer theory dictates that symmetrical flux linkage yields the most accurate ampere-turn balance.
What happens if I use a 5A secondary CT with a 1A relay?
You will get inaccurate readings and potentially damage the relay input. However, if you are stuck with existing 5A secondary wiring and buy a modern 1A relay, you can use an auxiliary 5:1 interposing CT. Alternatively, many modern digital meters (like the Socomec DIRIS or Schneider PM5xxx series) allow you to configure the software scaling factor. If you wire a 5A CT into a meter with a 1A nominal input, you can often set a software multiplier, provided the physical current doesn't exceed the meter's absolute maximum thermal rating (usually 2A continuous for a 1A input).
How do I handle bar type CTs on DC systems or VFD outputs?
You don't. Standard bar type CTs rely on Faraday's law of induction, which requires a changing magnetic field (AC). They will not measure DC current, and the high-frequency PWM carrier wave from a VFD output will cause massive eddy current heating and core saturation in a standard silicon steel CT. For DC busbars or VFD output monitoring, you must use Hall-effect sensors (like LEM modules) or specialized high-frequency Rogowski coils with appropriate integrator circuits.
