Auxiliary Current Transformer Sizing and Selection for Relay Retrofits

An auxiliary current transformer is a secondary instrument transformer that scales, isolates, or adapts the output signal of a primary current transformer to match the specific input requirements of protective relays, digital meters, or control circuits. When you upgrade legacy switchgear with modern microprocessor relays, the auxiliary CT is the critical bridge that prevents you from having to rip out and replace the high-voltage primary CTs. It changes the circuit by stepping down a standard 5A secondary current to a 1A or milliamp-level signal, drastically reducing wiring burden and I²R heating while maintaining strict galvanic isolation. The most common mistake in the field is confusing an auxiliary CT with a shunt resistor; while a shunt can convert current to a voltage signal, it offers zero galvanic isolation and ties your sensitive relay inputs directly to the high-energy secondary loop.

What an Auxiliary CT Actually Changes in Your Circuit

Older electromechanical relays (like the classic GE IAC or Westinghouse CO series) were designed with heavy copper coils that required a robust 5A nominal input to generate enough magnetic force to trip a mechanical latch. Modern digital relays, such as the Schweitzer Engineering Laboratories (SEL) SEL-351 or GE Multilin series, use solid-state analog-to-digital converters. These modern inputs prefer 1A or even milliamp-level signals.

By inserting a 5:1A auxiliary CT between the primary CT and the relay, you achieve three things:

• Signal Scaling: You map the 5A secondary loop down to a 1A loop that the digital relay expects.
• Burden Reduction: Lower current on the relay side means less heat dissipation inside the relay chassis.
• Galvanic Isolation: You maintain a physical magnetic barrier between the high-energy switchgear secondary wiring and the low-voltage relay logic.

The Math: A Worked Burden and Ratio Example

The most dangerous trap when adding an auxiliary CT is forgetting that it reflects its secondary burden back to the primary CT. If you miscalculate this, the primary CT will saturate during a fault, and your relay will never see the trip signal. For a deep dive on CT saturation curves, refer to the Electrical Engineering Portal's guide on CT basics.

Let's run a real-world numeric example. You have a 600:5A primary CT on a feeder breaker. You install a 5:1A auxiliary CT to feed a digital relay.

1. The Relay Burden: The digital relay's 1A input has an internal burden of 0.1 Ω.
2. The Turns Ratio (N): The auxiliary CT steps 5A down to 1A. Therefore, the turns ratio N = 5 / 1 = 5.
3. Reflected Burden: Impedance reflects across a transformer by the square of the turns ratio (N²).
   Formula: Z_reflected = Z_secondary × N²
   Calculation: 0.1 Ω × (5)² = 0.1 Ω × 25 = 2.5 Ω.

Even though the relay only presents a tiny 0.1 Ω load on the 1A side, the primary 600:5A CT 'sees' an additional 2.5 Ω of burden just from the relay alone. If your primary CT is rated for a total burden of 4.0 Ω (including wire resistance), this single auxiliary CT and relay combination consumes the vast majority of your available burden headroom. Always verify the primary CT's VA rating and accuracy class (e.g., C200 or C400) against the total reflected burden to ensure it won't saturate at 20x symmetrical fault current.

Where You Meet This in Practice

You will typically encounter auxiliary CTs in three specific field scenarios:

• Switchgear Retrofits: Upgrading 1980s air-magnetic breakers with modern solid-state trip units. The primary CTs are embedded in the buswork and cost thousands to replace; a $150 auxiliary CT mounted in the low-voltage compartment solves the interface mismatch.
• Ground Fault Summing: When a protection scheme requires a residual ground fault signal, but the relay only has one spare input. Three 5A phase CTs are wired into a 'summing auxiliary CT', which algebraically adds the three phase currents and outputs a single 1A signal representing the zero-sequence (ground) current.
• PLC Analog Isolation: Routing a power meter's 5A CT loop into a Programmable Logic Controller (PLC) analog input card. The auxiliary CT isolates the noisy, high-inductance power loop from the sensitive 4-20mA or 0-10V PLC transducers.

Decision Tree: Picking the Right Auxiliary CT

Use this decision matrix to select the correct auxiliary transformer for your specific application. Do not guess the VA rating; undersizing leads to saturation, while grossly oversizing wastes panel space and money.

Fatal Mistakes: Saturation and Open Circuits

Working with auxiliary CTs introduces the same lethal hazards as primary CTs. The two most common fatal mistakes (one to equipment, one to personnel) are:

1. Open-Circuiting the Secondary: Never open the secondary circuit of an auxiliary CT while current is flowing through its primary. If the 1A secondary loop is broken, the transformer core saturates instantly, and the collapsing magnetic field induces a massive voltage spike—often exceeding 2,000V to 5,000V—across the open terminals. This will arc across terminal blocks, destroy the relay, and can be lethal to nearby personnel. Always use a shorting block or shorting switch before disconnecting an auxiliary CT.

2. Ignoring Polarity (H1/X1 Orientation): For simple overcurrent metering, polarity doesn't matter. But if you are using the auxiliary CT for directional overcurrent (67), differential protection (87), or summing ground fault, reversing the H1/X1 polarity will cause the relay to see 180-degree phase shift, resulting in nuisance tripping or a failure to trip during an internal fault. Always mark your wires and verify polarity with a phase-angle meter before energizing.