When sizing a relay current transformer setup for motor or feeder protection, the direct rule is to match the current transformer (CT) primary rating to 100-125% of the load's Full Load Amps (FLA), and ensure the protective relay's AC-3 contact rating exceeds the motor FLA. In industrial control, the term 'relay current transformers' refers to the paired ecosystem of external CTs stepping down high line current (e.g., 400A) to a standard 5A or 1A secondary signal to feed a protective relay, or integrated solid-state current-sensing relays with internal CTs.
Getting this wrong means either nuisance tripping from inrush currents or, worse, a welded contactor and a burned-out motor winding. This guide breaks down the exact specifications, wiring topologies, and testing procedures you need to commission these systems safely.
Spec Sheet: Relay and CT Rating Tables
Before pulling wire, you must verify the utilization category. Which rating column governs this load? If you are switching a resistive heater, the AC-1 column governs. If you are switching an inductive contactor coil, AC-15 governs. But for direct motor starting and stopping, the AC-3 column is your governing metric, as it accounts for the 6x-8x inrush current during motor starting.
| Component / Model | Coil / Supply Voltage | Current / Primary Rating | Contact Rating (AC-3 / AC-1) | Breaking Capacity |
|---|---|---|---|---|
| ABB CR-M024DC2 (Control Relay) | 24V DC | N/A (Signal Relay) | 10A @ 250V (AC-1) | 16A / 250V AC |
| Schneider TeSys LRD340 (Overload w/ CT) | 24-240V AC/DC (Electronic) | 30A - 40A (Adjustable) | NC: 6A / NO: 6A (AC-15) | N/A (Pilot duty only) |
| Eaton EOCR-300W (Current Sensing Relay) | 24-240V AC | 1A - 10A (Internal CT) | 8A @ 250V (AC-1) | 10A Resistive |
| ABB ME575 (Toroidal CT) | N/A (Passive Sensor) | 100A Primary / 5A Secondary | N/A | N/A (Burden: 5VA, Class 5P20) |
Notice the distinction between the CT (ABB ME575) and the relay (TeSys LRD340). The CT's job is purely measurement; its 'breaking capacity' is effectively zero. The relay's job is to isolate the fault by dropping out the main line contactor. Always check the CT's accuracy class: use Class 5P or 10P for protection relays (they saturate predictably at high fault currents), and Class 0.5 for metering (they saturate early to protect delicate metering instruments).
Wiring the Sensor Side vs. the Output Contact Side
A common bench mistake is treating the entire assembly as a single block. You must mentally and physically separate the sensor (coil/CT) side from the output (contact) side.
The Sensor and Coil Side (Input)
If you are using an external CT, wire the secondary terminals (usually labeled S1 and S2) directly to the protective relay's current input terminals using 14 AWG stranded wire. The total wire loop resistance plus the relay's internal impedance must not exceed the CT's rated burden (e.g., 5VA). Exceeding the burden causes saturation and blind spots in your protection curve.
Never open-circuit the secondary of an energized current transformer. If the relay is removed while the primary conductor is carrying load, the CT will attempt to drive its secondary current into an infinite impedance, stepping up the voltage to lethal levels (often >1,000V) and causing catastrophic insulation failure. Always install a shorting block or shorting switch on the CT secondary circuit before disconnecting the relay for maintenance.
If your setup includes a standard control relay (like the ABB CR-M) driven by a DC supply, you must wire a flyback diode (e.g., 1N4007) in reverse parallel across the relay coil terminals (cathode to positive). When the DC circuit opens, the collapsing magnetic field generates a high-voltage inductive spike that will fry your PLC output transistors or microcontroller GPIO pins if not clamped by the diode.
The Output Contact Side
The relay's output contacts (typically a Normally Closed 'NC' contact for overload fault strings) are wired in series with the main line contactor's coil circuit. Keep this wiring separate from your CT secondary wiring to prevent induced noise from the contactor coil's switching transients from corrupting the sensitive 5A CT signal.
Selection Decision Path by Load Type
Selecting the right relay and CT combination depends entirely on the load's electrical characteristics. A frequent and dangerous error is treating fuses, circuit breakers, and overload relays as interchangeable. They are not. Fuses and breakers are designed for short-circuit protection (clearing faults in milliseconds). Overload relays are designed for thermal protection, utilizing an inverse time-current curve that mimics the heating profile of the motor windings, allowing harmless inrush currents while tripping on sustained overloads.
| Load Type | Governing Rating | Protection Strategy & Component Choice |
|---|---|---|
| Resistive (Heaters, Lighting) | AC-1 | Current tracks voltage linearly. Use standard current-sensing relays (e.g., Eaton EOCR) set to trip at 105% of nominal load. No inrush accommodation needed. |
| Inductive (Solenoids, Contactor Coils) | AC-15 | High inrush, low holding current. Use control relays with high AC-15 breaking capacity. Ensure flyback suppression on DC coils to protect upstream switching. |
| Motor (Pumps, Compressors, Fans) | AC-3 | High locked-rotor inrush (600% FLA). Requires an overload relay with an inverse-time thermal curve (Class 10, 20, or 30 trip class). Size CT primary to 125% of motor FLA. |
| Capacitor Banks (Power Factor Correction) | AC-6b | Massive transient inrush (up to 100x nominal). Requires specialized capacitor switching contactors with pre-insertion resistors; standard CT/relay setups will nuisance trip. |
For motor loads, consult the NEMA standard classifications to determine the correct trip class. A Class 10 relay trips in 10 seconds at 600% FLA (ideal for fast-accelerating centrifugal pumps), while a Class 30 relay takes 30 seconds (required for high-inertia loads like large fans or rock crushers).
Testing Dead vs. Live and Repair vs. Replace
Commissioning and troubleshooting relay current transformers requires a strict sequence. Always verify the system state before applying test equipment.
Dead Testing (De-energized)
Lock out and tag out (LOTO) the main disconnect. Verify dead with a CAT III/IV rated multimeter.
- CT Winding Continuity: Set your multimeter to the lowest ohms range. Measure across S1 and S2 on the CT secondary. You should read a very low resistance (typically < 1 ohm). An open reading means the internal winding is broken; the CT is dead.
- Relay Contact Integrity: With the relay in its un-energized state, check continuity across the NC terminals. It should read < 0.5 ohms. Manually actuate the relay's test button (if equipped) and verify the NC circuit opens and the NO circuit closes.
- Insulation Check: Use a megohmmeter (set to 500V DC for 600V systems) to test from the CT primary busbar to the secondary S1 terminal. It should read > 100 Megohms. Anything lower indicates degrading insulation that will eventually cause a ground fault.
Live Testing (Energized)
Restore power and run the motor under normal load. Wear appropriate arc-flash PPE as dictated by your facility's hazard analysis.
- Secondary Current Verification: Clamp an AC ammeter around the single S1 wire running from the CT to the relay. If your CT is 100:5 and the motor is drawing 40A primary, you must read exactly 2.0A on the secondary. If you read 0A while the motor is running, your CT secondary is open-circuited (a critical hazard).
- Voltage Drop Across Contacts: Measure AC voltage across the relay's closed NC contacts while the circuit is running. A healthy contact will show < 50mV. If you read > 200mV, the contacts are pitted, carbon-tracked, or suffering from loose terminal torque.
When to Repair vs. Replace
Electromechanical and solid-state relays are generally sealed units. Replace the relay if the internal contacts are welded (fails to open during a fault test), if the housing shows thermal melting, or if the solid-state logic board throws a persistent internal error code. Repair is only acceptable for field-wiring issues: re-torquing loose terminal screws to the manufacturer's spec (e.g., 1.2 Nm for standard M3 screws), replacing a crushed CT secondary wire, or cleaning dust and conductive debris from the relay's exterior ventilation slots. Never attempt to file or sand pitted silver-alloy contacts inside a protective relay; the altered contact geometry will ruin the breaking capacity and lead to catastrophic failure during the next fault event. For comprehensive installation tolerances and torque specs, always defer to the manufacturer's official support documentation for your specific series.
