Earth Fault Protection using Core-Balance CT

Earth faults — current leaking from a phase conductor to earth through insulation failure, moisture or damaged cable — are a leading cause of electric shock, equipment burnout and electrical fires. Ordinary overcurrent protection (MCBs, fuses) cannot see small leakage currents: a 30 mA fault to earth will not trip a 16 A breaker, yet it can be lethal. This is the gap residual-current protection fills. A core-balance current transformer passes all phase conductors through one magnetic core; under healthy conditions the vector sum is zero, and any imbalance is the earth-fault current itself. Utility and industrial panels implement this with dedicated protection relays, but the principle is rarely demonstrated hands-on in college labs. This project closes that gap: a working prototype that senses residual current with a CBCT, processes it on an Arduino, and trips a relay/contactor when leakage crosses a set pickup — with the trip logic, burden sizing and calibration procedure all documented for the student's report and viva.

1. The phase and neutral conductors of the test load pass through the core-balance CT, which outputs the residual (vector-sum) current.
2. The CT secondary feeds a burden resistor sized for the CT ratio; the resulting AC voltage is rectified, filtered and scaled to the Arduino's 0–5 V ADC range.
3. The firmware samples the conditioned signal, computes the residual current and compares it against the configured pickup threshold.
4. When the residual current exceeds pickup for the programmed confirmation time, the Arduino energizes the trip relay.
5. The relay drops the contactor coil, disconnecting the load; the LCD switches to FAULT TRIPPED and a buzzer sounds.
6. The contactor remains open until the manual reset button is pressed, after which the system re-arms and the LCD returns to NORMAL.

Tech stack:

• Arduino Uno (ATmega328P)
• Toroidal core-balance current transformer
• Burden resistor + bridge rectifier + filter conditioning stage
• 4-channel electromechanical relay module
• Single-phase contactor as trip actuator
• 16x2 character LCD
• Arduino IDE (C/C++ firmware)
• Single-phase test load rig with simulated fault injection

• [Core-balance CT sensing] All conductors pass through a single toroidal CBCT; the vector sum of phase currents is zero in healthy conditions, and the residual output directly represents earth-leakage current.
• [Configurable pickup threshold] The trip pickup is adjustable via a potentiometer/software constant, so the student can demonstrate sensitive versus coarse protection settings during the demo.
• [Relay/contactor trip output] On pickup, the Arduino drives a relay that drops a contactor coil, visibly disconnecting the test load — the same trip-chain architecture used in real panels.
• [LCD fault status display] A 16x2 LCD shows live residual current and the NORMAL / FAULT TRIPPED state, giving the examiner a readable demo.
• [Burden and rectifier conditioning] The CT secondary is loaded with a sized burden resistor, rectified and filtered to a DC level the Arduino ADC can sample — the conditioning stage is fully documented.
• [Manual reset logic] After a trip, the contactor stays open until the operator presses reset, demonstrating lockout behavior; auto-reclose is intentionally not implemented.
• [Test push-button] A test button injects a simulated imbalance so the trip can be demonstrated on demand without creating a real fault.

• Working earth-fault protection prototype (Arduino, CBCT, relay, contactor, LCD, test load rig)
• Complete firmware source code (sensing, pickup logic, trip/reset, LCD)
• Circuit and wiring documentation with burden-resistor sizing notes
• Component list with ratings
• Pickup calibration procedure (buyer-run: set and verify trip levels)
• Project report PDF (protection background, CBCT theory, methodology, test procedure)
• PPT presentation for final review
• Viva Q&A preparation document (residual current, CT theory, relay coordination basics)
• Setup and demonstration guide

• This is an academic demonstration prototype, not a certified protection relay; it must not be installed on real building wiring.
• Trip pickup and timing are design settings verified by the buyer's own calibration procedure — the prototype ships uncalibrated and final values depend on the CT and burden used.
• Single-phase test rig only; a three-phase CBCT pass-through is a possible extension, not part of the base build.
• CT accuracy and linearity depend on the burden resistor value and the CT core quality; saturation at high fault currents is not characterized.
• The Arduino ADC resolution limits how finely small leakage steps can be resolved; this is a teaching limitation, not a measurement instrument.