RCD Trip Time Tester

Residual Current Devices (RCDs) play a critical role in electrical safety by disconnecting power when leakage current to earth is detected. The speed at which an RCD trips is extremely important, as even small delays can result in serious electric shock.

This project demonstrates the design and working of a RCD Trip Time Tester, built without a microcontroller. The tester is capable of generating a controlled leakage current and measuring the time taken by the RCD to trip. This makes it suitable for educational, laboratory, and demonstration purposes.

Parts list:

IC NE555

IC 40110 (3pcs)

Bush button switch (2pcs)

ON-OFF switch

Optocoupler PC817

Transistor 2N2222

7-Segment display common cathode single digit (3pcs)

Bridge diode BD107

Capacitor 100uF, 100nF, 10nF

3V battery

Resistor 33K, 43K, 6.8K (2pcs), 2K (21pcs), 10K, 20K, 100K

Resistor 15K 2W

Resistor 7.5K 2W

Resistor 2.2K 2W

Resistor 1.5K 2W

IC Socket for 16 pin IC (3pcs)

IC Socket for 8 pin IC

Circuit Design

The circuit is designed to simulate an earth leakage fault and accurately measure the RCD response time. A resistor-based leakage current path is used to generate a controlled fault current when the test button is pressed. This action also triggers the timing mechanism, which starts counting at the exact moment the fault is introduced. When the RCD detects the imbalance and trips, the supply is interrupted, automatically stopping the timer. The recorded duration represents the RCD trip time.

Hardware Assembly

The circuit is assembled on a PCB to ensure mechanical stability and reliable electrical connections. All high-voltage sections are carefully insulated to prevent accidental contact and short circuits. LEDs and control switches are mounted on the front panel for easy operation and clear indication during testing. The completed circuit is enclosed inside a non-conductive enclosure to enhance user safety. All terminals and controls are clearly labeled to avoid connection errors during operation.

Safety Note:

This project operates at mains voltage. Proper precautions must be taken during assembly and testing.

Testing Procedure

1. Connect the tester to the RCD output terminals.
2. Switch ON the RCD.
3. Select the test current (if applicable).
4. Press the test button.
5. Observe the RCD trip.
6. Note the trip time displayed or indicated.
7. Reset the RCD before repeating the test.

Enclosure and Final Product

After completing the wiring and PCB assembly, the circuit is securely mounted inside a non-conductive enclosure. All external components such as switches, indicator LEDs, and terminal connections are aligned with the front panel openings and fixed firmly in place. The enclosure protects the internal circuitry from accidental contact and environmental damage while ensuring safe operation at mains voltage. Once closed, the final product forms a compact, portable RCD Trip Time Tester that is ready for practical testing and demonstration.

Expected Results

Under normal operating conditions, the RCD should remain ON without tripping, indicating that there is no current imbalance in the system. When a simulated leakage current is applied using the tester, the RCD should immediately detect the imbalance between phase and neutral currents and disconnect the supply. The measured trip time should fall within the standard safety limits, confirming that the RCD is functioning correctly and providing adequate protection against electric shock.

Test Condition : Expected Behavior

Normal supply : RCD remains ON, no trip occurs

Leakage applied : RCD detects fault and trips

Trip time : Within standard limits (typically less than 300ms for a 30mA RCD)