In an electrical installation an earthing system or grounding system connects specific parts of that installation with the Earth’s conductive surface for safety and functional purposes. This prevents users from the danger of contact with high voltages if electrical insulation fails. Earthing also limits the build-up of static electricity when handling electrostatic-sensitive devices.
This is a simple earth fault indicator to check if the mains wiring is correct or not. This tester can be used to check sockets before connecting appliances.
LED1 – 5mm green LED
LED2 – 5mm red LED
D1, D2 – 1N4007 rectifier diode
T1 – BC547 npn transistor
Resistors (1/4 watt, ±5% carbon, unless stated otherwise):
R1 – 33k ohm, 1W
R2, R3 – 470k ohm
R4 – 470 ohm
R5 – 1k ohm
CON1 – 3-pin connector for mains
Let us begin:
1(i) Connect T1’s Collector to LED’s -ve terminal. Connect LED1’s +ve terminal to R4.
1(ii) Connect R2 and R3 in parallel to T1’s Base.
1(iii) Connect T1’s Emitter and R3’s second terminal in parallel to TP0.
2(i) Connect R1, R5, D2’s n-terminal in parallel to R4’s other terminal.
2(ii) Connect R5’s other terminal to LED2’s +ve terminal.
2(iii) Connect D2’s p-terminal, LED2’s -ve terminal and CON1’s N-terminal to TP0 in parallel with T1’s Emitter and R3.
3(i) Connect R1’s other terminal to D1’s n-terminal. D1’s p-terminal and TP1 will be connected in parallel to L-terminal of CON1.
3(ii) Connect R2’s other terminal and TP2 in parallel to E-terminal of CON1.
Across TP0 and TP1 – Around 230V AC
Across TP0 and TP2 – 4V AC(depends on the load current and impedance of the neutral wire)
The red LED lights up if there is power in the socket and phase (L) and neutral (N) lines are connected correctly. The diode D2 is there to protect the green LED from damage when the polarity changes.
When the mains wiring is proper, a potential difference develops between the neutral (N) and earth (E) lines and transistor T1 turns on to light up the green LED (LED1). This indicates that the earth connection is perfect. At the same time, the red LED also glows, indicating that the phase (L) and neutral (N) lines are connected properly. In brief, if the phase, neutral and earth connections are proper, both the red and green LEDs light up. When the earth connection is broken, the red LED2 glows but green LED1 does not.
For the original project, go here.