In mains-powered equipment, exposed metal parts are connected to earth
wire in order to prevent users from contact with high voltages if electrical
insulation fails. Connections to ground through earth connection also limit the
build-up of static electricity when handling electrostatic-sensitive devices.
Earth in a mains electrical wiring system is a conductor that provides a
low-impedance path to the earth to prevent hazardous voltages from appearing on
equipment and hence the name.
Here is a simple tester to find whether the mains wiring is correct or not.
This tester can be used to check sockets periodically before connecting
appliances like heater and electric iron. It indicates the status of the mains
wiring through two LEDs
CIRCUIT DIAGRAM
Circuit and working: The circuit
is built around a BC547 transistor (T1), two LEDs (LED1and LED2), two 1N4007
diodes (D1 and D2) and five resistors (R1 through R5).The circuit takes advantage of the voltage that appears across the earth
and neutral terminals. Neutral-to-earth voltage as measured at the load for a
single-phase circuit is a function of the load current and the impedance of the
neutral wire. Various standards limit this voltage drop in a branch circuit to
3 per cent (5 per cent total for feeder and branch circuit) for a reasonable
efficiency of operation. Based on this, the neutral-to-earth voltage limit for
a single-phase 120V AC circuit is 3.6V AC and for a single-phase 230V AC
circuit 6.6V AC.
There is no additional power supply used to operate this circuit. The circuit
is directly powered from the 230V AC mains supply. The combination of diode D1
and resistor R1 reduces the 230V AC mains to a low voltage for the circuit. A
transistor switch is provided to light up the green LED (LED1) if earth is
correctly connected. The base of T1 is connected to earth pin of the mains
supply through a network of resistors R2 and R3.
Working of the circuit is simple. The red LED (LED2) lights up if there
is power in the socket and phase (L) and neutral (N) lines are connected
correctly. Diode D2 protects the green LED (LED1) from damage when the polarity
changes. Resistors R4 and R5 limit current through LED1 and LED2, respectively.
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.
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.
Construction and testing: An
actual-size, single-side PCB for the earth fault indicator and its component
layout. After assembling the circuit on a PCB, enclose it in a suitable plastic
case. The circuit can be housed in an adaptor box with three pins.To test the
circuit for proper functioning, check input supply at TP1 with respect to TP0.
Also check the voltage difference across neutral and earth pins as per the test
point table.
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