# Earth Fault Indicator. Design Note 17

This is my circuit published in Electronics For You Magazine. I have designed this circuit for checking the Earth connection in the power sockets ,whether it is intact or not. Earth connection is a must in power sockets especially if the gadget connecting has a metal body. If there is any leakage from live wire, the earth connection will bleed the current to earth and protect us from shock. So this circuit can be used to check the power sockets before using gadgets like Heater, Electric Iron etc. It also indicates whether there is power in the socket and the polarity of Phase and Neutral is correct.

As you know, the three pin power sockets have three pins- Line, Neutral and Earth. The Earth pin of Plug is large and long so that, it first enters into the socket and removes any leaking current. This pin is also large in size to handle high current. The circuit uses the potential difference across the Neutral and Earth lines. 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 % (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.

How it works

Simply plug the circuit in a power socket. If the Phase and Neutral are OK, a Red LED lights. If the Earth line is intact, a Green LED lights. So if all the lines-Phase, Neutral and Earth- lines are perfect, both Red and Green LEDs lights. If the Earth is broken, Green LED remains off but Red LED lights.

Image Credit : Electronics For You

The circuit need not require a battery or other power supply. It takes current from the socket itself. A diode (D1) and a resistor (R1) drop the 230Volt AC and converts to a low volt DC for the circuit. The base of Transistor T1 is arranged to connect with the Earth pin of socket, when the unit is plugged in. When the mains wiring is proper, a potential difference develops between the Neutral and Earth lines and transistor T1 turns on to light up the green LED (LED1). This indicates that the Earth connection is perfect. The red LED (LED2) lights up if there is power in the socket and Phase and Neutral lines are connected correctly.

Caution: Construct this circuit only if you are an experienced person working with AC circuits. The circuit is at mains lethal potential and gives fatal shock, if handled carelessly. Do not attempt to test or troubleshoot when the circuit is connected to mains.