There is a very high chance you have tried inserting a 3-pin plug into a socket. You must have noticed that the earth pin of the plug goes into the socket first, right? This happens because the earth pin is a little longer than the live and neutral pins. The pin isn’t just longer, it is bigger as well. Obviously, there are reasons for all that. To understand these reasons, you must first examine the essence of earthing and the earth pins.
Simply put, earthing provides safety and protection against leakage current from an appliance. Earthing is a technique of transferring leakage/discharge currents from an appliance to the earth/ground, via low-resistance materials such as copper. This technique reduces the chance of a human being getting electrocuted when they touch the metallic part of the appliance.
There are times when an appliance gets shorted and the metallic part of the appliance gets energized. When a human being touches that metallic part of the appliance, the human could get electrocuted. But, If the appliance is properly earthed, the short current would, most of the time, be transferred to the earth/ground instead.
You might be wondering why the current would prefer to flow through the earthing cable, right? Well, there is this ‘myth’ that electricity flows in the path of least resistance. Myth or not, there is an iota of truth in it. Since the earthing cable does have lower resistance than the human body, the current will flow through the earthing conductor to the ground.
Electricity always tries to find a path back to its source.
Key Points: Earthing helps reduce the chance of any human getting electrocuted from touching the metallic part of a plugged appliance. With proper earthing of a system (e.g. a house), leakage current from appliances tends to flow to the ground/earth rather than to the humans touching the appliances.
A 3-pin plug has the Live/phase pin, Neutral pin, and earth pin. The live and neutral pins have the same thickness and length. But, the earth pin is a little longer and thicker.
In a properly earthed system (earthing system of a building), when the 3-pin plug is connected to a 3-pinhole socket, the metallic part of the appliance gets a direct connection to the earth via the earth pin. When the metallic part of the appliance gets energised as a result of current leakages, the earth pin helps deliver the leakage current from the appliance to the ground. Thus, there is a reduced chance of a human being getting electrocuted from touching the metallic part of the appliance. So, the earth pin helps transmit leakage currents from the metallic part of an appliance to the earth.
Also, the earth pin serves as an opener for the live and neutral pinholes of sockets. Modern sockets have a safety shuttle in the earth pinhole. And the shuttle must be pushed down for the live and neutral pinholes of the socket to open. The earth pin helps push down the safety shuttle for the live and neutral pinholes to open.
Remember: The earth pin is useless if the building is not earthed.
Key Points: The earth pin helps connect appliances to the ground, and also pushes down the safety shuttle of modern sockets for the live and neutral pinholes of the socket to open.
Main Reason: The main reason why the earth pin of a 3-pin plug is larger is to prevent the insertion of the earth pin in the wrong pinhole of a socket. Just like the earth pin of a 3-pin plug is bigger than the rest of the pins, the earth pinhole of a socket is also bigger than the rest of the pinholes. Since the earth pin is bigger, it can’t be inserted into the live or neutral pinholes. This helps prevent the possibility of inserting the earth pin into the live or neutral pinhole of a socket.
Inserting the earth pin into the wrong pinhole can be injurious to humans. For example, if it were possible to insert the earth pin of a 3-pin plug into the live pinhole of a socket, electricity would flow to the metallic part of the appliance. It could even result in short circuits in the appliance. If you touch any metallic part of that appliance, you could get electrocuted. So, with the earth pin being bigger in diameter, there is no possibility of mistakenly inserting the earth pin in the hole of the socket where it doesn’t belong.
A common reason: Another reason the earth pin of a 3-pin plug is larger is that the higher diameter of the earth indicates a higher cross-sectional area pin. And a higher cross-sectional area of a conductor indicates higher conductance. Remember the law of resistance?
R = ρL/A
Where: ρ = Resistivity of conductor, R = Resistance of conductor, L = Length of conductor, and A = Cross-sectional area of conductor
From this law, the higher the cross-sectional area of a conductor, the lower the resistance of the conductor. Consequently, lower resistance means higher conductance. Current tends to flow on a path with more conductance. With the earth pin being very conductive, leakage current can easily flow through it to the ground.
Key Points: The earth pin of a 3-pin plug is thicker than the live and neutral pins because it helps prevent any possibility of mistakenly inserting the earth pin in the wrong pinhole of the socket. Also, the thickness of the earth pin indicates that any leakage current from an appliance would easily pass through the pin to the ground.
The earth pin of a 3-pin plug is a little bit longer than the other pins because it allows for the earth pin to connect to the socket first and also disconnect last. When the earth pin connects to the socket before the neutral and live pins, it allows the metallic part of the appliance to be connected to the earth first before the live terminal of the plug connects to the socket. Also, the earth pin disconnects from the socket last in order for the appliance to always be connected to the earth until the live pin is disconnected from the socket. These are all to ensure that electricity flows to the appliance only after the earth pin connects to the socket and that earth disconnects from the socket only after the live pin has been disconnected. Safety first, right?
Another reason the earth pin of a 3-pin plug is a little bit longer than the rest pins is that it helps easily push the safety shuttles of sockets, in order for the live and neutral pinholes of the socket to open. If the earth pin doesn’t push the safety shuttle first, the live and neutral pins won’t be able to connect to the socket.
Key Points: The earth pin of a 3-pin plug is a little longer than the live and neutral pins because it ensures that the earth pin connects to a socket first and disconnects last.