plugs

[madibug131]

why do plugs have three pins

[JudgeJ]

The electrical circuit needs two pins (one for the positive side of the circuit and one for the negative). The third pin is the earth connection. This is bonded to "earth" in your house (usually a water pipe) and is there to prevent devices becoming "live" (that is, incorrectly in contact with the elecrical supply) in the event of a fault.

[qapmoc]

Hi JudgeJ, you are correct except that under no circumstances should you connect an earth wire from anything to any water,  gas or whatever pipes. EXCEPT  that these if these are metal pipes they should have an earth wire connected (bonded) to them but this earth wire goes only to the main electrical earth point for the installation to ensure safety. There should also be a earth bonding connection to any metalwork in the house such as s/s sinks etc... etc....

This is to ensure that all metal parts in your installation are at the same potential as they are connected together, therefore you cant get a shock from one thing to another.  

[shaun1230_3]

hi all.

You are also right qapmoc except for the part about bonding stainless steel sinks. This method was abolished 5 years ago as it was deemed to create a different potential between the sink and the taps. The reason for this is that tap connections generally have a plastic insert so by bonding the sink the taps would not be earthed and thus if the taps were to become live and the person touched them whilst touching the sink there would be a different potential between poles and they would get a shock.

As an electrician myself i thought this would be of benefit to yourself.

[Eddiecurrent]

In the UK all mains plugs (bar a few exceptions) have three pins.
The top pin is the earth. It's primary function is to carry any current away to earth during a phase to earth fault. You will appreciate that having an electric appliance with a metal casing could be quite dangerous if a wire were to become detatched and touch the metal casing. Therefore all parts of the casing (metal) will be connected to this earth pin via the flex, if a wire were to come loose then it would be connected directly to earth and would allow a large current to pass through the fuse and blow it thus isolating the casing and the appliance from the mains. This would remove the possibility of anyone who touches it from receiving and electric shock, it also allows the earth potential (voltage)to rise thus prevent any potential difference (voltage difference) from being present across the person. If your hand is at 240 volts and your feet at 10 volts then you will have a potential difference of 230 volts across you allowing a large and potentially fatal current to pass through you. If the earth voltage (potential) rises at the same time as the metal casing you will not have a potential difference and there will be no danger ( hand at 240 volts and feet at 240 volts = 0 volts). This is also applicable if the earth's voltage outside the house rises (due to lightning strike for instance) then every appliance in the house will also rise to that voltage and again there will be no voltage difference. If you were to touch an appliance you would not notice anything
Where an appliance is 'Double insulated' there is no requirement for the earth pin to be connected as there is no exposed metal.This is usually done by the use of plastic casings. The earth pin is also used by certain socket manufacturers to open the shutters of the socket.