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All the neutral and ground (or "earth") wires in a building are tied (or "connected") together at the incoming service main breaker panel - and that is the only place they should ever be tied together - because it is "upstream" of all the fuses and/or circuit breakers which are there to protect the hot (or "live") wires for the various circuits installed in the building.

In the absence of an earth wire (= ground wire in US/Canadian English), if the appliance suffered some damage that caused a short circuit between the high voltage "hot" lead and the case of the appliance, the damage would make the case live and it would cause an electrical shock to anyone who touched it.

If the case is earthed by using a ground wire (= earth wire in British English), if that same damage occurred the hot lead would immediately be shorted to ground and in theory cause the fuse to blow or circuit breaker to open, thus eliminating the danger of a live case.

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In the USA I think we refer to what you are calling an "earth wire," as a GROUNDING CONDUCTOR, which in effect is a separate conductor which seems to be doing nothing but is in fact a protective wire. It is there, ready to take the current away to earth if it, or the body/frame of the electrical device it is connected-to, makes contact with any "hot" wire. [120 Volts mains power is carried in two current carrying conductors "hot" and "neutral".]

So, IF one of the "hot" conductors should contact the metal frame or housing - perhaps because the appliance got damaged by being dropped from a table, or similar accident - the third wire which is the "ground" or "earth" wire, which runs directly from the housing to the grounding [or earth bus] in the fuse or breaker panel, will in effect cause a short circuit which should blow the fuse or trip the breaker.

This third wire also guarantees a current path back to the load center where the fuse or circuit breaker protecting that circuit is located, in the event the hot wire should be in contact with the frame, but the other [neutral/return] conductor should happen to be cut, disconnected, or open.

The idea is to guarantee that if a part of the device should become "hot," which could be fatal to anyone who then came into contact with it, would trip/open the circuit protection device [fuse or breaker], turning off the flow of current to that circuit.

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All the neutral and ground (or "earth") wires in a building are tied or linked together at the incoming service main breaker panel. This is the only place they should ever be tied together because it is "upstream" of all the fuses and/or circuit breakers protecting the hot (or "live") wires for the various circuits installed in the building.

Warning: we must never assume that a neutral is safe to touch: it has to be checked with a voltmeter or a voltage indicator to be sure it is not "live". This is because a neutral wire is designed to carry current under normal circumstances.

So, if a neutral wire going back to the incoming main breaker panel has not been properly connected - or suffers a deliberate disconnection or some accidental damage which causes it to break - then it and any neutral wires connected to it further downstream will go live up to the break because of being connected to the downstream loads which still have hot feeds coming into them!

That is why we should never use a neutral as a substitute for a proper, separate, ground or "earth" wire.

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In addition to the above description it should also be mentioned that the presence of an earth wire allows a very sensitive safety device called a Ground Fault Circuit Interrupter (GFCI) to operate. It will cut off the power supply to the appliance even if a tiny current of a few thousandths of an Amp is detected flowing in the earth wire, which should normally carry no current at all.

In USA/Canada, and similar countries which use mains supplies running at 120 Volts 60 Hz to feed power sockets, it is probable that the latest designs of GFCIs which are fitted to all new wiring work actually operate in the same way as the RCDs described in the next paragraph, although they are still commonly called GFCIs.

In Europe, where 230 Volts, 50Hz mains supplies are standard in homes, offices, etc. such protective "trip" devices are called Residual Current Detectors (RCDs) because, in addition to being able to detect small earth leakage currents, they have the ability to detect very small differences between the currents flowing in the hot (or live) wire and the neutral wire. Such imbalances might be caused by minor damage to the appliance which allows a small current - known as a residual current - to leak to earth either via the user or via the earth wire (if one is connected) even though the appliance itself is still working. So using an RCD helps to prevent a serious shock hazard to users if ever the kind of minor damage occurs which, in the absence of an RCD, would cause the appliance's casing to become "hot" or live.

Thus RCDs give a very high level of safe operation even if no circuit breaker has tripped and no fuse has blown and the appliance appears to be working normally. (But it has really become unsafe!) Because of the enhanced protection they give to users of appliances the latest European wiring regulations (= wiring codes in US/Canada and elsewhere) make it compulsory to fit RCDs to all new power circuits.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.
For more information please see the answers to the Related questions and the Related link shown below.
Earth or ground at a device provides shock protection if the hot wire were to come in contact with a metal part that you could touch. At your main panel earth and neutral are bonded together and connected to a ground rod.

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12y ago
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15y ago

In general, the ground wire is not there to protect the appliance, it is there to protect YOU from electrocution. If there is a wiring fault inside the appliance, and a live wire touches a metal piece, it would become live. If you touch the metal, you would be shocked. The ground wire conducts the current to ground, tripping the circuit breaker and protecting you. Some small appliances, like hair dryers and electric hand drills, are designed so no metal parts are exposed that could become live. These do not need (or have) a ground wire. The nameplate on appliances that do not need a ground will say "double insulated". Don't worry about the appliance - worry about you! If an appliance is designed to have a ground, be sure it is there and intact. Never remove the ground prong on a 3-wire cord. It could save your life!

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15y ago

the power co provides protection for both. the earth wire, better known in this part of country as ground wire is driven 8' into the ground and connected to service from power company. the fuse is placed in line with service and connected to appliance thru wiring device. the fuse is rated to protect appliance generally 20 to 60 amps in line with 200 amp service.

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12y ago

This question means a lot to me because I was very nearly killed one time by an electric drill with a plug from which someone had cut off the grounding pin.

In most electric tools and appliances the "hot" wire or wires and the return are isolated from the metal outer parts. The case (body, chassis) are connected to the separate earth or "ground" connection. That way, if a live wire inside somehow becomes shorted to the case then the current flows back to the source through this alternate path. This usually causes such a rush of current that it blows the fuse or trips the circuit breaker that is in the circuit, cutting off the power entirely.

In my case, the electric drill had a "short circuit" to the case but since there was no grounding pin, the case became fully at the 120VAC (USA) potential. When I picked up the tool I felt nothing, probably because my shoes and the floor were not conductive, but when I touched a grounded object with my other hand, I was very severely shocked. So, DO NOT ever cut off those grounding pins!

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13y ago

If there happens to be a fault on that appliance the ground wire is the faults path back to ground.

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Q: If an appliance is metal how does the earth wire and fuse protect both the appliance and user?
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