Why is it necessary to earth metallic electric appliances?

Answer 1

You connect the housing of metallic electric appliances to earth ground so that, if an internal fault, such as a wire breaking loose and touching the housing, were to occur, then the housing would not become an electrocution hazard. This connection to earth ground is also accompanied with the neutral return being connected to earth ground at the distribution panel so that this internal fault causes high current in the circuit, with the result that the protective device (fuse or circuit breaker) trips.

Answer 2

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 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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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.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

If you do this work yourself, always turn off the power

at the breaker box/fuse panel BEFORE you attempt to do any work AND

always use an electrician's test meter having metal-tipped probes

(not a simple proximity voltage indicator)

to insure the circuit is, in fact, de-energized.

Answer 3

The ground wire from the appliance is connected to earth ground at the main electric panel through the cable that connects the outlet to the panel. There should be a grounding rod outside connected to ground in main panel.

Answer 4

If the metallic frame of an appliance was not earthed (grounded), should accidentally come into contact with the line (live) conductor, then that frame will assume a potential of 230 V (120 V in North America) with respect to earth -thus presenting a shock danger to anyone making contact with the frame.

By earthing the frame, the instant a line conductor makes accidental contact with the frame, a low-impedance earth fault exists, which will trip the protective device (fuse or circuit breaker) supplying the appliance, thus disconnecting the frame from the supply.

So the primary purpose of earthing an appliance's frame is to protect the user.

Answer 5

The case is attached to the earth wire in the cable. If the case becomes live electricity will flow to the earth via the earth wire and not through a person touching it

Answer 6

It doesn't need to be in order to function. The requirement is one of safety for people ... so that its less likely that you'll contact two appliances at once and shock yourself.