Green safety wires do not form a reliable single-point ground reference.
Have you every turned on a light switch, only to see a bright flash just before the whole room plunges into darkness? You just tripped a circuit breaker. That failure scenario requires a light-bulb filament that has almost, but not quite, burned through.
When you apply power to the bulb for the last time a rapid wave of heat stress breaks the filament, sending fragments of broken filament flying around inside the bulb. Imagine that a short fragment lodges across the power terminals inside the bulb, shorting those terminals together [see box below]. In that shorted condition the bulb would draw far more current than its designed operating level. The bulb would shine blindingly white-hot in its last moment before the building circuit breaker pops, disconnecting power to the bulb.
Sound unlikely? Yes, but ask around--someone you know has probably seen something like this happen.
In the brief moment before the circuit breaker pops, the power-circuit load comprises three things in series: the hot wire leading to the bulb, the shorted bulb, and one neutral wire coming back (Figure 1). In the worst case, a voltmeter touching points A and B could read a voltage drop as high as 60V rms. Under those conditions, the wiring easily dissipates enough power to burst the wiring into flames and toast your whole building. The circuit breaker prevents that scenario.
In addition to circuit breakers, most civilized nations require the use of modern, three-pronged grounding outlets. In that application, only the hot and neutral wires carry power currents. The green safety wire, or “third wire,” merely connects the metallic chassis of each product to earth at the ac power entrance. Under ideal, no-fault conditions, the green safety wire carries no current. An inexperienced designer might therefore conclude that the green wires form a single-point-ground reference system that provides a consistent voltage reference between different ac-powered products. It does not serve that function (Figure 2).
The box on the right represents an old vending machine with a metal body. One day, a hot wire inside the machine accidentally breaks free. It touches the metallic chassis, creating a potentially dangerous situation. Immediately, the product's green safety wire conveys a large fault current back to the power source. That action is the sole function of the green wire. The fault current trips the circuit breaker, shutting off power and possibly saving the life of the next person who wants a candy bar.
As the fault current surges through the green wire, other products plugged into the green-wire system at adjacent outlets C and D could experience voltage differences as large as 60V rms. Even though the original fault lies within the vending machine, if the surge blows out your equipment, you inherit all the blame. It would be better if you design gear that can sustain such extraordinary voltages without damage.
Suitable architectures for inter-chassis data transfer that can easily sustain 60V rms without disruption include well-balanced, transformer-isolated standards, such as Ethernet; fiber-based optical links; free-space IR (infrared) optics; and RF transceivers. You should use interfaces such as RS-422 that lack large common-mode immunity only between equipment that permanently connects to a common outlet or power strip.
A better explanation...
The idea that a broken filament might draw a huge current sounds superficially correct, but over-simplifies the physics. The short circuit isn't really caused by a short fragment of the filmanet lodged across the power terminals. What happens is considerably more complex, and more beautiful.
As time passes, atoms of hot metal dislodge themselves from the filament and float away into the surrounding space inside the bulb. The filament becomes thinner and thinner. At some point, the filament breaks. In the instant after the filament breaks, a powerful electric arc forms between the two broken endpoints. The arc is carried by the mixture of gasses and vaporized metal within the bulb. The arc rapidly consumes the filament, burning each broken half back towards the point of attachment. As it burns, the arc expands until it bridges the complete gap between the power terminals inside the bulb. In that state the bulb draws far more current than its designed operating level. The arc may be initiated at the original point of separation or by a segment of the broken filament touching the opposite conductor as it falls out of position. Either way, the whole mechanism results in a completely destroyed filament. This event is called a "burnout arc".
The picture at left shows an electrical arc jumping the broken tungsten wire and emitting a brilliant light as a bulb fails (credit: James D. Hooker, Lighting Equipment News.) Note that the arc shines brightly in the upper portion of the bulb, well away from the original filament location. The original filament has long since burned away. This event lasts only a fraction of a second. The image was captured on film by intentionally blasting the filament with a laser, causing it to break, initiating the arc.
Quality bulbs contain an internal fuse designed to prevent circuit overload in the event of a burnout arc. To find the fuse, wrap the bulb in a heavy cloth, smack it open, and look near the base of the bulb. Sealed inside the glass stem you may see a short section of skinny, low-melting-temperature wire. That fuse prevents circuit overload once the arcing starts. If the fuse on your bulb is blown, there is a good chance the filament is missing because the bulb experienced a burnout arc upon failure.
Cheap bulbs omit the fuse. When they fail, the resulting burnout arc pops the local circuit breaker.