Current flows through a conductor connecting points at different potentials.
Leaving aside multi-phase details, common/conventional AC systems use a 3-wire setup:
Wire-1: a line/live/hot/phase wire presenting a point that oscillates between 2 potentials.
Wire-2: a neutral wire presenting a point of unknown/unspecified and varying potential, that nevertheless presents some fixed/specified potential difference to Wire-1 at least some of the time.
Wire-3: a ground/earth wire presenting a point at 0V potential difference to its immediate physical surroundings.
Wire-1 and Wire-2, in addition to some device that is to be powered, are used to construct a closed electrical circuit. Wire-3, leaving aside EMI/shielding concerns, is used to ensure that current will flow through it, rather than the device's user, if there ever occur any faults and the device's user comes in contact with Wire-1 or Wire-2.
Additionally to this however, Wire-2 and Wire-3 are at some point connected. This is done to ensure that Wire-2's potential remains close to that of Wire-3 .. which seems to be important for some reason.
Now the part I don't understand is why there needs to be a distinction between Wire-2 and Wire-3 at the power socket, if there is none a few meters further down the line.
I have tried to look this up, but all answers I could find so far seem incomplete. The answers depend on how the question is phrased:
If the question is phrased as "Why do we need Wire-3 in addition to Wire-2" the answer is because "Wire-2 may be at a substantial potential difference to its surroundings/user and thus endanger him/her if he/she ever comes in contact with it or Wire-1".
If the question is phrased as "Why do we need Wire-2 in addition to Wire-3" the answer is because "Wire-2 is needed to form a closed electrical circuit" or phrased somewhat differently "Wire-2 is needed to create a potential difference to Wire-1 and thus for current to flow" .. with the argument further being that when taking practical considerations into account Wire-3 can't provide a reliable/stable potential difference to Wire-1 like Wire-2 can.
This doesn't really answer why there's a need to differentiate between Wire-2/Wire-3 though, considering how
- Wire-3 remains Wire-3 and maintains 0V potential difference to its surroundings/user, regardless of whatever else happens around it .. since that is what it's supposed to do, or phrased differently, since that is the reason for Wire-3 being useful in the first place .. right?
and
- Wire-2 is connected to Wire-3
What am I missing here?
Why is it safe to touch Wire-3 but not Wire-2, or why can Wire-3 provide a level of protection that Wire-2 can't?
Why differentiate between Wire-2 and Wire-3 at the power socket but then connect them further down the line?
Answer
If wires were 100% reliable and had zero resistance, there would be no difference between the neutral (groundED conductor) and the safety ground (groundING conductor). Neither condition applies, however.
Even if the neutral-grounded and safety-grounding conductors are connected at the breaker panel, a current-drawing appliance some distance from the box may cause significant voltage drop in the neutral-grounded conductor. Having any exposed parts of the device connect to ground using a separate safety-grounding conductor will avoid the voltage on its end of the neutral wire from appearing on its exposed parts.
Additionally, using separate conductors ensures that a variety of single failures may occur without creating an immediately dangerous situation (though a second failure which occurs without the first having been corrected could be immediately dangerous).
If exposed parts of a device are connected to the safety-grounding conductor, and a hot wire within the device accidentally touches those parts, short-circuit currents should trip the breaker.
If the hot wire fails between the breaker panel and device, the device would get no power, but there would be no dangerous voltages anywhere near the device.
If the neutral-grounded wire fails, the neutral wire in the device may be only a few ohms separated from direct hot potential, but no current would flow through it, and no path would exist from it to anything the operator might touch. Exposed parts would still be safely connected to the safety-grounding conductor.
If the safety-grounding wire fails, the device would no longer be protected against the possibility of a hot wire touching the case, but no immediate danger would be created.
If the case were not connected to anything, failure #1 would create an immediate potentially-lethal situation; if it were connected to neutral, failure #3 would create an immediate potentially-lethal situation. With both wires present, however, a single failure will not create immediate danger.
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