Why do you have to use a separated conductor for grounding and for neutral?
General information
There are multiple reasons why modern electrical wiring is done so that there are separate wires for neutral and ground. Two most important reasons are the following:
1. Safety
When there are separate wires for neutral and ground it is much less likely that a problem in electrical wiring causes a dangerous situation which will cause electrical shock or fire.
If the ground and neutral were the same conductor, the cutting only the neutral wire (for example by accident) would cause the grounded metal case of the equipment to be on mains potential just because there is only live connected to equipment and that voltage can go through the equipment to the cut neutral cable and from there to equipment case.
When ground and neutral are separate, then cut neutral causes only the equipment to stop working and no dangerous situation. If ground gets cut by accident, there is no danger caused before some equipment gets damaged. So when there is separate wires for neutral and ground, a singe wire fault (cut or short circuit to other wire) on any wire going to outlet does not cause immediate danger to the user of the equipment:
Live shorted to ground or neutral: fuse blows immediatly. When fuse is blown then the dangers of electricity are gone.
Neutral shorted to ground: No immediate danger to use, just nasty ground loop problems start to occur. If there is GFCI or other ground leakage detector in wiring, it will cut the power to the outlet.
Live or neutral cut: Equipment just stops getting the power
Ground wire cut: The safety ground to equipment is lost.It continues to work nicely and there is no danger as long as there is no fault inside the equipment. Potentially as dangerous situation as using ungrounded outlet in same place, bur no immediate danger.
So any single failure does not cause great danger. So the for safety sense separate ground and neutral wires are a very good idea.
2. Minimizing the ground potential differences between outlet grounds
In an ideal reparate grounding wire system there is no current flowing in ground wire network, so there is no voltage difference between grounds on different outlets. Unfortunately in real life systems there is always some current leakign to ground, but that current is very small (only propably milliamperes) compared to the current flowing on line and neutral wires (usually amperes).
If the neutral and ground were shared on same wire, the current flowing on neutral wire would easily cause a large voltage difference (up to many volts) on different outlets grounds. The ground potential on any outlet will then depend on the load current, neutral wire resistance and the mains phase it is connected to.
Conclusion
For the conclusion we can say that separate ground and neutral conductors are a very good thing in providing good electrical wiring security and minimizing ground potential difference between different outlets. So it is no wonder why modern electrical wiring regulations call for this arrangement in many applictions.
If the neutral to ground difference a problem when separate wires ared ?
In large computers, neutral to ground voltages are originated because nonlinear currents flow through the neutral wire. In general always when there is some current flowing in neutral wire, there will be a ground ot neutral wire voltage difference on output end, because there is no current flowing on ground wire. In some computer systems the voltage differences are considered to be problem, because it has been reported to cause cause improper behaviour like "hangouts" on some computer systems.
It is true that in some special cases neutral to ground voltages can cause this kidn of problems, but in those cases it is not to blame the separate wires, because the problem lies in some other place. In properly built electrical wirign network, the neutral to ground voltage differences will never be more than few volts and any properly designed electrical equipment should not be disturbed of this kind of small voltage differences.
If the equipment gets disturbed of this small main frequency voltage, claim the manufacturer/designer of that particular equipment for doing bad job. Computer equipments can be easily deigned to cope few volts of voltage differences between neutral and ground. Most of the modern euquipments are designed to cope this.
If the voltage difference between your neutral and ground is much larger than few volts or there is very good propability that there is somethign wrong on you main wiring. So it is best that an electrician goes and checks your wiring before more problem and damages happen.
In some cases the neutral actual to ground voltage is not the problem, but the wavefor of the voltage. Non-linear loads like computer power supplies and light dimmers can cause some considerable noise between neutral and ground. Such noise can be more problematic than just a simple voltage difference, but equipment should still be able to cope quite fair amount of it (EMC regulations etc.). If you have considerable amounts of noise on neutral wire used by nonlinear loads, then you can again blaim the improper design of the electrical wiring. This kind of problems can be solved by good wiring design practices: separate electrical wiring from main panel/transformer to the equipments which cause noisen and to equipments which are sensitive to it. This is good practice and keep problems away.
If you would have on those cases had a wiring with common neutral and ground wire, you would have terrible ground potential diffeences between different outlets. In cases were ther problems neutral to ground voltage differences in separate ground and neutral wire wiring start cause problem, a wiring with common ground+neutral wire there would be so much ground potential differences between outlets that any equipments connected to different electrical outlets and interconnected would work very erratically, you would propably cause many equipment damages. Computer sustems are generally much more sensitive to the ground potential differences than ground to neutral differences.
The bottom line is that separate ground and neutral wire is a good thing for many problems. It solves more problems that what it potentially causes. And in wery rare situations where separate ground and neutral wires cause some problems, in those same situations a wiring with connon neutral+ground wire would have caused much more but different kind of problems in form of ground potential differences. When taking account situation where computers are interconnected/networked, a good stable ground potential for aal equipment is much more important than very small neutral to ground potential. Separate ground and neutral wires is a good method to keep the outlet grounds at same potential, which is very important in networked computer systems.
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