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I'm not sure if I understand the concept of grounding correctly.

Suppose I live in an apartment's 10th floor. Now the main panel is installed in the 1st floor of the building, and I want to setup a ground for the whole building. I put a copper rod into the soil at the 1st floor, with a wire connected to the main panel. From the main panel the ground wire goes up to my room on the 10th floor, and connected to the ground pin of the 3-hole socket on the wall.

My first question is: When I'm standing in my room on the 10th floor, is my body at the same voltage potential as the ground pin? I don't think this is the case, because the ground wire doesn't make contact with the floor I'm standing, it only makes contact with the true ground wire on the 1st floor. So probably the 10th floor is at 110V and the ground pin is at 0V, and when I touch the ground pin I get a shock. Is that possible? If so, why is grounding safe?

Suppose somehow the 10th floor is also at 0V with respect to the ground wire. Now I operate a heavy pump with the chassis connected to that ground pin, but somehow the pump leaks current to the chassis, for example the hot wire might brush against the chassis.

Now how it the grounding supposed to protect me? The chassis is still at mains voltage (220V), and my feet touching the floor is at 0V.

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    \$\begingroup\$ If live touches the grounded chassis, a breaker, a fuse or a residual current device should trip. That is the whole point about grounding a chassis. \$\endgroup\$ – Andy aka Sep 13 '14 at 9:45
  • \$\begingroup\$ So the point is that if the load leaks, it should stop working. What if my pump gives me electric shock during operation, should I discard it? That would be so wasteful. \$\endgroup\$ – seilgu Sep 13 '14 at 9:52
  • \$\begingroup\$ And what about operating a HV transformer and connecting the chassis to ground? I think it's supposed to make touching the chassis safe. \$\endgroup\$ – seilgu Sep 13 '14 at 9:54
  • \$\begingroup\$ Anything that gives you an electric shock, sounds potentially lethal and either the pump chassis is meant to be grounded or it needs throwing into the garbage or investigating further. \$\endgroup\$ – Andy aka Sep 13 '14 at 9:58
  • \$\begingroup\$ I think that's not the whole picture. Consider a high-voltage source with the HV lead brushing against the chassis resulting in a connection of several kOhms, the current is thus not enough to trip the fuse, but assuming your body has resistance of about 10kOhm, if you touch the chassis, the voltage across you is the HV voltage times 0.9, which is still lethal. \$\endgroup\$ – seilgu Sep 13 '14 at 12:35
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When I'm standing in my room on the 10th floor, is my body at the same voltage potential as the ground pin? I don't think this is the case, because the ground wire doesn't make contact with the floor I'm standing

A building has usually rather low ground resistance. Think of all the plumbing and other metal structures, for example.

So probably the 10th floor is at 110V and the ground pin is at 0V, and when I touch the ground pin I get a shock.

If the ground resistance is (unusually) high, you won't get a lethal shock, because there will be no current flow. Note that you may get a small shock from your own electrostatic discharge, but that is not dangerous.

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When speaking of safety the purpose of ground in buildings and installations is to allow protections to trip (circuit breakers, fuses, etc.) in all circumstances, thus cutting the supply in case of fault (a fault may be a short circuit or a leakage; the supply scheme, insulation level, and other factors influences decision on design of protection by tripping).

If there is a short circuit (failure of insulation), e.g. inside equipment, with 1 phase touching enclosure, the enclosure gets live: if not properly grounded, this loss of insulation will never be detected.

  • If it is a bolt short circuit (major loss of insulation), the overall resistance of the fault path is low, the enclosure is at the phase potential, a large short circuit shall flow through the earthing connection, and a protection will trip (fuse will blow, magnetic circuit breaker will trip, etc.).
  • If the insulation failure is a leakage may be dangerous or not, depending on the overall insulation resistance that is still there, and if the operator/user is insulated from ground: so we have the typical approach for household appliances that is to detect any dangerous leakage a trip in the same way (residual current detection, RCD, also called "zero sequence current relay"), protecting humans with a suitable threshold (e.g. 30 mA); for medical environment, with patient in already bad conditions, probably much more exposed, threshold is more restrictive (5 mA).

However, one may desire that the user is protected by making him/her float, with feet insulated from earth (e.g. 100 kohm of insulation resistance, insulation sized for more than the largest voltage that might appear): in that case no danger, because 220 V / 10 kohm = 2.2 mA, that we assume safe here (see IEC 60479). Also the human body has a resistance that is summed to the fault circuit (let's say about 1-2 kohm, but depends on which part is touching ... foot, hand, ... if the person is heavy or thin, if it is a man or a woman, sweating, etc.): see IEC 60479-1 "Effects of current on human beings and livestock" for info on resistance and on perceivable/dangerous thresholds of current, for both DC and AC.

When the system is floating, we have a distribution scheme that is called "IT": the reason is continuity of service, mainly, so after a first fault a warning lamp flashes, but power supply is still there and the situation is not dangerous. It is important when the function of the involved equipment is mission critical, e.g. a data elaboration center or equipment protecting people.

The situation of a large fault path resistance, enough to protect the operator may occur, but you cannot rely on it if it is just due to a combination of factors that might occur or not (e.g. it is wet or dry, the wire you say is touching gently or boldly, the surface is oxidized, there is some paint protecting the surface). For safety it is not acceptable, and the worst case shall always be considered: in this case the problem might be the opposite, i.e. the short circuit is too weak to be detected by the main circuit breaker. This is why the farther from the point of common coupling (where the utility line gets in), the higher the sensitivity of protection: combined with the time of intervention (tripping time) this is called selectivity. It's a trade-off between availability of the supply system and protection of people even for "small events": the IEC 60479 and other electrical safety standards (e.g. IEC 60364) establish thresholds (for current through the body or touch voltage at hands, feet, ...) considered safe, on which protection are tuned and that are confirmed by calculations with worst case scenarios (design).

Of course any earthing connection of an enclosure, chassis, metallic part, shall be sized for the estimated worst sort circuit, not to blow during the mishap: sizing is done with long time intervals during which the conductors heats up, but keeps its integrity. The long time interval is usually 1 second or the time of the second protection upstream, assuming the the first one nearest to you has failed its mission for some reason (again, see selectivity). Standards are BS 7430, IEEE Std. 80, IEC 60364.

[Already quite long. Please, if you need details, just ask. Cheers]

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To see the "fundamental rule of grounding" check this Circuit Cellar post. "Ground" in that case is the common 0 volt potential.

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  • \$\begingroup\$ Link only answers are not acceptable. The idea is to form a body of information, link only answer are often rendered useless once the link rots. Summarize the information, provide attribution and add material. \$\endgroup\$ – placeholder Sep 13 '14 at 16:06
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My first question is: When I'm standing in my room on the 10th floor, is my body at the same voltage potential as the ground pin?

This depends.

Note the "ground" conductor is really called "safety earth" or "protection earth" abbreviated "PE".

Say your wet hand rests on the stainless steel kitchen sink, which if it was installed according to local code here, must be properly earthed with a wire. In this case there is a very low impedance between yourself and the sink, and there is copper from the sink to the rod in the ground, so everything is at the same potential.

If you ask why the sink must be grounded: this is to make the RCD trip when some mains-powered kitchen implement is dropped into the sink, or the kids try to wash the toaster when it's plugged in, that kind of scenario. For the same reason, the metallic bathtub is earthed, also the faucets, etc.

On the opposite, if you are wearing sneakers (thick insulating rubber soles) and walk on carpet on a very dry day, your body's potential will be floating, and very insulated from PE. Thus you are one plate of a capacitor, the other side being whatever conductive material is around. You can build a static charge of several thousand volts, and then you get zapped when you touch something conductive.

This does no harm, because the capacitance is very low (some pF) so the amount of energy is extremely small. It feels like a pinprick. However if the conductive thingy you touch is part of some electronic equipment, the spark will find its way in and if no appropriate countermeasures were implemented, it will blow a random chip, which is why everything that is CE- or UL- approved must pass strict ESD tests...

So probably the 10th floor is at 110V and the ground pin is at 0V, and when I touch the ground pin I get a shock. Is that possible?

Well as mentioned above if you are insulated by your shoes you can build impressive static charge voltages on yourself, so you get zapped, but it is just a capacitive discharge. Once the capacitor is discharged, there is no more energy to zap you. 2pF charged to 2kV is a pinprick, but if you had a 2kV power supply capable of delivering sustained current, it would be lethal.

If so, why is grounding safe?

See andrea's answer, he gets my upvote for mentioning the RCD, which is what usually saves your life. Fuses and breakers are more about preventing fires when short-circuits occur.

By the way, do you have a RCD installed in your home, and do you check it still works by using the TEST button once in a while?

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