How does grounding prevent shock?

Hello, everyone.

I am reading the material on this website, and have a question regarding grounding, discussed here:

http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

I understand that the purpose of the grounding point is to make any person standing on the ground electrically common with earth ground, and that there is no voltage between electrically common points.

What I fail to understand is the specifics of how this helps.

The book gives scenarios to illustrate the potential danger in not grounding a circuit: if a tree touches the wire, and creates a ground path, then when a person standing on the ground also touches the wire, the circuit is complete, and the person will be shocked.

The second of these scenarios involves a case where there are two people, each standing on the ground. One touches the high wire and one touches the low wire. A tree touching the high wire completes the circuit for the first person, through a ground path. But the second person is also shocked, since the two people are both touching the ground.

What I fail to understand is how adding a ground wire would solve the problem! In all of the danger scenarios that the book depicts, I don't see how adding the grounding wire would prevent the shock.

I thus fail to understand how in any scenario, a person would receive a shock in the absence of a grounding point, but due to the grounding, the person does not receive a shock.

http://www.word-detective.com/howcome/birdsonwires.html

How come a bird on a wire doesn't get shocked? When the bird perches on a live wire, her body becomes charged--for the moment, it's at the same voltage as the wire. But no current flows into her body. A body is a poor conductor compared to copper wire, so there's no reason for electrons to take a detour through the bird. More importantly, electrons current flow from a region of high voltage to one of low voltage. The drifting current, in effect, ignores the bird.
But if a bird (or a power line worker) accidentally touches an electrical "ground" while in contact with the high-voltage wire, she completes an electrical circuit. A ground is a region of approximately zero voltage. The earth, and anything touching it that can conduct current, is the ground.
Like water flowing over a dam into a river, current surges through the bird (or person's) body on its way into the ground. Severe injury or death by electrocution is the result.
That's why a squirrel can run across an electrical line, but sadly die when its foot makes contact with the (grounded) transformer on the pole at wire's end.
It's also why drivers and passengers are warned to stay inside the car if it runs into a downed power line. Touching the ground with your foot would complete the circuit: Electrons would flow from the wire, into the car, and through you on their way into the earth. (Inside the car you are usually protected by the car's four rubber tires, which act as insulators between car and ground.)
Likewise, birds can get in trouble with power lines if wing or wrist bones--or wet feathers--connect bare wires and grounds.

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Ground therefor should complete the circuit and kill you?

superconductor said:

What I fail to understand is how adding a ground wire would solve the problem! In all of the danger scenarios that the book depicts, I don't see how adding the grounding wire would prevent the shock.

I thus fail to understand how in any scenario, a person would receive a shock in the absence of a grounding point, but due to the grounding, the person does not receive a shock.

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The original scenarios without gounding lead to treating the wires as "safe to touch" and the examples show how that can go wrong. The system would be virtually allways grounded somewhere by something, but you would think it is safe.
Adding a grounding wire leads to the general idea "touching electrical wires is very dangerous", or at least that is the way i see this solution.

The other part of grounding is that whenever a hot wire touches the grounded metal case it creates a short and blows the fuse, so there is no possibility of getting shocked by touching a presumably safe metal part of an appliance.

cjdelphi said:

Ground therefor should complete the circuit and kill you?

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Yes, but it also makes you try your best not to touch the wires.

The concept of the grounded metal appliance case is important. Kubreek has the right of it, with no grounding or other safety then the chance the circuit will accidentally ground where you don't expect it is high. With the current system the breakers or fuses take the brunt, and you know there is a problem to be fixed.

The current system is a result of some trial and error. The third ground wire is a relatively new concept, it wasn't around during the 60's. I suspect when electricity was brand new there wasn't a hot or a neutral either, but experience has taught it is better to have known dangerous wire then hope both are safe (and find out you were wrong).

It is a little like aluminum wiring. Aluminum is cheaper than copper, so there was a period where houses were wired with aluminum wire. Quite a few house fires later the practice was abandoned, and when people find aluminum as wiring they are encouraged to upgrade ASAP.

Thanks to all posters.

After reading the responses so far, I'm not sure whether my question has been answered.

The book asks at the beginning of the section: "Wouldn't a ground-less circuit be safer?"

So far, it really seems that a ground-less circuit would be safer. That way, at least someone in common with the ground touching the high part of the wire would be safe! So the ground wire seems to do more harm than good.

Apparently, I'm missing something, since the book attempted to refute exactly this thinking, but, again, I don't see how.

Thanks to kubeek for his scenario, but from reading the book, I'm not sure that's what the book had in mind for the scenario that the ground wire is supposed to protect us from.

I encourage other posters to read the scenarios discussed in the book, and see if I have missed something explained there.

I once had an electric can opener that, after several years of use, gave me a mild shock when I touched its metal case with one hand and something that was grounded with the other. The can opener case was not grounded (it had a 2 wire cord). I unplugged it and opened it up to see what was going on. I don't remember if it was double insulated (when did they start doing this?), but it turned out that the motor had carbon brushes, and carbon dust had built up a resistive path from one of the brushes to the case.
If the case had been grounded using a 3 wire plug, power would have been wasted, but I would not have gotten shocked, because the voltage would have been too low between the case and the ground I had touched to even be able to feel a tingle.

I drew some pictures of the book scenarios as I think superconductor elaborated ....

I don't see how the first scenario definitely electrocutes the person. This may be because I didn't understand the description. There is no obvious return path as described in superconductor's post #1. Perhaps what was meant was that the 'high' wire touches the tree and the person touches the 'low' wire - in which case the person would be electrocuted. If the supply 'low' wire was grounded at the source, then in that case the person would be safer and probably survive - depending on the ground return resistance.

The second scenario appears to permit electrocution of both parties - although I think the tree is redundant and might possibly save the 1st person since the tree and first person are in parallel .... and which has the lower resistance - man or tree? Since I've made the supply 10kV I have probably rather callously ensured the person will die anyway. In this case it's the 2nd person who completes the circuit by providing a path from the low wire to ground. This ensures the 1st person and the tree both then have a return path.

If you ground the low side (as a third scenario) then for scenario B the 2nd man probably survives as long as the ground resistance isn't too high back to the supply ground point. A ground at the supply in the case of scenario A [per my drawing] will now increase the risk of electrocution for the single person touching the 'live' [hot] wire. In this latter case the tree would also be in parallel with the person so while it's still likely the person will die it's not absolutely certain.

The point of all this is that coming into contact with an unprotected live wire in a grounded system does indeed increase the risk of electrocution. That's why the RCD was invented.

In reality the earthing system simply makes it easier to protect connected equipment, cables and overall supply continuity by ensuring a sufficiently high fault current flows that will cause a fuse or circuit breaker to interrupt the supply as near as possible to the fault location.

superconductor said:

So far, it really seems that a ground-less circuit would be safer. That way, at least someone in common with the ground touching the high part of the wire would be safe! So the ground wire seems to do more harm than good.

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No, it wouldn't be safer. There will ALLWAYS be something like in this picture, could be a tree, another guy tinkering with the wires, or some mis-wired appliance that completes the ground path.
Then you then come to it, thinking that the system is ungrounded so nothing bad can happen and start touching the wires, which will 99.9% of time result in an electric shock.

That is way less safe than coming to it and thinking you will DEFINITELY get a shock if you touch it because the system is grounded.

I suspect, also, that differential breakers weren't very common at that time, so if a device malfunctioned and sent a wire to the metal case, you would want the fuse to blow, instead of the device to have an undetected fault forever.

What would happend if you had a faulty hair dryer with the metal chassis connected to netral, and your house had a bare live wire touching the metal tubes? This doesn't needs to happend at the same time, it could take years for every fault to develop, but all your house could have faulty devices everywhere after years. Maybe you die by touching the TV and the Heater at the same time, or the wall. Not good.

kubeek said:

No, it wouldn't be safer. There will ALLWAYS be something like in this picture, could be a tree, another guy tinkering with the wires, or some mis-wired appliance that completes the ground path.
Then you then come to it, thinking that the system is ungrounded so nothing bad can happen and start touching the wires, which will 99.9% of time result in an electric shock.

That is way less safe than coming to it and thinking you will DEFINITELY get a shock if you touch it because the system is grounded.

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Maybe so. But the main issue is that I'm looking for a scenario where in the non-grounded state, the person would receive a shock, and in the grounded state, the person would not.

You're saying that in the scenario you're presenting, the person would be shocked in either case, and the only differences is that in the grounded case, they might act more carefully. I'm looking for a case where the person would actually be saved from a shock, not due to extra careful behavior on their part, but rather due to the protection offered by the grounded state.

I've been re-reading the next chapter of the book, and I'm starting to think that I have indeed missed something in my thinking, that, indeed there are scenarios where grounding does prevent shock, but unfortunately, the explanation eludes me.

Move into a structure and consider wiring faults in appliances. With a grounded case, the fault results in the associated breaker tripping, removing the lethal voltage. Even if you are in contact with the case at the moment of the fault, the case remains at ground potential. There is no voltage difference between you and the case, so no shock is felt.

The purpose of grounding equipment is to remove the possibility of that fatal potential difference from giving you a lethal shock. That is why exposed wiring must be considered dangerous unless verified to be safe with a meter.

superconductor said:

I'm looking for a case where the person would actually be saved from a shock, not due to extra careful behavior on their part, but rather due to the protection offered by the grounded state.

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In your original post the second scenario puts both people at risk of electrocution. If the "low" (neutral?) wire was grounded at the electricity service connection point - as in the MEN system - the 2nd person touching the "low" wire would not be at risk of electrocution. Alas, the other person touching the "high" (active?) wire will still be at risk. In an effort to protect people in the latter situation, many improved electrical installations include RCD protection - particularly for GPO (wall socket) outlets.

superconductor said:

Maybe so. But the main issue is that I'm looking for a scenario where in the non-grounded state, the person would receive a shock, and in the grounded state, the person would not.

You're saying that in the scenario you're presenting, the person would be shocked in either case, and the only differences is that in the grounded case, they might act more carefully. I'm looking for a case where the person would actually be saved from a shock, not due to extra careful behavior on their part, but rather due to the protection offered by the grounded state.

I've been re-reading the next chapter of the book, and I'm starting to think that I have indeed missed something in my thinking, that, indeed there are scenarios where grounding does prevent shock, but unfortunately, the explanation eludes me.

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Did you read my post on the previous page?

I've been re-reading the next chapter of the book, and I'm starting to think that I have indeed missed something in my thinking, that, indeed there are scenarios where grounding does prevent shock, but unfortunately, the explanation eludes me.

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Yes I'm not suprised, the grounding section of the book is a bit simplistic.

Been There has it in a nutshell as usual:

The purpose of grounding equipment is to remove the possibility of that fatal potential difference from giving you a lethal shock. That is why exposed wiring must be considered dangerous unless verified to be safe with a meter.

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Regulations now require that all terminals, wires, connections etc at shockable voltages are covered.

But electrical apparatus often has exposed metal parts, that are not normally connected to the electricity (eg the side of your cooker).

It is earthed so that if a fault condition develops so that this metal part becomes connected to a shockable voltage supply the supply will be diverted straight to earth with a large enough current to operate the supply disconnection device (fuse or breaker), thereby removing the threat.

The metal housings of electric fires were once particularly suceptible to this issue.

As the regards the difference between humans and birds touching live wires, most humans can't float several feet above ground. Those that can are as safe as the birds standing on the power lines.

studiot said:

It is earthed so that if a fault condition develops so that this metal part becomes connected to a shockable voltage supply the supply will be diverted straight to earth with a large enough current to operate the supply disconnection device (fuse or breaker), thereby removing the threat.

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Also, if a resistive connection between the electricity and the case develops, lethal current could flow from the case to a grounded human. Minimum lethal current levels are way below the threshold of circuit breakers and fuses (but not GFCIs). Grounding the case will reduce the voltage on it to a level which cannot force lethal current through the normal skin resistance of a human.

The following link might assist you:



http://www.thewaycorp.com/power/PwrM-ground NEC.pdf