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Some say Ground is just a reference point for measuring voltages, some say ground is a safety device for appliances and some say ground is just a bare piece of metal regardless if its even connected to the actual earth (as in dirt)

I have multiple questions about ground:

This one is about appliances and is taken from howstuffworks, "Let's say that a hot wire comes loose inside an ungrounded metal case,the metal case then becomes hot so anyone that would touch the case would be fatally shocked. By having a ground wire attached to the metal case, electricity from the hot case will flow straight to ground and will trip the breaker"

  1. Can you not be safe just by touching a hot wire alone, granted you don't touch neutral?
  2. What if you touch a hot wire then, stick you other finger in the dirt (soil)?
  3. So its telling me that the current flow from a hot wire, to the grounding wire-> (connect to some rocks and dirt/poop) is so high, it can trip a breaker?
  4. In a DC situation there is no hot wire, and (correct me if i'm wrong) but if I have a 350v Capacitor fully charged, there is absolutely no way for me to get electrocuted unless I touch both terminals. Why doesn't AC behave this way?

And how about this:how to jump a car people say step 4 is to ground that wire, How? The car is insulated from the earth All I see is an open circuit.

Please help, Thanks!

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  • \$\begingroup\$ Ground is pretty confusing. One thing I want to point out is that the amount of current necessary to cause injury or death is much smaller than the amount necessary to trip a circuit breaker. Also, I want to point out that in AC systems in houses, there is a connection from the neutral wire to the earth. This would normally be made near where the wires come into your house. There may actually be a spike hammered into the earth with a bare or green wire connecting the spike to neutral. This is how it is in the US, and I imagine details are slightly different in other countries. \$\endgroup\$ – mkeith Oct 14 '16 at 5:02
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The reason this is confusing is because the ideas are confused, mixed up in the word "ground".

Sometimes “earth(ing)” is used to refer to the concept of electrically connecting to the earth, allowing “ground” to be used for the concept of “a part of the circuit we consider to be 0 volts”.

Some say Ground is just a reference point for measuring voltages

That's one of the meanings. You have some device, some circuit, and say that this conductor is 0 V and measure everything else relative to it.

If the device is isolated (battery powered and not hooked up to anything else), then it's truly arbitrary, but if you have connections, signal or power cables, to something else — power supply, digital signal, analog audio signal, whatever — then the expected electrical characteristics are generally such that the two devices share the a common 0 V reference — a common ground.

(If there's a problem so that one of the devices is causing current to flow along different parts of the system that should be the same “ground”, that's called a ground loop.)

some say ground is a safety device for appliances

The safety device here is the presence of a connection between the chassis of the device (generally, any exposed metallic parts) and the “ground” conductor of the electric circuit, which is also joined to earth. The safety comes from the idea that if somehow the chassis gets joined to line voltage (“hot”) and current will flow to the “ground” (and perhaps trip a breaker or GFCI/RCD device) instead of through you when you touch it (as you are a higher-resistance connection to earth).

some say ground is just a bare piece of metal regardless if its even connected to the actual earth

A bare pice of metal is never “a ground” by itself. It's just common, for safety reasons and for electrical design convenience, to design things so that all exposed metal is joined to, or a part of, the circuit's “ground”.

Can you not be safe just by touching a hot wire alone, granted you don't touch neutral?

If you jump in the air and poke the hot wire, you're certainly safe. If you're touching anything else, like what you're standing on, you might be able to make a circuit.

What if you touch a hot wire then, stick you other finger in the dirt (soil)?

So its telling me that the current flow from a hot wire, to the grounding wire-> (connect to some rocks and dirt/poop) is so high, it can trip a breaker?

Depends on the kind of dirt, and how wet it is. If it's sufficiently conductive, the circuit consists of the incoming electric service (hot branch) → house wiring → you → dirt → service ground connection at the breaker panel → service neutral. (That is if your wiring is like USA wiring, at least.)

But safety grounding isn't just about “dirt” — think about the kitchen. You've got electric appliances, salty (conductive) water involved in cooking, possible spills and/or damaged wiring —

In a DC situation there is no hot wire, and

This is true but misleading. The reason we use the term “hot” with AC only is because AC voltages reverse all the time and so we can't just say “the positive wire” or “the -5V wire” or such. A way to describe either case would be to say “not at 0 V”. (Nominally in that resistance in wiring means that almost nothing is at exactly 0 V.)

(correct me if i'm wrong) but if I have a 350v Capacitor fully charged, there is absolutely no way for me to get electrocuted unless I touch both terminals.

If you had a 350 V AC power source that was isolated, then you could touch one terminal and be just as fine as in the capacitor case.

(Well, mostly fine. Because AC can pass through capacitors, and lots of things are a little bit like capacitors, there could be some current flow, though not much if the other terminal isn't connected to anything and therefore has no opportunity for capacitive coupling of its own.)

But your household AC line supply is not isolated — the neutral side is joined to earth — so it is not safe in this way.

people say step 4 is to ground that wire, How? The car is insulated from the earth All I see is an open circuit.

What they're not showing in the picture is that the “Yours” car is assumed to have the negative battery terminal connected to the car's metal frame, thus completing the circuit in the picture.

The reason for that hookup scheme is not anything about the electric circuit itself — it's because the final connection when current starts to flow may make an arc for a moment, and you want that arc to be away from the battery so it doesn't ignite any hydrogen gas that may be in the vicinity. (Mistreated lead-acid batteries electrolyse their water and thus produce hydrogen gas.)

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  • \$\begingroup\$ So if you don't jump in the air, touch a live, then touch what you're standing on, please explain how that is a complete circuit. Explain the electron flow (pretend it's dc) so from the live wire-> through your body-> into the wet dirt then.. \$\endgroup\$ – user126613 Oct 14 '16 at 11:50
  • \$\begingroup\$ @user126613 I've edited to explain the complete circuit. \$\endgroup\$ – Kevin Reid Oct 14 '16 at 15:06
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1.Can you not be safe just by touching a hot wire alone, granted you don't touch neutral?

Your question is not complete. A hot wire means hot in reference to somthing else. In most situations Earth or Ground. With no reference you can not use the term hot or live wire. Hot has also nothing to do with polarity but only with a potential relative to a reference.

2.What if you touch a hot wire then, stick you other finger in the dirt (soil)?

The effect of such an action depends on if you are closing an electric circuit or not. As long as you do not close the circuit there will be no current and no risk.

Please bear in mind that the term hot or live wire mostly is used in mains situations where the neutral is connected to ground. In that case the circuit will be closed at the moment you touch the hot or live wire. Meaning you can be electrocuted in such situations. So "Do not even try this"

3.So its telling me that the current flow from a hot wire, to the grounding wire-> (connect to some rocks and dirt/poop) is so high, it can trip a breaker?

Yes but only if the circuit is closed. Otherwise there is no current and the breaker does not trip. But also here attention: If you touch a live mains wire and you close the circuit there will flow a lethal current trough your body. That current is however not large enough to make te breaker trip. Again "Do not try this".

4.In a DC situation there is no hot wire, and (correct me if i'm wrong) but if I have a 350v Capacitor fully charged, there is absolutely no way for me to get electrocuted unless I touch both terminals. Why doesn't AC behave this way?

See the answer under 1.

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  • \$\begingroup\$ For your answer to number one, what I meant was the wire you would touch along with neutral that would electrocute you. For number 2, you are having electrons dissipate into the ground without the need of completing a circuit \$\endgroup\$ – user126613 Oct 14 '16 at 4:09
  • \$\begingroup\$ When you use the therm "Neutral" you indicate already a reference. Actually it is "Neutral" in respect to a common reference. For two electrons can not flow unless there is a closed circuit at some time. Therefore closing a circuit is required. \$\endgroup\$ – Decapod Oct 14 '16 at 4:24
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    \$\begingroup\$ When we talk about the hot wire, we're most likely talking about mains wiring. In which case the neutral is in many cases bonded to earth ground, making your answer to #2 dangerously unhelpful without more explanation. \$\endgroup\$ – The Photon Oct 14 '16 at 5:05
  • \$\begingroup\$ @ThePhoton: I added the information. Thanks for the comment. \$\endgroup\$ – Decapod Oct 14 '16 at 5:14
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Just a partial answer about "Point 4":

This has nothing to do with grounding but with safety!

In modern cars "unpainted metal" is connected to the minus pole of the battery.

Imagine the following situation:

You connect the two wires directly to the poles of the battery, you accidentally touch the end of the "plus wire", the "plus wire" falls off the battery and touches some "unpainted metal" of the car.

This would mean: The "minus" poles of both batteries are connected ("minus wire") and they are connected to the unpainted metal of the car and this is now connected to the "plus" pole of one battery - so you have a short-cut. A short-cut of a lead-acid (or even worse: lithium) battery is very dangerous!

To avoid this situation you connect both "plus" poles first and when connecting the second end of the "minus" wire you don't connect directly to the minus pole of the battery but to some unpainted metal on the car (which is connected to the minus pole) located far away from the battery.

Chosing a piece of metal far away from the battery you avoid accidentally touching the end of the "plus wire"...

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There have been many unfortunate maintenance souls who had spontaneous combustion using a DMM on a 480Vac busbar with insulated probes and isolated shoes, until the industry got wise about arcflash protection.

If an instrument rated for an AC line voltage and a transient or stray leakage occurs that causes an voltage breakdown and shorts out the instrument, the follow-on current is like a crowbar and will take as much current as it can give. (>>10kA)

I have personally vaporized heavy duty screw driver tips removing ac cable staples thinking it was safe in my newbie home rec.room construction days 40 yrs ago. I never got a shock, but the vaporized copper left sputtered copper spray on my plastic lenses after shorting out the line to neutral under a heavy duty staple.

Enough about that.

  • Ground is only intended to absorb leakage current, line filter current, stray RF noise, stray SMPS noise, ESD/EOS discharges etc.
  • Otherwise Ground we define as a reference point for 0V which may be found nearby with near 0 OHms resistance between them. It may have a large area capacitance and low resistance or be up to 100 Ohms through the soil to the power transformer in dry areas.
  • It is intended to shunt these unwanted low currents from causing harm due to people and equipment or reduce interference to equipment operation.
  • if you have the well-paid job of cleaning insulators on UHV transmission lines from a helicopter, you would be wearing a braided "faraday cage" suit and touch a 500kV power line to charge up the helicopter and then hop on the line to start cleaning dust causing corona on bushings.

  • Thus on schematics is normally 0V and 0 Amps flowing thru it, unless there is a fault somewhere in the power to ground network

    • or a design flaw where digital currents crossover to analog ground and cause interference because of a poor layout having a shorter path or inductive or capacitive electromagnetic coupling
  • much more can be said about it, but this is just a supplement to the other fine answers

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You are talking about two totally different things

That's why you are confused.

First is the concept of "GND as ordinary current return" commonly used in low voltage DC electronics and automobiles. In this case, nobody bothers segregating "safety shield" from "normal current return path" since you are dealing with safe low voltage. So yes, your silicon electronics circuit returns current to the backplane. Yes, your automobile returns tail-light current to chassis.

Second is the concept of equipment safety earthing as a protective shield to protect people from dangerous high voltages. In this regime, the safety shield is absolutely forbidden to be used as a current return, since that, plus a single point of failure, could create a lethal situation unnecessarily. In fact, using earth as current return defeats the purpose of earth, and wastes the money spent on the third wire.

In the second case, what do they do about current return? They return it to a different hot wire. Now to keep the various hot wires from floating at thousands of volts (say, due to capacitive coupling in a transformer), they create an equipotential bond to force all hot wires to sensible voltages compared to earth. If this results in one of the hot wires being quite near earth, e.g. within 1-2 volts in normal operation, then that hot wire is smited "neutral". Here's the trick: It's still a hot, and should be insulated as one for many reasons.

So...

Can you not be safe just by touching a hot wire alone, granted you don't touch neutral?

A local light rail line has an ancient maintenance car built in 1890. It is prized. Back then, trolley cars had wooden sides. This also has a deck on its roof and a platform that can telescope upward - entirely made of wood. This deck and platform allows servicing the trolley wire, and the car itself is electric - it has a trolley pole and self-propels. When you work on the roof, you are at trolley wire voltage, and that's no big deal. The rails and earth are lethal. You are forbidden, say, to feed a wire up from the ground.

So when you touch hot, you take your life in your hands. If you come in contact with neutral, ground or any path back to neutral, even inadvertently, you will get nailed.

So the question is... "Do you feel lucky today?"

What if you touch a hot wire then, stick you other finger in the dirt (soil)?

It depends on the impedance/conductance of the soil. If it is able to flow more than about 20mA, it could kill you. At more than about 5mA, it could stun you, and then you will have the knock-on effects of being stunned, such as falling or drowning.

So... Do you feel lucky today?

So its telling me that the current flow from a hot wire, to the grounding wire-> (connect to some rocks and dirt/poop) is so high, it can trip a breaker?

It depends on the impedance/conductance of the soil. If it is able to flow significantly more current than the breaker trip, then yes. However, there's a reason we mine copper, dirt is a poor conductor, so it's not the way to bet.

In a DC situation there is no hot wire, and (correct me if i'm wrong) but if I have a 350v Capacitor fully charged, there is absolutely no way for me to get electrocuted unless I touch both terminals.

Dirty Harry asking if you feel lucky

Why doesn't AC behave this way?

AC and DC behave the same in that respect, except that DC is nasty in greater extremes.

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