21
\$\begingroup\$

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?

\$\endgroup\$
  • 1
    \$\begingroup\$ You should really clarify that you're asking about the setup in the US (most likely) as in other parts of the world it's quite different, especially the "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" part. \$\endgroup\$ – Fizz Sep 10 '15 at 21:25

10 Answers 10

19
\$\begingroup\$

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).

  1. 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.

  2. 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.

  3. 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.

  4. 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.

\$\endgroup\$
  • \$\begingroup\$ That's right .. I forgot about the RCD. Wire-3 is there to ensure that all current stops flowing (and thus all devices lose power and fail) if Wire-1 ever touches anything it shouldn't. For some reason I kept imagining that Wire-3 was there to allow for safe & smooth operations even if faults occurred. \$\endgroup\$ – BVN Sep 11 '15 at 9:19
  • 1
    \$\begingroup\$ @BVN: Even without an RCD, wire-3 (groundING) is intended to carry away enough fault current to trip the primary overcurrent protection device. Such protection is not 100% reliable, but in the days before overcurrent protection devices it was far better than nothing. \$\endgroup\$ – supercat Sep 11 '15 at 15:32
  • \$\begingroup\$ Your list of 4 single-fault failures would be easier to understand if it were either 1) Description of a possible failure and explanation of how a grounding conductor prevents immediate danger, or 2) Description of a possible failure only, with the following paragraph serving to explain how a grounding conductor prevents immediate danger. Currently it seems to be a mix of both and for the electrical newbie it's hard to understand. Great answer nonetheless! \$\endgroup\$ – I say Reinstate Monica Sep 24 '17 at 18:16
  • \$\begingroup\$ @Twisty: Only in the first scenario does the grounding wire play a useful active role. In the second scenario, it will play a dangerous role if connected to neutral, and be harmless otherwise. In the third and fourth roles, the grounding wire is essentially irrelevant, but those scenarios are described to make clear that there are no single-fault scenarios where it is harmful. \$\endgroup\$ – supercat Sep 24 '17 at 22:22
13
\$\begingroup\$

TL;DR:

The ground wire is a safety feature to keep you safe in case things aren't working right.

You have a neutral wire as a current conducting wire to provide power.

You have the ground wire as a safe ground point for equipment with conductive (metal) housings and as a safe short circuit path for current when things go bad.


Now, some background. In the US, power is delivered to the house at higher voltage and is stepped down to provide 230 VAC with a center tap.

The neutral is connected to the center tap.

From the two ends of transformer output you have 230VAC.

From either end to the center tap you have 115VAC.

There are thus 2 circuits that provide 115VAC. These 2 circuits each provide power to half the lights and half the outlets in the house.

The neutral is thus floating, and at some unknown voltage above the voltage of the (literal) ground beneath your feet. Touching the neutral would be very dangerous. Touching either of the live wires is also very dangerous.

To keep the neutral from floating, it is connected to the house's ground - there's a large metal conductor in the ground beneath the house that provides a real connection to the real ground.

There are two points of danger when dealing with a power system.

One is the danger of connecting yourself between two voltage carrying lines - this will obviously cause current to flow through your body.

The other danger if of connecting yourself between a voltage carrying line and the ground - literally, the ground beneath your feet. If the power system is not grounded it will always have a voltage difference as measured to the ground.

The first danger can be worked around by never touching more than one wire at a time - usually pretty easy to do.

The second is much more difficult. If you touch any wire from an ungrounded power system, there will be a voltage difference between it and the ground and current will flow through your body=ouch/dead.

To reduce this second danger, power systems are grounded.

In the US, you ground the neutral wire. It is now (nearly) at ground potential. Now, there's one wire that it should be safe to (accidentally) touch. This is the reason to connect the neutral to ground.

The two live wires are now at 115VAC as measured to ground, but there's only one live wire in each outlet, so the wiring is somewhat safer - there's only one wire in the outlet box that can kill you.

BUT we aren't through yet. If there is a large current flowing through the neutral then (thanks to Ohm's law) there will be voltage difference between it and ground, so neutral is no longer really at ground potential.

Given that the two 115VAC circuits in an American house can never be balanced, there is almost always a current flow through the neutral line therefore it is not really at ground potential.

Now, imagine you are using a device with a grounded metal housing. If you are using the neutral as a safety ground, then the housing isn't really at ground potential so you get a (hopefully only) low level tingle if you touch the housing - not good, can still hurt.

If there's a short from the live wire to the metal housing then the voltage on the housing will rise== Ouch,Ouch, Ouch. If the neutral wire nows breaks in the power cord or has a bad connection in the outlet then metal housing is now at line voltage= dead user.

Now, imagine the same device with a safety ground wire. The safety ground is connected to the metal housing. Since there's never any current flowing through the safety ground (except when it is protecting you from a short circuit) the housing of the device is really at ground level=perfectly safe, no tingle.

If there's now a short from the live wire to the housing, the voltage on the housing will only go up a little bit (resistance of the ground wire) before the circuit breaker disconnects. The voltage might get high enough to tingle, but not enough to kill= user gets to keep on living.

\$\endgroup\$
  • 1
    \$\begingroup\$ This is a pretty reasonable answer, but it doesn't explain why NEC mandates connecting neutral and ground inside the house only at the main panel but forbids any other connections of neutral and ground (like at sub-panels or outlets). This I think is the main finesse in the question. \$\endgroup\$ – Fizz Sep 10 '15 at 20:35
  • 2
    \$\begingroup\$ @Fizz If the neutral and ground are connected at more than one point, current will flow on the ground wire: it will essentially just be one big neutral wire, which defeats the purpose. \$\endgroup\$ – user56384 Feb 8 '16 at 3:02
  • \$\begingroup\$ Touching a single point in a non-grounded (thus floating) system does not result in (sustained) current flowing through you and is generally unable to kill you. That's the selling point of isolation transformers. \$\endgroup\$ – Michael Karcher Nov 29 '16 at 23:54
  • \$\begingroup\$ Given that the two 115VAC circuits in an American house can never be balanced, there is almost always a current flow through the neutral line therefore it is not really at ground potential. ... This really cleared up the confusion for me, so much so that some bold formatting might be in order. \$\endgroup\$ – I say Reinstate Monica Sep 24 '17 at 18:23
5
\$\begingroup\$

Based on the various premises, this is most likely a question about the US NEC (National Electrical Code) requirements, I think.

Why differentiate between Wire-2 and Wire-3 at the power socket but then connect them further down the line?

Because if you connect them further upstream from the main panel, then you have a normal return current path through the grounding wire, which creates an unsafe situation for anyone touching it or anything connected to it, which is a lot of metal casings. As further detailed in one decent book on the topic.

The neutral is a grounded conductor by virtue of the connection at the service, but is not a grounding conductor because it is not used to connect anything else to ground. It is only used to carry the normal load current of lights, outlets, or other devices that are connected from phase to neutral. The grounded conductor remains isolated from ground everywhere except for the bond at the service. If more than one connection to ground is made, load neutral currents will divide between the grounded conductor and the EGCs (equipment grounding conductors). This can result in continuous current flow on conduit systems or metal structures and piping, which can cause electrolytic corrosion and interference with sensitive electronic equipment due to radiated magnetic fields.

Actually the US setup isn't all that foolproof as a pig pole (transformer) is shared by several houses (in the burbs) and an open neutral in one house creates the following current return path through the grounding of a nearby house, something that's not terribly easy to debug (image from the same book):

enter image description here

As for the other question:

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?

Well, it's safer most of the time. It's surely just as unsafe during a fault. The same book says (p. 104):

Never assume that a grounding electrode conductor is dead.

Finally, this NEC-mandated setup is called a TN-C-S earthing system in IEC lingo. In Europe (including UK), particularly in urban areas, the TN-S system, in which the earth is split from the neutral all way to the substation is common.

\$\endgroup\$
  • \$\begingroup\$ After talking to a friend who works for a utility company in the EU, there's a practical reason that you won't find textbooks why TN-S system is only used with buried cables: copper/conductor theft, which is rather common in poorer parts of EU. The earhing conductor is generally safe to touch thus steal. It's rather hard to steal the sheathing of a buried cable, which is normally used for earhing, but stealing an un-energized (aerial) cable is a piece of cake, I'm told. \$\endgroup\$ – Fizz Sep 15 '15 at 12:30
  • \$\begingroup\$ Well, apparently copper theft is a cross-pond problem: cnbc.com/id/100917758 \$\endgroup\$ – Fizz Sep 15 '15 at 12:41
3
\$\begingroup\$

As you've already read elsewhere, Neutral is required to carry the return current from Hot, but it's tied to Ground at exactly one point to make it somewhat safer to touch by accident. It only swings a few volts instead of a hundred or two. That's why you always switch the Hot side of a load even if it makes the controlling circuit more difficult.

Because Neutral does swing a few volts, and could heat up and fall off in extreme cases, thus making the entire mess Hot, Ground is required to provide a true 0V compared to dirt. This means that it can't carry any current because that would make it not ground anymore at the appliance end, just like Neutral. However, even though it doesn't carry operating current, it is required to carry fault current so as to trip the fuse/breaker if the user would otherwise be exposed to Hot.

\$\endgroup\$
1
\$\begingroup\$

You make a good point and I think it is reasonable to consider that wire 3 (earth) could be got rid of completely. After all it's not that it's like a screen stopping emissions emanating - it's just a wire and usually of smaller cross section than either live or neutral.

But then how would an earth leakage circuit breaker work to protect a user? It (an ELCB) sits there looking for an earth current flowing back down the earth wire - this current tells it that something unusual is happening at the load (TV, washing machine, ceiling fan etc..). If a current flows then some insulation is breaking down and potentially exposed parts of an appliance (connected to earth wire) might be in danger of getting connected to the live wire due to degredation or misuse. If this happens then only wire-3 can tell us this.

Modern installations (in EU) use RCBs to do the same thing but don't rely on measuring an earth current - they imply it by measuring the "difference" current between live and neutral. This is done by feeding L and N thru a toroidal core and having a multi-turn secondary winding that can trigger the reset should the "difference" rise above (say) 30mA.

Now think about the poor electrician wiring someone's house up - if neutral were not indistinguishable from live by a simple voltmeter then his (or her) job is much harder.

Just a few thoughts.

\$\endgroup\$
1
\$\begingroup\$

If there is no current flowing through Wire-3 there will be no potential across it (Ohms law). That way any case which is connected to Wire-3 is also at ground potential, meaning it is safe to touch as there is no potential between case and ground.

Wire-2 carries current and if connected to the case it would lead to a potential from the case to ground, which might be harmful. Also it is possible that if Wire-2 breaks and the device plugged in the wrong way (Wire-2 and Wire-1 interchanged, easily possible) the case suddenly has full phase potential to ground.

If you were to connect Wire-2 at different points to ground, you would also make it impossible to detect residual currents reliably - and those can kill you already.

Further down the line only trained personal will be in contact with the lines and the additional safety might not be needed any more. And you save a lot of money not having a fourth wire all the way back to the generator. (There are systems which work like this)

That's at least the things I understood - or hope so at least.

\$\endgroup\$
1
\$\begingroup\$

The understanding I have maintained and like to reinforce is that "ground" is more clearly referenced as "safety ground". It was added as a secondary grounding sytem for safety in case, say, the "hot" lead went to the device's (metallic) case. There were not always 3-wire, single phase circuits. The "neutral" or "return" wire is the return conductor to the generator plant. I always try to keep this as my reference. (I may be way off here, but I am under the impression that the return conductor is not actually necessary; return through earth. Please advise.) No regular service is to be applied to the ground circuit (green) although I suspect this convenient wire is beginning to be used as electric control circuits' (timers etc...) return. The electrician and owner also will not want to run a separate wire for this and will cheat with allowing a small, small current. Traditional lighted wall switches are achieved by tickling a small current through the incandescent light bulb circuit of which no light is noticed. The CFLs will flicker (as charge is built up). A separate ground would allow the lighted wall switch to work if it were constructed with an additional wire. So, do we allow this use of the ground circuit or not? I say no because it was intended to assuredly not be live. The trickle method was a tricky cheat but was not an open circuit. A bit of a tangent, but this is the practicality of the situation.

\$\endgroup\$
1
\$\begingroup\$

Neutral (wire-2) is not ground. It only measures the same as ground at the outlet if you are not drawing current.

As soon as you draw current, the voltage at the neutral connection at the outlet will not be zero.

Lets use an example:

Outlet is 50 feet from breaker you are drawing 12 amps, (a hair dryer,) 14G wire. This site, says the resistance is 0.13 Ohms.

12A, * 0.13 Ohm = 1.56V at the blade in the outlet. Not much, but not zero.

Also, there are newer breakers and GFI plugs that detect if there is current flowing in the ground wire and will trip if so.

\$\endgroup\$
0
\$\begingroup\$

Depending on the country, Wire-2 is not always neutral and connected to Wire-3. Wire-1/2 can also be a two phases (both live). Another aspect is that Wire-3 is always grounded, protecting metal casing from being live in case of fault.

\$\endgroup\$
0
\$\begingroup\$

Here is the exact reason in plain English. If you are using a tool or appliance and somehow the regular "ground" becomes disconnected, YOU could then become the default ground, shocking, to say the least. However, if the shell of your power tool, frame of your lamp, or all the metal in your electric stove is also grounded separately, you're safe. Under normal circumstances, the ground would never become disconnected, but once in awhile it might, so it's a good idea to have the third wire correctly connected to ground. Also, in faulty wiring, it's possible for some idiot to connect the two wires in reverse, many appliances would run anyway, but the casing could then be LIVE! But if the casing is grounded separately, it will immediately blow a fuse.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.