Question About Mains Ground and Neutral

[clansd99]

Hi all,

I understand that neutral (at least where I live in Canada) is tied to ground in the main service panel, and that the purpose of the neutral line is to carry return current from the hot 120V line.
My confusion starts with the ground wire. Its' purpose is to tie into exposed conductive elements and provide a last resort incase they end up energized, right? At least in theory, couldn't we tie neutral to these exposed elements instead? The ground seems a bit redundant, although I'm sure it's there for safety purposes I haven't yet thought of.
Most of all, I'm wondering: does current through the ground wire instantly-trip the main breaker? Or is that only something that a GFCI in the wall outlet would do? It feels like that would be the main reason to have a separate ground - in case something exposed ends up hot, shut off the power.

I've taken a beginner course on power electronics before but never quite wrapped my head around this part.

Thanks!
-Chris

[bdunham7]

The ground wire not carrying (significant) current ensures that everything connected to it is at the same potential.  It also provides a return path for any leakage current, or in case of a failure, short circuit current which should trip the breaker.  Using the neutral as the ground would mean that exposed grounded metal surfaces would be energized to the extent of the return voltage drop (bad) and in the case of a break in the neutral wire, every exposed metal surface connected to ground beyond the break would become live at full mains voltage (very bad).  Two wire system simply don't have grounds and rely on insulation.

[m k]

You have only one ground.

Connection there is Y.
The leg is finally usually a large and good conductive metal lump somewhere in the soil.

One of the horns is neutral and the other is protective earth.
Between neutral and line is a load, when the other horn is sort of just hanging there.
So if line connects to the grounded chassis there will be a short and the fuse blows.

[Psi]

It's best not to call it a ground wire so as not to get confused with a local ground on your circuit.
If it's part of the mains voltage plugs/wiring in a building it's an earth wire / earth pin / earth connection

My confusion starts with the ground wire. Its' purpose is to tie into exposed conductive elements and provide a last resort incase they end up energized, right? At least in theory, couldn't we tie neutral to these exposed elements instead? The ground seems a bit redundant, although I'm sure it's there for safety purposes I haven't yet thought of.

In theory yes but in practice no it's a very bad idea. 
2 reason that come to mind.

1)- For older 2-pin plugs/sockets it was common that the phase/neutral polarity did not matter. (You could plug them in upside down in some countries).
Because of the public expectation that phase/neutral order did not matter you have to expect that sometimes people will wire up a 3-pin plug with the phase and neutral wires backwards. If you were using the neutral pin as earth on an appliance and someone has a plug or extension cord wired backwards like this then you now have Phase on this appliance's metal box and neutral on all the correctly wired appliances metal boxes ! 
(The same thing could happen if you used a 3pin to 2pin adaptor followed by a 2pin to 3pin adaptor with the 2 pin version plugged in upside down)
So it's best to have a dedicated earth pin with good labeling so it's obvious where the earth wire should go.

2)- You may mess up GFI/RCD devices ability to detect faults

[EPAIII]

As you seem to mention, the primary purpose of the ground wire is to hold the OUTSIDE case or enclosure at the safe, earth potential. That outside enclosure is what people touch, what they handle the device by. And yes, as you guessed, if a live conductor does touch that outside enclosure, then the fuse or circuit breaker in the service panel should instantly blow or trip, removing all power from the device. And keeping the user safe. Fire prevention is also tied up in here. This type of protection comes from a time when most electric devices had metal outer enclosures, not the plastic of today. It is still applicable to most commercial and industrial equipment where metal is still the primary material.

The problem with using the neutral for double duty, for the return path for the current and for a safety ground is if the neutral wire should break or somehow become disconnected between the device and the service panel. Then the outside enclosure that is still tied to the neutral will become energized at or somewhat below the line potential. And then the user can/will be shocked and possibly injured or killed. The path for the line Voltage would then be from the hot wire, through the internal circuitry of the device, to the neutral wire inside the device, then to the outer metal enclosure, and finally to the person holding the device. And through that person to any ground he/she may be in contact with. The degree of the shock would be determined by the resistance through the device's internal circuitry and how well that ground contact may be. It could vary from a tickle to a lethal shock.

According to the US code the ground wire can be eliminated if the outer shell of the device is DOUBLE insulated from any current carrying conductors. Thus, some devices like lamps or hand held power tools are equipped with a two conductor cord. In theory, at least two separate insulators need to fail before the user can get shocked. In these devices, if the outer shell is metal, then it can NOT be connected to the neutral or hot conductors or any internal parts connected to either of them. So a break in the neutral, as I described above, will only cause the device to stop working. It will not result in any shock hazard.

The electric codes must account for many possible failure scenarios, not just the simplest or most obvious ones. Oh, and according to some sources, the fuses or circuit breakers are originally there for prevention of fires when wires are shorted, not for the protection of human lives. But protection of humans has become one of their purposes today. I tend to believe this.

[Siwastaja]

Thinking starts with language, and terminology is important as it either enables successful communication, or sets the stage for confusion.

Hence, my strong suggestion is: completely get rid of the word "ground" when it comes to mains wiring. It's endless source of confusion. Ground is used in electronics design as an arbitrary net label and has no place in mains-related terminology.

In single phase mains, you have two live (live = attribute, not a name of a net) wires, namely line (aka Phase) and neutral. Additionally you have Protective Earth a.k.a. PE.

Just get rid of all other terminology. Better be verbose than to guess.

Normally, Neutral equals PE - they are connected together at some point, but the distinction is made and Neutral is considered live and as dangerous as Line because of nonpolarized plugs, wiring mistakes, wires coming loose, etc.

[IanB]

According to the US code the ground wire can be eliminated if the outer shell of the device is DOUBLE insulated from any current carrying conductors. Thus, some devices like lamps or hand held power tools are equipped with a two conductor cord. In theory, at least two separate insulators need to fail before the user can get shocked.

In theory, at least. This seems to be open to some interpretation, as can be seen from the typical lamp holder shown below. The two layers of insulation separating the live screw terminals and the outer shell consist of a layer of cardboard and a thin plastic film. It seems somewhat creative to consider this as two layers of insulation rather than merely one, but apparently UL thinks it is OK.

[m k]

the fuses or circuit breakers are originally there for prevention of fires when wires are shorted, not for the protection of human lives. But protection of humans has become one of their purposes today. I tend to believe this.

True dat.

If fuse blows everything is safe again.
But if not, then you die, active fuse is not going to save anything with a bumping heart.

Bad situation here is that missing N.
Then load is off but live is still there, between L and PE.
For that you need a RCD of some sort that will trip before a bumping heart will collapse.

I think the goal here is the idea of one failure is still safe.
So the rule number one, do not cross boundaries.
(would have saved lives through the years)
It means that you do not use your new and heavy duty rubber extension cord and pull the juice from living room to the kitchen.

[EPAIII]

Oh, yes. The common lamp sockets. What can I say?

I have never liked that design. The most common thing you see go wrong, pre LED bulbs, was a 100 Watt bulb in a fixture rated for only 60 Watts. Or worse yet, a 150 Watt bulb. The great unwashed did not understand or even, in most cases, know that they were misusing the lamp. But after years of that extra Wattage, the cardboard insulator would crumble from the heat. And for many, many years there was no second layer of insulation unless you counted a half mm air gap. If they are adding a second insulation layer today, that is a long overdue good thing.

And yet for all their faults, I have never had a shock from one or seen anyone get shocked. I'm not saying it didn't happen, just I never saw it. I have replaced more than one of those sockets. Or in some cases where a more decorative outer shell was used, I replaced the inner, cardboard insulation which was falling apart.

I was not defending or justifying anything in my post above. Just explaining some of the reasoning behind the way things are the way they are.

According to the US code the ground wire can be eliminated if the outer shell of the device is DOUBLE insulated from any current carrying conductors. Thus, some devices like lamps or hand held power tools are equipped with a two conductor cord. In theory, at least two separate insulators need to fail before the user can get shocked.

In theory, at least. This seems to be open to some interpretation, as can be seen from the typical lamp holder shown below. The two layers of insulation separating the live screw terminals and the outer shell consist of a layer of card and a thin plastic film. It seems somewhat creative to consider this as two layers of insulation rather than merely one, but apparently Underwriters Laboratories thinks it is OK.

[Alti]

I understand that neutral (at least where I live in Canada) is tied to ground in the main service panel(..)

There are tens of different distribution and grounding concepts all around the world. I bet even in Canada there are many of them. Depending on how you define "ground" in above statement, experiences may vary.

[Siwastaja]

I understand that neutral (at least where I live in Canada) is tied to ground in the main service panel

The key is to understand this tie is not infallible.

Draw a schematic with, say, a heater element between L and N. Remove connection between N and the Earth. Now imagine the voltage between the disconnected N (remember, it's now connected to L through the heater!) and the Earth.

Separate Earth wire which does not normally carry current circumvents this scenario. That is something you can more safely tie to metal chassis, pipework etc. If you cut the PE - N connection, N becomes dangerous because L connects to it through loads, but PE does not become dangerous, there is no connection to L.

[Alti]

Short introduction to IEC 60364 and various earthing concepts (with pictures).

[james_s]

Thinking starts with language, and terminology is important as it either enables successful communication, or sets the stage for confusion.

Hence, my strong suggestion is: completely get rid of the word "ground" when it comes to mains wiring. It's endless source of confusion. Ground is used in electronics design as an arbitrary net label and has no place in mains-related terminology.

In single phase mains, you have two live (live = attribute, not a name of a net) wires, namely line (aka Phase) and neutral. Additionally you have Protective Earth a.k.a. PE.

Just get rid of all other terminology. Better be verbose than to guess.

Normally, Neutral equals PE - they are connected together at some point, but the distinction is made and Neutral is considered live and as dangerous as Line because of nonpolarized plugs, wiring mistakes, wires coming loose, etc.

This is location specific though. In North America where the OP is located, "ground" is the correct term for what many parts of the world refer to as protective earth. If you mentioned the PE conductor to an American electrician they might not know what you're talking about because that isn't a term we typically see here.

[Siwastaja]

This is location specific though. In North America where the OP is located, "ground" is the correct term for what many parts of the world refer to as protective earth. If you mentioned the PE conductor to an American electrician they might not know what you're talking about because that isn't a term we typically see here.

Yeah. It's still useful to understand why less confusing terminology is used elsewhere, even if you have to come back to your local confusing terminology.

For strictly electrician viewpoint, it doesn't matter much what you call it. But electrician's "ground" (i.e., PE) can be very confusing for electronics hobbyists and even professionals for whom "ground" is something else.

[jonpaul]

think worst case scenario, fault tolerant design, "what if" for safety situations.

In RF design, my old friend Dick Sequerra said "there's no such thing as a ground".

A good electricians forum is the best place to discuss these issues, try Mike Holt.

Jon