Relationship between ground and neutral

Reply

  #1  
Old 12-27-20, 03:44 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
Relationship between ground and neutral

Over the past year I've gotten myself much more familiar in how household electrical service wordk, including things like 240v, GFCI, AFCI, shared neutral, breaker box wiring, and the different although somewhat similar function of the neutral and ground wires. Deeper thoughts into that last topic led me to a a concern regarding the safety of ground wires, and that's I what I'd like to touch on here.

Maybe I'm misunderstanding something here, but it seems like it might be possible for an unsafe current to run through a ground wire, even when there are no faults. The reason is because both ground and neutral attach to the same grounding bar in the main breaker box. The idea is that from there any current on any ground or neutral wire will run directly to the ground via the main grounding wire. However, since neutral and ground share the same grounding bar, that means any current flowing through a neutral wire to the grounding bar could wind up flowing from there to one of the grounding wires and back to, for example, an outlet. Of course that would only happen if that path eventually itself led to ground, something that normally isn't the case. But I summized that if I were touching a grounded part of a plugged in grounded appliance, and I was myself well grounded (standing barefoot in a puddle let's say), then potentially some of the current on *any* neutral wire could end up flowing through me to ground rather than to ground through the main grounding wire in the breaker box.

So I set out to prove this (no I did not stand in a puddle). I instead got out my multimeter, set it up to measure current, stuck one lead in the ground socket of an outlet and clipped the other onto a heater duct that was a few feet away which I had already verified was grounded. I measured about 1.2mA, and it was only about 1.5V, so I guess not a concern. I'm assuming these low numbers are because the path to ground from the main breaker is only a few feet, whereas the alternate path to ground back through a ground wire to the outlet I was using, and then into the heater duct and eventually to ground was probably 100'. So almost all of the current chose the shortest distance, but I guess due to some resistance on the main ground wire, some decided to take the longer route to ground through the outlet I was using.

Any thoughts on this? Am I understand things correctly? Although my measurements didn't show any hazard, is their potential for a problem here that could result in you getting a noticeable shock when touching something that is grounded off your household wiring? I'm was thinking maybe if there was a very heavy electrical load, and only on 1 pole, that this would increase the current, and therefore resistance, on the main breaker's grounding wire, possibly causing more current to flow through any alternate route to ground.
 
  #2  
Old 12-27-20, 04:45 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 13,799
Received 252 Votes on 220 Posts
Although the ground a neutral are connected to the same point in the main panel they are separated (or they should be) everyplace else. That separation is key to the electrical system operating correctly.

If the ground and neutral are connected together elsewhere you will get unwanted current flowing on the ground wire which could be a hazard. This is why it is so important to make sure the ground is never touching a neutral wire. This can happen when pushing a receptacle into a box or if something is accidentally (or intentionally) wired incorrectly.

I do not think it would be out of the norm to have a small amount of difference of potential between the ground and the ductwork which is why you are reading the 1.5v. This is nothing to be concerned about.
 
  #3  
Old 12-27-20, 04:50 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 15,218
Received 103 Votes on 89 Posts
To help, the current is trying to get back to its source, not the ground. Earth is not a great conductor. The current is not going to run uphill to a grounded receptacle.
 
  #4  
Old 12-27-20, 05:18 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
But the only path back to its source is the ground, right? There's no wire running back to the source. I get the running uphill part. The short and thick wire running to ground from the main breaker is clearly the easy path. The long and thin ground wire running back to an outlet is not. However, if that short ground wire in the main breaker were to lose contact with the ground, then suddenly the uphill paths become the only paths, and every grounded appliance becomes a shock hazard. And not just a potential hazard, but a real one just waiting for someone to touch it.

And yes, I realize that with the main ground wire no longer serving as ground, and no other good paths getting you to ground, appliances would actually get little to no power. But if you were to provide a path to ground by touching a grounded portion of an appliance, and make sure you were also at least somewhat ground, then the neutral return path would start running through you to get to ground, and it would do so by taking the very long route along the netural wire back to the main breaker, then back to the appliance via the ground wire, and then to you by touching the appliance. And it's not just the neutral path of this one appliance you touch that would have its neutral path end up running through you. Everything using power in your household would attempt to get to ground through you. Basically if you make contact with something that is grounded (back to the main breaker box), then you would become the sole path to ground for everything, taking the place of that short ground wire in the main breaker box that is no longer doing its job.
 
  #5  
Old 12-27-20, 05:52 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 15,218
Received 103 Votes on 89 Posts
The normal path is back through the neutral. In the service panel the grounding and grounded conductors are bonded together.
 
  #6  
Old 12-27-20, 06:18 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
Yes, I realize that. I am hypothesizing what would happen if the ground in the service panel was severed, so it was no longer providing a path to ground. Then the path through the neutral back to the service panel no longer has the normal path to ground, so it would look to some other path. Normally there are no other (good ones). However, there is a path alone any of the ground wires back to the various appliance that are plugged in. If they are grounded to the ground wire, then basically the grounded parts of the appliance become "hot", with the current looking for some place to go. The current would flow through you if you touched it, but only to the extent that you are yourself grounded. So this could mean, for example, a small shock if you are wearing shoes and are standing on a hardwood floor, but could be a deadly shock if you are barefoot and literally standing on the dirt ground.
 
  #7  
Old 12-27-20, 06:32 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 15,218
Received 103 Votes on 89 Posts
The system ground is for high voltage event like lightning and surges. The system will work just fine without it.

When you say lose the ground are you talking about the grounded or grounding? If the grounded is lost the circuit stops working. If you mean grounding the safety would be compromised and could allow metallic parts to remain energized.
 
  #8  
Old 12-27-20, 06:43 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
If the ground and neutral are connected together elsewhere you will get unwanted current flowing on the ground wire which could be a hazard. This is why it is so important to make sure the ground is never touching a neutral wire. This can happen when pushing a receptacle into a box or if something is accidentally (or intentionally) wired incorrectly.
Sorry I missed your initial response. So this really caught my eye because it seems there is a high likelihood of this happening. The ground wire is completely exposed (I've always wondered why that was considered safe). The ground wire could easily come in contact with the neutral terminal screw. Can a 3-wire receptacle tester detect this? As far as I can tell they can't even test if neutral and ground are reversed (which makes sense), so I don't see how they can tell if neutral and ground are essentially the same wire.

I see two consequences of this happening. The first is that both the ground and the neutral are now both acting like a neutral. So roughly half of the neutral current will run back to the break box along ground, and touching it would be almost as bad as touching a hot wire. The 2nd is that the grounded appliance will essentially be grounded to neutral. That means return current that would normally flow through the neutral wire back to the breaker box can now instead flow through anyone that touches the grounded part of the appliance. This is much like the situation I described above that would happen if the grounding wire at the service panel no longer went to ground (was severed).
 
  #9  
Old 12-27-20, 06:52 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
The system ground is for high voltage event like lightning and surges. The system will work just fine without it.
Can you explain what you mean by "system ground". If you mean the wire running to ground at the main service panel, this doesn't sound right to me. There is no neutral wire running into this panel, only the two 120v hot wires. They go through the 200A breaker, and then from there to the breaker box in the house, in additional to the neutral wire and the grounding wire, both of which originate off the grounding bar. So I assumed the wre is running to ground is also serving as the return path for the neutral wire.

When you say lose the ground are you talking about the grounded or grounding? If the grounded is lost the circuit stops working. If you mean grounding the safety would be compromised and could allow metallic parts to remain energized.
At the main service panel where power enters the house, the neutral and ground wires are bonded together, and there's what looks to be about an 8g bare copper wire running from the grounding bar to the ground. I was speculating on what would happen if this grounding wire was cut, so now the service no longer has its normal path to ground, but that doesn't mean the electricity won't seek out another path to ground. A potential path is any grounded appliance. It has a path back to the grounding bar via the ground wire. If this ground wire it attached to the appliance, and the appliance also provides some other path to ground (such as a person touching a grounded part of the appliance), then this provides an alternate path to ground.
 
  #10  
Old 12-27-20, 09:04 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 60,786
Received 1,324 Votes on 1,224 Posts
Coming in from your meter are three wires. Two hot wires and one neutral wire. This is a known fact.
In your main panel.... the neutral and the ground connect to the same bar. It's a common bar. This is the only place neutral and ground are connected together.

The neutral coming from the poco is what the grounds in your house are connected to. If your house ground fails..... your circuit grounds and neutrals are still connected to the neutral bar. A short to ground at a receptacle will connect the hot to ground which goes to the neutral bar. That will trip the circuit.

The house ground between the main panel and the grounds rods and water service are for lightning protection. They do not help in protection of the circuit.
 
  #11  
Old 12-27-20, 09:42 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
Coming in from your meter are three wires. Two hot wires and one neutral wire. This is a known fact.
In your main panel.... the neutral and the ground connect to the same bar. It's a common bar. This is the only place neutral and ground are connected together.
That's not the case in my house. I just checked for the 3rd time now. From the meter there are 2 hot wires running into the service panel from the side. The only other wire coming into the service panel is the bare copper wire running to the grounding bar. This comes in from below, so I assume runs directly into the ground. Running out of the top of the service panel are the two hots (after going through the 200a breaker), the neutral (coming off the grounding bar), and a thick bundle of bare aluminum wires (also coming off the grounding bar). In the breaker panel in the house I see these same 4 wires coming in. The two hots leading to the two hot busbars, the neutral to the neutral bar (where all the white neutral wires also connect), and the bare aluminum bundle to the grounding bar (where all the bare copper ground wires also connect).
 
  #12  
Old 12-27-20, 09:48 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 60,786
Received 1,324 Votes on 1,224 Posts
It could be two insulated wires and a bare wire.
Service cable uses two insulated wires and a spiral wrapped bare neutral.
 
  #13  
Old 12-27-20, 09:56 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
It could be two insulated wires and a bare wire.
Service cable uses two insulated wires and a spiral wrapped bare neutral.
Yes, I've been googling a bit and have been seeing examples of that, but that's not my case. I'll get some pics.
 
  #14  
Old 12-27-20, 10:06 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
I think I know what might be going on. The service panel is part of a larger panel that includes the meter. Is it possible that the incoming neutral is bonded to the panel itself in the area I can't access. I'll get pics uploaded soon.
 
  #15  
Old 12-27-20, 10:26 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
  #16  
Old 12-27-20, 10:51 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 60,786
Received 1,324 Votes on 1,224 Posts
This is your main disconnect panel. The grounds and neutrals are combined here and sent to your inside panel via four wires..... two hots, a neutral and ground. Your inside panel is being treated as a sub panel. The poco neutral is connected to the bar behind the meter.

 
  #17  
Old 12-27-20, 11:16 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
The poco neutral is connected to the bar behind the meter.
Ok. Close to what I eventually concluded, although I didn't realize that the neutral bar was extending into the meter half of the panel. Thanks!
 
  #18  
Old 12-28-20, 07:42 AM
D
Member
Join Date: Mar 2020
Posts: 29
Received 0 Votes on 0 Posts
Please correct me if I'm wrong, but if you were to unbond the ground wire, you would not notice any difference except in the case of a fault. Older houses don't have a ground and they function fine (until they don't).

The ground is connected to the earth near the main panel to allow a return path for lightning strikes to return to earth (the source) without going through your house. It's never been intended as a return path for household current.

If your neutral and ground are disconnected at the panel, I think it's pretty hard to get any current to flow in the hot wire except for a lightning strike trying to return to earth.

Is my understanding correct? Sorry if this has nothing to do with your original question.
 
  #19  
Old 12-28-20, 05:26 PM
A
Member
Join Date: Mar 2010
Location: USA
Posts: 4,283
Received 109 Votes on 101 Posts
Your understanding is correct.

If the ground bond were broken then normally everyng works the same, current flows from hot through the load to neutral.
oti

If both ground and neutral were disconnected than there is no complete circuit from hot through the load and back to the panel. No current flows.


Except if there is a fault to exposed metal, then a person might complete a ground fault path not via the disconnected ground wire ultimately getting back to the pole transformer. Then current will flow. It takes only a few milliamps of current to kill someone.

Ground reference or system ground is the pole transformer neutral or metal member attached thereto in the panel enclosure where the first whole house disconnect means is located, the term "main panel" referring to this panel. Ultimately, equipment grounding conductors and grounding electrode conductors are interconnected (bonded) here. But neutrals are not to be bonded to EGCs downstream of this point.

< < < That's not the case in my house. > > >

The large panel in your house where the neutrals are attached to one bus bar and the ground wires attached to a different bus bar is a subpanel. The main panel is outside where the 200 amp breaker is located, said 200 amp breaker being upstream of the top breaker in the large house panel..
 

Last edited by AllanJ; 12-28-20 at 06:03 PM.
  #20  
Old 12-28-20, 06:39 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
< < < That's not the case in my house. > > >

The large panel in your house where the neutrals are attached to one bus bar and the ground wires attached to a different bus bar is a subpanel. The main panel is outside where the 200 amp breaker is located, said 200 amp breaker being upstream of the top breaker in the large house panel..
Yes, but that's not what I was referring to when I said "That's not the case in my house.". However, I was wrong when I said that. I did not realize that the busbar in the part of the main panel I could see extended into the other half of the panel with the meter, and I could not see what was going on there, therefore did not see the poco neutral attached to it.
 
  #21  
Old 12-28-20, 06:48 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
I'm still curious about the situation where the ground makes contact with the neutral, more specifically when this happens in a receptacle, such as the ground touching the neutral terminal screw. I think I understand what happens in this case. Basically both ground and neutral wires start to serve dual purposes of both ground and neutral. So that means some of the current normally destined for the neutral wire can end up on the ground wire. If you touched a grounded appliance, that current could flow through you. However, given the likely very high resistance the current would have trying to get to ground through you, the shock will likely be minimal, but providing a strong path to ground through you could result in a deadly shock.

What I'd really like to know is how to detect this situation. It seems that you would need to test one circuit at a time by running some appliance off the circuit, and then use some other outlet to see if you can detect unsafe current to ground, something like I described in my very first post in the thread where I when I put the multimeter between the ground plug of an outlet and a grounded heat duct.
 
  #22  
Old 12-28-20, 08:01 PM
D
Member
Join Date: Mar 2020
Posts: 29
Received 0 Votes on 0 Posts
I'm still curious about the situation where the ground makes contact with the neutral, more specifically when this happens in a receptacle, such as the ground touching the neutral terminal screw. I think I understand what happens in this case. Basically both ground and neutral wires start to serve dual purposes of both ground and neutral. So that means some of the current normally destined for the neutral wire can end up on the ground wire. If you touched a grounded appliance, that current could flow through you. However, given the likely very high resistance the current would have trying to get to ground through you, the shock will likely be minimal, but providing a strong path to ground through you could result in a deadly shock.
I don't think any appreciable amount of current would travel through you. Current will to return to the source in proportion to the easiest path. The path via neutral and/or ground wires is infinitely easier than through you and then through another neutral/ground wire. If the neutral comes in contact with metal box and you touch the metal box, you probably won't feel anything because the easiest path back to the source is still the neutral or ground.

If you are the only path, then you could be in big trouble.

That's my understanding.
 
  #23  
Old 12-29-20, 05:45 AM
A
Member
Join Date: Mar 2010
Location: USA
Posts: 4,283
Received 109 Votes on 101 Posts
With a proper neutral and a proper equipement grounding conductor except there is a fault between neutral and ground well downstream such as in an appliance, yes, neutral current will return using both the neutral and the EGC. But the resistances of those paths is low enough that the voltage difference between the location of the fault and the house ground (main panel neutral bus bar) is near zero. So if someone touches the ground wire with current flowing through it there is not enough voltage to allow enough current to take a third and higher resistance path, through the person's body, to electrocute him with.

How to detect the current flowoing in the EGC? We don't bother to do that. Except where it is possivble to obtain a higher voltage differential between an energized exposed piece of metal and other nearby metal such as a water faucet, we use ground fault circuit interupters to indirectly detect curent flowing on a separate path to ground by sensing that current returning on the proper path, the neutral, is different from the current supplying the circut via the hot condutor(s).

Sometimes a fault in an appliance is between some other energized part not part of the neutral path, and exposed metal. Here the current returning via the EGC can be greater than the current returning on the neutral. The EGC is supposed to be robust enought that the branch circuit breaker would trip before the amount of current could result in significantly elevating the voltage present at any of the exposed metal.
 
cplum voted this post useful.

Last edited by AllanJ; 12-29-20 at 06:11 AM.
  #24  
Old 12-29-20, 12:58 PM
C
Member
Thread Starter
Join Date: Dec 2020
Posts: 23
Received 1 Vote on 1 Post
Thanks Allan. That was a good explanation.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: