Neutral to Ground Fault (Theory Question)

[JMBSD]

I'm trying to understand the principles behind a neutral to ground fault:

An OCPD would trip in a Line to Line, Line to Neutral, Line to Ground fault, and an overload where the current flow would trip the device.

But let's say a neutral becomes dislodged from an outlet pigtail and is resting against a metal box which is properly grounded.

Once someone plugs something in, the current would flow from line and return on the neutral which is now faulted to ground which would be the return path for the circuit. This wouldn't trip the OCPD because it's not an overcurrent situation.

However wouldn't this energize the entire EGC? Including all the metal studs in a commercial building without tripping the OCPD? Now in a GFCI circuit this would be stopped, but in a standard non GFCI 20A circuit isn't this an issue?

Am I understanding the theory correctly? If so what is the protection or standard to guard against this (aside from GFCIs)?

If I'm missing something please enlighten me.

 

[GoldDigger]

It is indeed an issue, and is one reason that the NEC generally prohibits neutral to ground connections downstream of the main panel or the upstream service disconnect if there is one. And there is no way, short of a suitable GFCI device, to mitigate the danger.
GFCI devices, by using a voltage injection toroid on the neutral, are able to detect a downstream neutral to ground connection even if no current is flowing in the circuit.

 

[charlie b]

If the neutral wire becomes disconnected, then the circuit becomes inoperable, and presents no hazard. But I don't think that is what you meant.

If the neutral wire remains connected, but somehow makes contactvwith the metal box, then the rest of your description is accurate. I agree with GoldDigger that this is the reason we aren't allowed to connect neutrals and grounds in subpanels. I agree with you that a GFCI device would trip and that a non-GFCI circuit would not.

How much of a hazard does this present? As you mentioned, all the EGCs (including conduit) and all metal enclosures would form a parallel path with the neutral, and all would carry current. If you touched a conduit, your body would be in parallel with the the current flowing along the conduit from the point you touched it back to the source. But the voltage drop along that path is minimal. The vast majority of voltage dropped in a circuit takes place within the load. Very little voltage remains at the point the neutral wire leaves the load. Thus, the voltage that would be driving current through your body is very small. You would likely not feel it, and it would likely cause no damage to your body.

All that said, however, if this condition remains undetected (how could it be detected anyway?) for an extended period, it could setup a very dangerous situation down the line. I am not sure exactly what would set this up. But I can envision some future electrician disconnecting a neutral while replacing an outlet, and doing it live (bad idea!). Now, instead of the neutral and ground wires sharing current in parallel, the ground wire will carry all the return current, and the voltage drop along that path gets larger. Would it be enough to create a shock hazard? I don't know for certain. Would I want to touch that energized conduit? I rather think not.

 

[kwired]

I'm trying to understand the principles behind a neutral to ground fault:

An OCPD would trip in a Line to Line, Line to Neutral, Line to Ground fault, and an overload where the current flow would trip the device.

But let's say a neutral becomes dislodged from an outlet pigtail and is resting against a metal box which is properly grounded.

Once someone plugs something in, the current would flow from line and return on the neutral which is now faulted to ground which would be the return path for the circuit. This wouldn't trip the OCPD because it's not an overcurrent situation.

However wouldn't this energize the entire EGC? Including all the metal studs in a commercial building without tripping the OCPD? Now in a GFCI circuit this would be stopped, but in a standard non GFCI 20A circuit isn't this an issue?

Am I understanding the theory correctly? If so what is the protection or standard to guard against this (aside from GFCIs)?

If I'm missing something please enlighten me.

It allows return of current to source neutral which has a connection someplace (often multiple places when utility source) to "ground/grounded objects" That alone won't fully "energize the EGC and other grounded items, but will raise the voltage to true ground because of any voltage drop in the path - and since there is no perfect conductors there is always some resistance resulting in some voltage drop any time current flows on a conductor.

Normally the resistance is minimal and not enough voltage to be noticed, but when abnormal conditions occur and that path is opened or has a dramatic increase in resistance then you may end up seeing pretty high voltage rise on those items.

 

[JMBSD]

I appreciate your responses, gives me the background information I was looking for.