Re: Ground Fault....Short Circuit??
I am going to speak in terms of our most common systems: 480 volts and below. My answer would change, if we were talking about medium or high voltage transmission and distributions systems. The difference between "short circuit" and "ground fault" has to do with the paths for current flow.
Originally posted by infinity: . . . what about a line to neutral fault?
Originally posted by iwire:A ground fault is an ungrounded conductor contacting grounded / bonded metal objects.
I have to disagree here, or at least to split a hair here. Normally, current goes from ungrounded, through the load, to grounded (neutral), and back to the source. A "short circuit" is a low resistance path that bypasses the load. Ungrounded to grounded (i.e., hot to neutral) is a short circuit. Current will go from ungrounded, through the short circuit path (i.e., bypassing the load), through the grounded, and back to the source. The fact that the grounded (neutral) wire has a connection to planet Earth is irrelevant. Current is not heading towards dirt; it is heading towards the source.
I don't know if there is a code article that will contradict the following statement, but I am going to risk making it anyway. I would say that a failure that causes a connection between a hot wire and the case of a piece of equipment is also just a "short circuit," and not a "ground fault." My reasoning is the same. Look at the path for current flow. The EGC will carry the current back to the source, and dirt will not be part of that path.
For supporting evidence to this statement, I refer the reader to the definition of "ground" in Article 100. A "Ground" is a connection to Earth (whether by intent or by accident). It is also a connection to "a conductive body that serves in place of the earth." I interpret this as not meaning a bonding connection to a non-current-carrying metal case. For example, when you connect the bare or green wire of a light fixture to the metal box to which the light is mounted, the metal box is not "serving in place of the earth." Rather, the metal box is part of a low-resistance path through which the metal parts of the fixture are connected to the source.
By counter-example, I think the phrase "serves in place of the earth" would apply to a mobile diesel generator. Under certain circumstances, we are allowed to use the frame of the truck body as the connection point for all equipment grounding conductors. Under those circumstances, we do not have to drive ground rods, as the truck body is serving "in place of the earth."
By contrast, a "Ground Fault" brings planet Earth into the circuit. The two most commonly discussed faults of this category are the "Single Line to Ground" and the "Double Line to Ground." Current will flow from the source, via the ungrounded conductor, then through the fault point into dirt, and via planet Earth to the Grounding Electrode System, up the Grounding Electrode Conductor to the ground bar in the main service panel, via the Main Bonding Jumper to the neutral bar, and thus back to the source.
