230.95 GF protection of Equipment 480/277 only?

[kwired]

I'm only speaking of the fault circuit path itself, not the dynamics of the source. Two wires of equal length versus one wire and basically a zero ohm conductor. The ground fault circuit is almost half the impedance of the line to line short, everything else being equal.

That is a conditional thing though. If you have metal raceways, lot of structural steel, lots of metallic piping, etc. you may have a nearly zero ohm return path through those objects. If you have wood framed building, non metallic piping systems and only real path back to the grounded service conductor is via wire type EGC's you often will have higher ohm path through that EGC than you will if you have a fault from an ungrounded conductor to the grounded conductor (grounded phase) because the EGC likely is smaller than the grounded conductor.

A fault between a grounded conductor (with no incidental parallel paths) and an ungrounded conductor will have same fault current as if you had a fault between the two ungrounded conductors the same distance away from the source.

With a true neutral conductor as the grounded conductor you will have less ohms in a line to ground fault because you are only passing through half the source windings as you would with a line to line fault. But this only ends up applying at short conductor lengths from the source as resistance of the conductor along with the lower voltage to neutral than to another phase eventually works out to leaving you with higher available fault current on line to line faults as you get far enough away from the source.

But we were recently talking corner ground delta - it is a very balanced system if full delta and of all same size coils, it just happens to have one point that is grounded but other than that is still equal potential and equal impedance from each corner to any other corner of the source itself.

 

[CoolWill]

That is a conditional thing though. If you have metal raceways, lot of structural steel, lots of metallic piping, etc. you may have a nearly zero ohm return path through those objects. If you have wood framed building, non metallic piping systems and only real path back to the grounded service conductor is via wire type EGC's you often will have higher ohm path through that EGC than you will if you have a fault from an ungrounded conductor to the grounded conductor (grounded phase) because the EGC likely is smaller than the grounded conductor.

A fault between a grounded conductor (with no incidental parallel paths) and an ungrounded conductor will have same fault current as if you had a fault between the two ungrounded conductors the same distance away from the source.

With a true neutral conductor as the grounded conductor you will have less ohms in a line to ground fault because you are only passing through half the source windings as you would with a line to line fault. But this only ends up applying at short conductor lengths from the source as resistance of the conductor along with the lower voltage to neutral than to another phase eventually works out to leaving you with higher available fault current on line to line faults as you get far enough away from the source.

But we were recently talking corner ground delta - it is a very balanced system if full delta and of all same size coils, it just happens to have one point that is grounded but other than that is still equal potential and equal impedance from each corner to any other corner of the source itself.

You're not wrong in any of that. But the question about why 480 volt wye connected services require GFPE when other types of 480 volt services do not is what my post was addressing. Don surmised that it was perhaps because a corner grounded delta has a higher voltage to ground and could allow enough current to flow to open an OCPD during a ground fault. I believe this is fundamentally correct and it is tied to the idea that the ground fault circuit will USUALLY be a low impedance through many parallel paths, especially considering that a corner grounded delta is a very specialized system that you won't find very often in wood framed structures with NM wiring.

 

[jim dungar]

You're not wrong in any of that. But the question about why 480 volt wye connected services require GFPE when other types of 480 volt services do not is what my post was addressing. Don surmised that it was perhaps because a corner grounded delta has a higher voltage to ground and could allow enough current to flow to open an OCPD during a ground fault. I believe this is fundamentally correct and it is tied to the idea that the ground fault circuit will USUALLY be a low impedance through many parallel paths, especially considering that a corner grounded delta is a very specialized system that you won't find very often in wood framed structures with NM wiring.

At 277V to ground a large arcing fault, hundreds of amps, will often not self extinguish, leading to severe equipment damage and building damage. This has been known for close to 80 years. Higher voltages to ground, as little as 480V, does not usually have this issue.

 

[kwired]

You're not wrong in any of that. But the question about why 480 volt wye connected services require GFPE when other types of 480 volt services do not is what my post was addressing. Don surmised that it was perhaps because a corner grounded delta has a higher voltage to ground and could allow enough current to flow to open an OCPD during a ground fault. I believe this is fundamentally correct and it is tied to the idea that the ground fault circuit will USUALLY be a low impedance through many parallel paths, especially considering that a corner grounded delta is a very specialized system that you won't find very often in wood framed structures with NM wiring.

You often have similar low resistance paths in facilities supplied with a wye secondary though. Just it is at square root of three factor of the line to line voltage.

I can show you center pivot irrigation machines and associated well with many of them that are on 480 volt corner ground delta. The machine is a lot of metal structure, but there often is a long run of conductor back to the source, maybe up to 3000 feet in some cases with only parallel path to the grounded or EGC being earth which will normally not carry enough current to help clear ground faults.

If anything the reason for GFP on Wye systems is maybe because 277 volts is enough it tends to sustain arcing easier than even lower voltages do but also doesn't tend to draw enough current when doing so to trip overcurrent devices.

I've seen many more meltdowns or at least more severe meltdowns on 277 to ground than any other voltage system. Lower volts tends to burn itself out enough it can't continue arcing before it does much damage, higher volts tends to similarly burn itself out enough to stop arcing or it maybe even draws enough current to trip overcurrent devices. This leads to an off topic discussion on why do they require AFCI on 120 volts to ground, as it typically can't sustain an arc for very long at all without some outside help to keep feeding the arc.

 

[CoolWill]

You often have similar low resistance paths in facilities supplied with a wye secondary though. Just it is at square root of three factor of the line to line voltage.

I can show you center pivot irrigation machines and associated well with many of them that are on 480 volt corner ground delta. The machine is a lot of metal structure, but there often is a long run of conductor back to the source, maybe up to 3000 feet in some cases with only parallel path to the grounded or EGC being earth which will normally not carry enough current to help clear ground faults.

If anything the reason for GFP on Wye systems is maybe because 277 volts is enough it tends to sustain arcing easier than even lower voltages do but also doesn't tend to draw enough current when doing so to trip overcurrent devices.

I've seen many more meltdowns or at least more severe meltdowns on 277 to ground than any other voltage system. Lower volts tends to burn itself out enough it can't continue arcing before it does much damage, higher volts tends to similarly burn itself out enough to stop arcing or it maybe even draws enough current to trip overcurrent devices. This leads to an off topic discussion on why do they require AFCI on 120 volts to ground, as it typically can't sustain an arc for very long at all without some outside help to keep feeding the arc.

Seems like we agree then.