NEC Table 110.34(A)

[houchin]

Assume a 4160V System. If the system is solidly grounded I assume that the voltage to ground to be used for determining the proper working distance is 2400V. If the system is a delta I have always assumed that the voltage to ground to be used 4160V since one phase could be grounded and the system would not experience faults - I not sure that this is correct and would like verification.

The real question is for resistance grounded systems. When a phase goes to ground on these systems the phase-to-ground voltages of the other two phases elevate to the phase-to-phase voltage much like an ungrounded system. What voltage to ground should be used in determining the proper working distances? A ground fault on low resistance grounded system will typically be relayed to cleared in 10 seconds or less where the high resistance grounded system may be energized for a prolonged period of time. I'm unclear whether this distinction would make any difference in the answer.

 

[rbalex]

Voltage to Ground is a defined term in Article 100:

For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.

In absence of working text to the contrary, the voltage to ground for solid or impedance grounded systems is the same.

 

[skeshesh]

This question has come up in the past and there's some disagreements among members. I think the term "Nominal Voltage to Ground" used in the work space tables is intended to relate to the definition of "Voltage, Nominal" as defined by Article 100. The definition does not address fault (much less the time to clear a fault), but it does establish that the term "Nominal Voltage" refers to "a nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class". I would think the voltage variance during a fault should not be considered when applying the tables in 110.34.