Service systems rated at 480 V in power plants may be ungrounded, solidly grounded, or resistance grounded. There conditions unique to this service voltage. The possibility transient overvoltage on this system exists as for a higher voltage system. However the insulation of equipment at this voltage level has more inherent per-unit voltage withstand capability. For example, the overpotential test voltage of 2E+1000 for a 460 V motor indicates a withstand strength of 1920 V ac rms, or 4.17 times the motor voltage rating. Comparatively, a 4000 V motor has a withstand strength of 9000 V ac rms, or 2.25 times the voltage rating. It is for this reason and for reasons of service continuity that some system designers will operate a 480 V system ungrounded but will ground a higher voltage system. In spite of the greater percent margin inherent in 480 V insulation, there have been cases of motor failure as a result of arcing grounds at that voltage level. There have also been incidents of voltage transformer damage from neutral instability on grounded 480 V systems.
The 120/240 V systems are always solidly grounded. The 480 V systems may be solidly grounded, resistance grounded, or ungrounded. However, even on solidly – grounded 480 V systems, the circuit and fault impedance at location remote from the source may limit the fault currents to level that will be difficult to detect.
Medium voltage systems (2400 to 13800 V), are usually resistance or high resistance grounded, but this could constitute an equipment hazard. The ground fault current to be permitted is related to acceptable ground fault damage. Generating station service system loads are primarily motors. When a ground fault occurs in a motor, the fault current path may include the magnetic steel core of the stator. If so, damage to the core may require repair and partial or complete replacement of the laminations. It may, therefore, be prudent to limit the amount of fault current and the duration of the fault.
During a fault, the metal in the immediate area of the fault is usually vaporized. Since a constant arc voltage is assumed, the magnitude and time duration to which ground fault currents should be limited, to minimize damage, can be determined by relating fault energy to ampere-seconds. It requires approximately 4000 KJ of energy to vaporize one pound o copper or steel. Arcing fault tests, which conducted on 480 V aluminum bus in steel enclosures indicated that 30 KJ is a threshold below which insignificant damage occurs. These same tests confirmed that arc voltage is essentially independent of fault current for a given gap. At 1000 A, the arc voltage varied from 50 V with a 0.2 in gap to 100 V for a 1 in gap. Accordingly, the 30 KJ threshold value of fault energy translates to 600 and 300 A, respectively. Calculations (Baldwin.M.S. "Grounding can stop insulation failures," industry and power, Feb.1955) indicate that 53.5 KJ will vaporize 1/8 in3
of aluminum or 1/20 in3