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