IEEE 1584 Guide Equation for calculation of incident energy has a factor K2 for system grounding. It is zero for ungrounded and high or low resistance grounded systems and −0.113 for the solidly grounded (earthed) systems. The incident energy release is higher for the ungrounded and high – or – low – resistance grounded systems as compared with solidly grounded systems. However, it is not always recommended that solidly grounded systems should be adopted to reduce arc flash hazard. Minimal equipment damage and continuity of supply can be achieved with high impedance grounded systems.
In a solidly grounded system, there is no intentional impedance between the system neutral and ground. A power system is solidly grounded when the generator, power transformer, or grounding transformer neutral is directly connected to the ground. A solidly grounded system is not a zero impedance circuit due to the sequence impedances of the grounded equipment, like a generator or transformer itself. These systems, in general, meet the requirements of an “effectively grounded” system in which ratio X0/X1 is positive and less than 3.0, and ratio R0/X0 is less than 1, where X1, X0 , and R0 are the positive – sequence reactance, zero – sequence reactance, and zero – sequence resistance, respectively.
Solidly grounded systems have an advantage of providing effective control of over-voltages, which become impressed on or are self-generated in the power system by insulation breakdowns and re-striking faults. Yet these give the highest arc fault current and consequent damage and require immediate isolation of the faulty section. Single line-to-ground fault currents can be higher than the three-phase fault currents.
These high magnitudes of fault currents have a two-fold effect:
- higher burning or equipment damage; and
- interruption of the processes, as the faulty section must be selectively isolated without escalation of the fault to unfaulted sections.
An impedance grounded system has a resistance or reactance connected in the neutral circuit to ground. In a low resistance grounded system, commonly adopted for medium voltages, the resistance in the neutral circuit is so chosen that the ground fault is limited to approximately full load current or even lower, typically 200–400 A. The arc fault damage is reduced, and these systems provide effective control of the overvoltages in the system caused by resonant capacitive-inductive couplings and restriking ground faults. Though the ground fault current is much reduced, it cannot be allowed to be sustained. Selective tripping must be provided to isolate the faulty section. High resistance grounded systems limit the ground fault current to a low value, so that an immediate disconnection on occurrence of a ground fault is not required.
Reference:
Arc Flash Hazard Analysis and Mitigation by J.C. Das (IEEE Press/Wiley – 2012)