A grounded system is one in which at least one conductor or point (usually the neutral point of a transformer winding) is intentionally grounded, either solidly or through a current-limiting device. To evaluate the advantages obtained by system grounding, first consider the characteristics of ungrounded systems.
An ungrounded system is defined as a system without an intentional connection to ground, except possibly through potential indication or measuring devices. The neutral of an ungrounded system under reasonably balanced load conditions is usually close to ground potential. However, in any practical system there is always a capacitive coupling between the conductors and ground, so even the so-called "ungrounded system" is usually a capacitively grounded system.
However, tests have shown that ungrounded systems are subject to transient overvoltages which can be caused by normal switching, switching of a circuit containing a ground, or by repeated restrike of an arcing line-to-ground fault. Highest voltages are obtained when arcs are involved in the circuit disturbance. These overvoltages may have magnitudes up to six times the normal line-to-ground voltage. The line-to-ground insulation on all equipment connected to the system is also subjected to these overvoltages, which may weaken it so that it will eventually fail. It is not uncommon to experience multiple instantaneous equipment failures on different parts of an ungrounded system when this condition occurs. Due to standard insulation practices, this condition is more likely to occur on system voltages of 2,400 volts and above.
One important advantage (in some applications) with an ungrounded system, is that the phase-to-ground fault current is negligible and does not result in load interruption. However, if nothing is done to clear the fault, a safety hazard exists. In the meantime, if a fault occurs on another phase on any feeder in the system, relatively high line-to-line currents could flow, which would result in equipment damage and tripping of one or both feeders (because there will be a phase-to-phase fault under these conditions). The recommended operating procedure for an ungrounded system is to install some type of ground fault indicator using lights and/or audible alarms when a phase-to-ground fault exists. When one develops, an immediate check should be made to locate and correct the fault condition. Locating the fault could be difficult since it is necessary to disconnect all the loads, one by one, until the fault is found.
However, high resistance grounding can be supplied so that the ground current is not appreciably higher than the capacitive charging current, and zero sequence relaying can be applied to indicate fault location. This is one of several schemes available to systematically and promptly locate the ground fault without interfering with production. This concept is desirable when system shutdowns are intolerable.
The NEC indicates that practically all wye-connected alternating current circuits and systems shall be grounded. There are very few exceptions. The NEC further indicates equipment ground fault protection shall be provided for grounded wye electrical service of more than 150 Volts to ground, but not exceeding 600 Volts phase-to-phase for any service disconnecting means rated 1,000 amperes or more (NEC 230-95). The NEC 215-10 also requires ground fault protection for a feeder disconnect rated 1,000 amperes or more in a solidly grounded wye system with electrical service of more than 150 Volts to ground, but not exceeding 600 Volts phase-to-phase. On any ungrounded circuit, it is very strongly recommended that a ground alarm be provided. While the NEC does not require this ground alarm, it provides protection against equipment damage from overvoltage (providing a diligent search is made to find and correct the ground fault when the alarm indicates the presence of the fault).
Grounded systems are designed to limit the transient overvoltages to values within the capabilities of equipment insulation and thereby extend equipment life. Ground faults on a grounded system activate a protective device and are automatically removed from the system, without disturbing other system components, and without requiring a search for the faulted component. Some benefits derived from system grounding are:
The two methods normally used to ground a wye system are solid grounding and resistance grounding.
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