A short circuit occurs when one or more energized conductors make contact with each other or with a grounded conductor or object, resulting in a large current. Although there are many types of faults, such as three-phase and line-to-line, the line-to-ground short circuit accounts for 70%–80% of faults.
Why is this the most common type of fault? Probability. As an analogy, have you ever had all four tires go flat on a car at the same time? Although it can happen, it is quite rare and usually requires some other contributing factor like road debris. It is more likely that a single tire fails as it wears out or is punctured by a nail (I know this one too well).
The same logic can be used to explain line-to-ground faults. Although there are three energized phases, all three faulting simultaneously is uncommon and usually depends on outside factors such as leaving safety grounding jumpers on a circuit or an unfortunate encounter with a crane or backhoe.
Ground faults and arc flash
A ground fault may begin as an intermittent, low-magnitude arcing current. The phase overcurrent devices might not be sensitive enough to respond quickly, if at all, leading to extensive damage. Because of this situation, the National Electrical Code defines requirements for ground-fault protection. On the other end of the spectrum, it is possible to have very high-magnitude currents produced by bolted ground faults and the phase overcurrent device would likely respond.
Arc flash escalation
When fault current flows across a gap between conductors, it results in an arc flash, producing many additional hazards such as extreme thermal energy, sound pressure, ultraviolet light, shrapnel and more. If an arc flash begins as a line-to-ground event, the conducting plasma that results can quickly reach the second and third phase, escalating the event into a three-phase arc flash.
This escalation can occur almost instantaneously, in just a few cycles. Is it possible to know when a single-phase arc flash will escalate into a three-phase arc flash? No—which is why arc flash incident energy calculations assume the arc flash will be a three-phase event.
Impedance grounding
What if the arc flash from a single-phase fault can be prevented? There would be no opportunity for it to escalate to three-phase. Is this possible? Yes—by greatly limiting the ground-fault current with a neutral grounding resistor (NGR). This is a large power resistor connected between the power system neutral, such as the wye connection of the source, which could include a transformer or generator and the grounding electrode system.
The NEC provides specific requirements regarding the NGR, and sections 250.36 and 250.187 state that the following conditions must be met to use an NGR:
- The conditions of maintenance and supervision ensure that only qualified people service the installation.
- Ground detectors are installed on the system.
- Line-to-neutral loads are not served.
High-resistance grounding
A high-resistance grounding system can be used where continuous operation of the distribution system is critical. The resistor is normally sized to limit the ground-fault current between 1A and 10A, which is such a low current level that the system can continue to operate. If a ground fault does occur, alarm indications and lights help the user identify there is a ground fault and potentially locate and correct the problem or allow for an orderly shutdown of the process. If the faulted condition remains, someday another fault may occur on a different phase and a phase-to-phase fault could result—leading to a nonorderly shutdown.
Low-resistance grounding
Low-resistance grounding is used predominantly on medium-voltage systems due to the magnitude of capacitive charging current that can be present. This current is due to various components such as higher voltage cables that can have significant capacitance due to their shielding. In general, the larger the distribution system, the more shielded the cable and the more capacitive the current.
In these cases, high-resistance grounding is not recommended, and a low-resistance grounding system is used. The value of resistance is selected that will allow the flow of sufficient current equal to or greater than the capacitive charging current, typically between 200A and 800A. This allows conventional relays to detect and isolate the ground fault in a timely manner.
In addition to the many advantages of using an NGR, there is also a very compelling benefit regarding electrical safety. By reducing the ground-fault current to a very low value, the single-phase arc flash would become negligible, eliminating escalation into a three-phase arc flash.
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About The Author
PHILLIPS, P.E., is founder of brainfiller.com and provides training globally. He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E. He can be reached at [email protected].
