با دقت در شکل زیر جریانهای ارت فالت I1 , I2 در صورت بروز اتصال کوتاه فاز-زمین می توانند مستقل از یکدیگر شکل بگیرند. بنابراین کلیه الزامات متوجه جریان I1 مستقل از شرایط شکل گیری جریان I2 همچنان به قوت خود باقیست. میزان اهمی یا خازنی بودن جریان ارت فالت I2 در شکل گیری ولتاژهای مخرب گذرا موثر است. مطابق استاندارد چنانچه مولفه اهمی جریان ارت فالت کوچکتر از یک سوم جریان خازنی ناشی از خازنهای پراکنده سیم پیچی ژنراتور یا اتصالات بعد از آن باشد ، ریسک وقوع این اضافه ولتاژها قابل توجه خواهد بود.
Resistance grounding solves the problem of transient over-voltages, thereby reducing equipment damage. Overvoltages caused by intermittent (arcing) faults, can be held to phase-to-phase voltage by grounding the system neutral through a resistance which limits the ground current to a value equal to or greater than the capacitive charging current of the system. Thus the fault current can be limited, in order to prevent equipment damage.
In addition, limiting fault currents to predetermined maximum values permits the designer to selectively co-ordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault. The reason for limiting the current by resistance grounding may be one or more of the following, as indicated in IEEE Std. 142-1991, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems. pp. 25-26.
(1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines.
(2) To reduce mechanical stresses in circuits and apparatus carrying fault currents.
(3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path.
(4) To reduce arc blast or flash hazard to personnel who may have accidentally caused (or who happen to be in close proximity) to the ground fault.
(5) To reduce the momentary line-voltage dip occasioned by the occurrence and clearing of a ground fault.
(6) To secure control of transient overvoltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault.
It is preferable to measure the magnitude of the charging current on existing power systems for correct grounding equipment selection. The measured values must be adjusted to obtain the maximum current if not all system components were in operation during the test.
The measurement of system charging current 3IC0 is a relatively simple procedure, but, as on all occasions when one deals with energized distribution systems, a careful consideration of the problem, followed by the use of the proper precautions, is essential.
On low voltage systems, the charging current can be measured, by intentionally grounding one phase as shown below. The apparatus required for measurement on low voltage systems consists of an Ammeter, with ranges up to 10 amps, an HRC fuse and a disconnecting switch with adequate continuous and interrupting rating, such as a QMQB switch or a circuit breaker connected in series as shown in the diagram. The fuse is provided for equipment and personnel protection against the occurrence of a ground fault on one of the other phases, whilst the measurement is being made. For this test the entire system should be energized if possible.
It is recommended that a properly rated variable resistor should also be connected in the circuit to minimize transient changes in the system charging current when the phase conductor is brought to ground potential by progressively decreasing the resistance to zero. With the resistance set for Maximum, the current should be limited to half the estimated charging current.