250.36 High-Impedance Grounded Neutral Systems.
To limit fault current to a very low value, high-impedance grounded neutral systems have a resistor installed between the system and the metal parts of the system. Figure 250–90
This is only permitted for three-phase ac systems (typically rated 480V) where all the following conditions are met:
(1) Conditions of maintenance and supervision ensure that only qualified persons service the installation.
(2) Continuity of power is required.
(3) Ground detectors are installed on the system.
(4) Line-to-neutral loads aren’t served.
FPN: The system bonding resistor is selected to limit fault current to a value slightly greater than or equal to the capacitive charging current of the system. This value of impedance will also limit transient overvoltages to safe values. For guidance, refer to criteria for limiting transient overvoltages in ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book).
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