250.36 High-Impedance Grounded Neutral Systems
- The conditions of maintenance and supervision ensure that only qualified persons service the installation.
- Ground detectors are installed on the system.
- Line-to-neutral loads are not served.
Informational Note: According to Annex O of NFPA 70E-2018, Standard for Electrical Safety in the Workplace, high-impedance grounding is an effective tool to reduce arc flash hazards.
The grounded system conductor shall have an ampacity of not less than the maximum current rating of the grounding impedance but in no case shall the grounded system conductor be smaller than 8 AWG copper or 6 AWG aluminum or copper-clad aluminum.
Informational Note: The impedance is normally selected to limit the ground-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 142-2007, Recommended Practice for Grounding of Industrial and Commercial Power Systems.
- If the grounding electrode conductor connection is made at the grounding impedance, the equipment bonding jumper shall be sized in accordance with 250.66, based on the size of the service entrance conductors for a service or the derived phase conductors for a separately derived system.
- If the grounding electrode conductor is connected at the first system disconnecting means or overcurrent device, the equipment bonding jumper shall be sized the same as the neutral conductor in 250.36(B).