Overview

It is estimated that approximately 30,000 arc flash incidents occur each year. An effective method for lessening arc flash hazards is to use an impedance grounded system. Impedance grounded systems insert an impedance device, generally a resistor, in place of the main bonding jumper. In the event of a grounding fault, the impedance limits the amount of current that can flow, which will decrease the amount of energy dissipated. This will lessen not only the damage dealt to the equipment, but will decrease the potential for personnel injury. The 2023 NEC has updated Section 250.36, which deals with impedance grounding, making the language clearer.

Applying the 2023 Code

The 2023 NEC has updated Section 250.36 to remove the words “High” and “Neutral” from the section title. The title has been renamed to “Impedance Grounded Systems – 480 Volts to 1000 Volts.” This change directly indicates the voltage requirements for systems utilizing impedance grounding. Previous editions of the NEC referenced the impedance throughout section 250.36 numerous times. The 2023 NEC has added the term “device” in conjunction with impedance to clear up any confusion as to whether the value of the impedance, or the physical device was being referred to. By directly stating the voltage ranges associated with such systems, and providing clarity in the language, these changes make the text much more accurate, and will aid the electrical professional when interpreting the requirements for impedance grounded systems.

What’s New for the 2023 NEC?

2020 NEC

In the 2020 NEC Section 250.36 read as follows (excluding informational note):

250.36 High-Impedance Grounded Neutral Systems.
High-impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value shall be permitted for 3-phase ac systems of 480 volts to 1000 volts if all the following conditions are met:
(1) The conditions of maintenance and supervision ensure that only qualified persons service the installation.
(2) Ground detectors are installed on the system.
(3) Line-to-neutral loads are not served.

High-impedance grounded neutral systems shall comply with 250.36(A) through (G).

2023 NEC

In the 2023 NEC Section 250.36 reads as follows (excluding informational note):

250.36 Impedance Grounded Systems — 480 Volts to 1000 Volts.
Impedance grounded systems in which a grounding impedance device, typically a resistor, limits the ground-fault current shall be permitted for 3-phase ac systems of 480 volts to 1000 volts if all the following conditions are met:
(1) The conditions of maintenance and supervision ensure that only qualified persons service the installation.
(2) Ground detectors are installed on the system.
(3) Line-to-neutral loads are not served.

Impedance grounded systems shall comply with 250.36(A) through (G).