Since the 2018 edition, the Canadian Electrical Code has required all impedance-grounded power systems to be protected against impedance-grounding-device failure. Usually, the impedance-grounding device is a neutral-grounding resistor (NGR), which is why this blog will use NGR terminology. While the 2021 CE Code was amended to no longer require immediate tripping, both open and short-NGR failure detection is mandatory. Power-system engineers should consider the application limits of commercially available NGR monitors, such as Bender’s NGRM500 and NGRM700, when designing a Code-compliant resistance-grounded system.
Historically, NGR monitoring devices were first required in mining applications per CAN/CSA-M421 Use of Electricity in Mines. In mobile and moveable substation applications NGRs are required to control touch potential on equipment fed by trailing cables. CSA-M421 only requires open-NGR failure detection, the more-common NGR failure mode. As a result, early NGR monitors were not designed for short detection.
Technically easier, an open failure changes an NGR resistance from a finite value to infinity—a very large change. Short detection is comparatively difficult because the resistance change is small—from a low value, typically tens of ohms—to zero. Complicating this fact, NGR monitors typically connect to the power system through a coupling device, a high-value resistor (10’s or 100’s of kΩ), that is measured in series with the low-ohm NGR; the monitor must be able to accurately resolve the NGR-only resistance across a wide temperature range while connected to an operating industrial-scale power system.
Bender now has two devices capable of open- and short-NGR detection, allowing power system designers, service companies, and end users to comply with the CE Code. However, there are limitations that must be considered when designing a resistance-grounded system. Single-phase isolation transformers must not be used, and low-resistance grounding should be avoided when possible. Use system charging current as a guide for NGR specifications that minimize prospective ground-fault current while maintaining voltage stability. Bender NGRM500 and NGRM700 NGR Monitors enable CE Code compliance when the NGR rating is at or above a minimum resistance value (ie: maximum current value) if used within either a restricted temperature range or a wide temperature range.
At 0.6 kV, the minimum resistance is 17 Ω (20 A) within 0 to 40°C, or 35 Ω (10 A) within -40 to 70°C.
At 4.16 kV, minimum resistance is 24 Ω (100 A) within 0 to 40°C, or 61 Ω (39 A) within -40 to 70°C.
At 13.8 kV, minimum resistance is 159 Ω (50 A) within 0 to 40°C, or 266 Ω (30 A) within -40 to 70°C.
At 25 kV, minimum resistance is 144 Ω (100 A) within 0 to 40°C, or 289 Ω (50 A) within -40 to 70°C.
The power-system designer should consider these limitations when deciding upon NGR specifications.