Power System Design Challenges to Meet CE Code Rule 10-302

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.

The evolution of NGR monitors

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.

Meeting the CE Code NGR-monitoring challenge

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.

Maximum NGR current ratings

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.

For more information about this application or to learn more about Bender technology related to your specific application, contact our team of experts.


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