New requirements introduced in Section 10 of the 2018 CSA 22.1-18 Canadian Electrical Code will impact the application of impedance-grounded systems within Canada’s various industries, including mining.
The importance of NGR monitoring in mining
In North America, impedance grounding is commonly achieved by inserting a neutral-grounding resistor (NGR) between the system neutral and ground at the distribution transformer or the generator. Doing so lowers the prospective ground-fault current to a predetermined value dictated by the impedance of the NGR. This is advantageous in mining as it reduces or eliminates the chance of an arc flash or hazardous ground-fault voltages on portable trailing-cable-fed equipment during a phase-to-ground fault.
The use of NGR monitoring relays is mandated by CSA M421, Use of Electricity in Mines. By providing continuous monitoring of the neutral-to-ground path, NGR monitors verify that the NGR is intact. This is important, as an open NGR renders current-sensing ground-fault protection inoperative and could result in a false belief that the system is properly functioning.
CE Code revisions and NGR monitors
In addition to current CSA M421 requirements, the 2018 CE Code requires that:
The integrity of an impedance grounded system shall be monitored, and the system shall have an audible or visual alarm that corresponds to the occurrence of:
a) a ground fault on current-carrying conductors, including the neutral conductor where line-to-neutral loads are served;
b) a ground fault on the conductor connecting the impedance grounding device to the source; and
c) a loss of continuity of the impedance grounding circuit from the system source through the impedance grounding device to the grounded non-current-carrying conductive parts of the electrical system.
To meet the intent of the CE Code’s new requirements, the NGR monitor must provide detection of a ground fault on the neutral-to-NGR conductor or on a distributed neutral where line-to-neutral loads are served. Although NGRs do not typically fail shorted, a person who is accustomed to solidly grounded systems can mistakenly wire the neutral directly to ground, which effectively bypasses or shorts out the NGR. Shorted NGRs can result in dangerously high currents during a phase-to-ground fault that can often lead to arc flash incidents.
NGR monitors in mining and industrial applications were introduced in Canada by Littelfuse Startco more than 30 years ago. Littelfuse Startco engineers have served on the CSA M421 Use of Electricity in Mines committee for decades, helping to develop the mining standards that ensure the safety of people and equipment.
Impedance-grounded systems must use continuous monitoring devices in order to meet code requirements. To help customers meet the requirements of the Canadian Standards Association’s publication of the 2018 CE Code, Littelfuse developed an update to its SE-330 series’ firmware that provides the additional protective functions that are fully compliant with new CE Code. Littelfuse tested the update in select mining sites throughout Canada.
The new CSA requirements do not necessarily require the procurement of new protective equipment. Facilities that were already equipped with the SE-330 NGR Monitor, for example, can upgrade their equipment in the field to the latest firmware update. The free update ensures that workers and equipment across the Canadian mining industry are not only compliant with the 2018 CE code, but safe.
