Protecting personnel and equipment from damage is a top priority for most companies, especially those whose work involves industrial power systems. The neutral grounding resistor (NGR) with integral Sigma relay from I-Gard, Mississauga, Ontario, Canada, is helping employers achieve this goal by incorporating grounding circuit monitoring along with a ground fault relay. According to the company, the NGR is the only product currently available that limits both the duration of a fault as well as the fault magnitude.

“This hasn't been done before,” says Tony Locker, I-Gard's director of business development. “Normally, you'd have two separate products — one would monitor the resistor, and the other would be a ground fault relay. We've combined them into one system.”

Introduced to the market last spring, Locker explains two main factors prompted the development of the product. “In the past, there were resistors out in the field with wires connecting them to the power system,” he says. “Either the resistor itself would fail due to things like loose or corroded connections, or the copper wires running from the power system to the resistors were stolen because copper scrap was becoming so valuable. So even if the resistor was intact and functioning well, it couldn't operate if the copper ground wires were missing.”

Compounding the dilemma was the fact that the only time a resistor or relay comes into play is if a problem occurs on the power system. “During normal operation, there is nothing going on with the resistor — 98% of the time it's literally just sitting there,” says Locker. “So you don't know if you have a bad resistor until you have a problem on the power system. This product alerts you about a resistor's status prior to an anomaly on the power system.”

Bernard Geurts, an operations supervisor with ABB, Zurich, Switzerland, says the device is being used on a 600V unit substation application in Canada to monitor the ground fault resistor and ground fault current on the distribution transformer. “Along with auxiliary relays, it is used to alarm on the presence of a ground fault and trip the main secondary 600V breaker or loss of resistor,” he says. “It also fulfills the requirements of the Ontario Mining Code by constantly monitoring the condition of the grounding resistor and the possible presence of an active ground fault. It provides separate contacts to allow independent operations, depending on the type of fault.”

Initially geared toward the mining industry, I-Gard expanded the product offering to industrial customers and utilities. Branching out into new markets brought different design considerations. “The atmosphere and different types of environments — such as heat, cold, high humidity, and salt air — had to be taken into account,” says Locker. “Also, we had to develop a human machine interface (HMI) that could attach to the resistor enclosure so users didn't have to open the enclosure to get inside to look at or operate the relay.”

In addition, all of the NGRs are standardized on the edgewound element design. According to I-Gard, the edgewound element material is mounted on porcelain supports that are not affected by high temperatures or high voltages. Thus, the coiled element is able to expand, will not deform when heated, and offers consistent current density. Made of a special grade of electrical alloy with a low-temperature coefficient of resistance, the element material prevents the resistance value from increasing sharply as the resistor operates through a wide temperature range.

“Customers have been impressed with the controlled change of resistance,” confirms Locker. “They also see the benefit of having a resistor and relay in one package. It allows one product to be used in lieu of several, which lowers the system cost and reduces space requirements.”

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Product Specs

Main trip relay rating: 10A at 240VAC, 10A at 30VDC, ½ hp at 240VAC
Auxiliary ground fault relay and NGR fault relay rating: 10A at 240VAC, 8A at 24VDC, ½ hp at 240VAC
Operating temperature range: -40°C to +60°C
Ground fault circuit CT input: non-isolated; one side of the CT input, terminal 22, is internally grounded
Ground fault circuit CT ratio: T2A, T3A, or equivalent
NGR current setting trip time: 32 settings, 0-60 msec; 150 msec to 3.15 sec in 100 msec steps
NGR current setting thermal characteristics: short time withstand 400A for 1 sec