There are effective tactics for reducing risk, but most attack the problem in a piecemeal fashion: a fuse here, a relay there, at equipment all over the building. Many electrical professionals are seeing the limitations of traditional protections, and they’re considering a systemic approach that prevents phase-to-ground arc flash from forming anywhere in the system.
Resistance grounding systems are getting a fresh look; their use is increasing 23% a year. If a phase faults to ground, the resistance limits current to just a few amps, not enough to cause downtime by tripping the overcurrent protection device, and not enough to produce an arc flash.
Electrical professionals must consider many things when converting the facility’s solidly grounded system or ungrounded system to a resistance-grounded system. That’s why it’s important to know about resistance grounding, how to implement it, where it can be used and where it isn’t appropriate.
Figure 1. Ungrounded systems can continue to operate if one phase faults to ground but can be subject to transient overvoltages and difficulty locating ground faults.
Some older plants use ungrounded electrical systems, with a delta-connected transformer (Figure 1). Their advantage is they can continue to operate if one phase faults to ground, which is particularly important if continuity of power is critical. For example, chemical plants or refineries involving processes that can’t be interrupted without extensive dollar or product loss might have ungrounded systems. However, ungrounded systems have major problems, including transient overvoltages — a series of arcing ground faults can increase overall system voltage relative to ground to several times its normal value — and difficulty locating ground faults. For these reasons, many have been replaced by grounded systems.
Solidly grounded systems A grounded system (Figure 2) is wye-connected, with the neutral — the center of the wye — solidly connected to ground. Three-phase loads are connected to three phases and single-phase loads are connected either to phase-to-neutral or phase-to-phase. It’s straightforward, and everyone understands how it works. But it also has some drawbacks.
Figure 2. A grounded system is wye-connected, with the neutral solidly connected to ground. While it is simple, ground faults can shut it down and allows severe arc-flash hazards.
The main drawback has to do with ground faults. If a phase conductor shorts solidly to ground, the overcurrent protective device will operate and shut off the affected phase. The other phases might not shut down immediately as well, which can mean that some motor loads might single-phase for a while. In either case, the loads connected to the affected phase get shut down, and production is disrupted.
But the ground fault might not be a solid short to ground; it might be an arc, and not draw enough current to trip the overcurrent protective device. In addition, the arc can initiate a dangerous arc flash, which is why power panels are required to have warning labels, and anyone working on an energized power panel must wear personal protective equipment (PPE).
The severity of an arc flash depends on system voltage and, more importantly, the available current, which can reach 100,000 A. Current-limiting fuses allow only a certain amount of energy to pass before they open the circuit, but they respond best to a solid short circuit, a so-called bolted fault.
There has to be a better way
Another way to ground a plant power system allows it to continue operating even if one phase faults to ground, while getting around the drawbacks of the ungrounded system. Overcurrent protective devices won’t trip, loads continue to operate and maintenance personnel can schedule an outage when it’s convenient rather than having to rush repairs because production has stopped. This grounding method also helps to eliminate of the danger of arc flash, and it protects against transient overvoltages. We’re talking about a high-resistance grounding (HRG) system, also known as a resistance grounded (RG) system, high-impedance grounded neutral system, or a neutral grounding resistor (NGR) system. HRG systems aren’t new; in fact they’ve been mandatory for power wiring in mines for years, and they’re widely used in the petrochemical industry, but only now are they being considered for general industrial use. HRG systems are most often used on systems at or below 600 Vac, which the National Electrical Code (NEC), NFPA 70, article 250.36 permits.
Figure 3. In a high resistance grounding system the neutral point of the wye transformer is connected to ground through a high-value neutral grounding resistor, which normally carries little or no current.