Filling In The Electrical Safety Picture
EP Editorial Staff | May 17, 2011
Is your organization missing some major pieces in thisvery important picture? These tips will help close the holes.
As plants strive to meet the electrical-safety standards of NFPA 70E, plant maintenance managers have found gaps in their safety programs, especially in regard to arc-flash hazards. As a result, managers are completing the sometimes puzzling picture of electrical safety with fresh strategies, new types of safety devices and improved work practices. This article discusses some of the ways to reduce arc flash and shock hazards.
Arc flash defined
An arc flash is a possibility whenever/wherever there is energized electrical equipment. A short circuit between live conductors or between a live conductor and ground, caused by unmaintained or faulty equipment—or such missteps as crossing two conductors with a voltmeter probe, being clumsy with a screwdriver or slipping with a wrench—can produce a violent reaction as the electricity vaporizes some of the metal parts involved and blasts through the air, releasing large amounts of light, heat, sound and debris.
Arc-flash hazards are a major concern of OSHA, which depends on standards set by the National Electrical Code® (NEC®) and National Fire Protection Association (NFPA) that cover the construction of electrical panels and recommends the types of PPE users are required to wear (and when they are required to wear it). The standards recommend that electrical panels be labeled with such things as the available fault current at each location, the category of arc-flash risk at that location, the arc-flash boundary (defining the distance from the panel within which PPE must be worn) and the level of PPE required.
Levels of arc flash
Energy involved in an arc-flash event is determined by the available fault current and total clearing time of the overcurrent protective device during the fault. The greater the current and the longer it lasts, the greater the incident energy. Calculating the level of arc flash possible at a specific enclosure calls for an arc-flash analysis—a multi-step procedure requiring engineering calculations that’s usually best left to specialists. Elements in the analysis include one-line drawings, available fault current from the utility or generator, wire resistance, calculating maximum available bolted fault currents, minimum self-sustaining arcing current at each location and the clearing times of the overcurrent protective devices.
Table I shows five risk categories based on the thermal energy reaching a victim. It’s important to note that the category classification does not include the effects of arc blast and ejected material.
There are a number of good ways to mitigate arc-flash danger: These include the use of resistance grounding systems, current limitation, protection relays and better work practices. Let’s look at them.
The most important strategy to emerge is resistance grounding, also known as neutral-resistance grounding or high-resistance grounding (HRG). Originally used only in hazardous environments, HRG is becoming popular for general manufacturing, driven by the desire to lower the risk of arc flash.
In an HRG system the center point (the neutral) of the wye transformer is connected to ground via a resistor (Fig. 1), instead of being tied solidly to ground as in a conventional solidly grounded system. If the system uses a delta transformer, then the neutral point for connecting the resistor can be created by using a zig-zag transformer. Under normal conditions, the resistor carries just a small residual current caused by variations in the distributed capacitances among the three-phase feeders, but if one phase shorts to ground, then the voltage at the neutral point will jump up to the system’s normal phase-to-neutral voltage. The fault current under such condition is determined by the resistance to ground. In a solidly grounded system, very low resistance results in large currents and the potential for arc flash. In an HRG system, the resistor is chosen to limit the current to just 5 or 10 amps. This is not enough current to cause an arc flash. Moreover, it will not blow the fuses or trip the breakers, and will allow equipment to keep running until repairs can be made. Since 95% [Ref. 1] of arc-flash events begin as ground faults, converting to an HRG system can dramatically lower the risk of such incidents.
Increasing numbers of companies are seeing the value of HRG systems—the use of which is growing by about 23% per year. Maximizing the advantages of an HRG system, however, requires more than just adding a resistor. For one thing, if no breakers trip and no fuses blow, how do you know if there has been a ground fault?
One way to detect a ground fault is to install a Resistance Grounded Relay (or Neutral Grounding Resistor Monitor) that verifies the neutral-grounding resistor continuity to ground. A relay on each three-phase feed also can detect a ground fault and indicate the faulted feeder. While it’s possible to run with a ground fault temporarily, if that fault isn’t removed, and a second one occurs, a phase-ground-phase fault will result. Because a second ground fault may cause a shock or arc-flash hazard, the first fault should be removed as soon as possible. (A portable zero-sequence meter plus a pulsing contactor can locate the fault.)
There are other important considerations around the installation of an HRG system, including how to do it properly. Your best approach may be to consult with an expert in the field.
Fig. 2. A current-limiting fuse will open in less than half of an AC cycle (8.3 ms), which greatly limits the amount of energy released during an arc-flash event. (Click to enlarge)
Additional ways to deal with arc flash
There are many other methods that reduce the risk of arc flash—directly or indirectly. The simplest is to put current-limiting fuses in the feeders. As shown in Fig. 2, a current-limiting fuse will open in less than half of an AC cycle (8.3 ms), which greatly limits the amount of energy released during an arc-flash event. Current-limiting fuses have another advantage: They can contribute to a plant’s selective coordination, which is the selection of overcurrent protective devices in such a way that an overload or short circuit on one branch circuit will cause only the fuse or circuit breaker feeding that circuit to open, without causing those “upstream” to open. While the NEC requires selective coordination only where loss of power could be hazardous (emergency circuits and legally required standby systems), it’s a good idea for any plant to avoid unnecessary downtime or damaged product caused by avoidable power interruptions.
A faster way to detect an arc flash is with an arc-flash relay. These devices respond to the light created by the arc. Some also look for excessive current (see Sidebar). Advanced models can detect an arc-flash incident very quickly and send a signal to a circuit-interrupting device in a few milliseconds.
Another way to reduce the risk of arc flash is to use ground-fault monitoring relays. These devices identify low-level ground faults before they can grow into large ones, so faults can be addressed while they are small. Similarly, relays that monitor motor insulation can spot ground currents caused by gradual breakdowns in motor insulation before they become major problems. (This capability is a built-in function of some motor-protection relays.)
Motor-protection relays can also be useful during maintenance on energized equipment. Some of these (Fig. 3) have a maintenance mode setting that increases sensitivity temporarily during maintenance or can be used to spot incipient problems. While these types of relays have been considered costly in the past, their prices are now declining. They’re also gaining features—which make them worth considering even for smaller motors.
Better work practices
Improved work practices are another means for helping reduce arc flash and shock hazards. One of the easiest and most practical is just to leave electrical panels open for less time. Remember that personal protective equipment (PPE) is required when opening any energized panel in which there is a possibility of an arc-flash event—so anything that can reduce the need to open a live panel increases safety. If the panel utilizes indicating fuses or indicating fuseholders (see Fig. 4), it will not be necessary to probe with a voltmeter to find which fuse has opened. Even with the power off, indicating fuses will show which fuse is open. And installing a power-shutoff switch on the outside of a panel makes it simple to shut down power before opening the cabinet.
For those specifying an electrical cabinet or panel that will house a motor-overload relay, it’s useful to remember that most overload relays are available with remote-reset devices that mount on the front of the cabinet. The box does not have to be opened to reset the overload relay, which eliminates danger of electrical shock and arc flash. Some overload relays are also available with remote display/keypad interfaces that can be mounted behind cutouts on the face of the panel. These show load condition during operation or can output signals, which can help operators forestall unwanted shutdowns.
Sometimes there’s no alternative to opening a live panel, which means the worker MUST wear the appropriate PPE. Too much gear, though, can obscure visibility and make workers uncomfortable, paradoxically decreasing safety. Furthermore, the more gear that needs to be worn, the longer the job takes and the less likely it is that workers will want to wear it. The level of required PPE varies with the level of arc-flash hazard. Thus, anything that can decrease the required level can reduce work time and the amount of PPE the worker must wear. Reducing the potential energy, using methods ranging from simply using current-limiting fuses in the feeders to more elaborate methods like HRG, reduces the level of potential arc flash.
There are a number of ways to cut the danger from arc flash and shock hazards. Some are more elaborate than others; some help prevent or mitigate other problems. Remember that right after protecting workers from arc flash and shock hazards, comes the responsibility to protect your operations and keep them up and running. Fortunately, there are ways to help ensure both. MT
1. Dunki-Jacobs, et al. Industrial Power System Grounding Design Handbook, Dexter; Thomsom-Shore, 2007.
Understanding Arc-Flash Relays
Light is one of the earliest warning signs of an arc-flash event. An arc-flash relay, like the new Littelfuse PGR-8800 pictured here, can trip in less than 1 ms at light intensities as low as 10,000 Lux. In environments with high amounts of ambient light, it’s possible to increase the trip threshold to prevent nuisance tripping. Because energy is a function of time, a fast response limits the amount of energy released and can significantly reduce the damage caused by an arc flash. Advanced arc-flash relays also measure current by using current transformers on each phase. Not only does this provide overcurrent (or short-circuit) protection, but by combining overcurrent and light detection it is possible to recognize dangerous situations faster than standard overcurrent protection devices. This also minimizes nuisance tripping associated with heavy current draws due to normal operation or from activities that involve light, such as welding.
Typically, arc-flash relays are installed in a switchgear electrical panel. The PGR-8800 allows for up to 24 strategically placed light-detecting sensors to comprehensively monitor any panel configuration. Alternatively, this device allows the use of a 360° fiber-optic cable looped throughout the panel for challenging spaces. An arc-flash relay augments existing electrical-safety devices, so its implementation is straightforward. The ability to log data and the ability to communicate status and alarms to other systems are also important factors in a well-integrated protection solution.