- With the current focus on arc flash safety and the availability of means to upgrade obsolete and poorly maintained equipment, the path to safety and improved reliability is easier to navigate.
- The first step toward electrical safety is to be aware of the implications of poor maintenance and make a decision to improve it.
- Today there is a significantly heightened awareness of arc flash hazards, and consulting engineers who design power systems for new facilities are striving to design the safest possible system for their clients.
Electrical arc flash accidents that result in a serious injury or fatality occur five to 10 times every day in the United States. Approximately one fatality per day results from an electrical arc flash accident. The human consequences of these accidents are devastating to the victims and their families. If the victim survives a serious electrical burn injury, the injury is often so disabling that the victim is never able to recover sufficiently to return to work. The financial consequences are also very damaging to the employers and their insurers.
Electrical accidents have been occurring for more than 100 years, and the accident statistics have been fairly consistent until recently. The electrification of facilities that began during the early industrial revolution has continued with facility growth and process demands constantly requiring more and more electrical power density to sustain operations. At the same time, the manufacturers of electrical equipment improved on their products, developing improved designs to more safely control the electrical energy, protect workers and others from injury, and provide high reliability, reduced maintenance requirements, and long equipment service life.
Figure 1. With the current focus on arc flash safety and the availability of means to upgrade equipment, the path to safety and improved reliability is easier to navigate. (Source: Fluke)
With the current focus on arc flash safety and the availability of means to upgrade obsolete and poorly maintained equipment, the path to safety and improved reliability is easier to navigate, but the first step is to be aware of the implications of poor maintenance and make a decision to improve it (Figure 1).
Development of the electrical safety standards
The hazards of electric shock have been fairly well understood by most qualified electricians, and they have been reasonably well trained in the techniques of working safely to avoid shock and electrocution hazards. Arc flash and blast hazards were not really well understood until the late 1990s.The reason for the lack of awareness stems from the fact that, until 1999, consistently accurate methods to calculate potential arc flash hazards did not exist.
In spite of the statistical frequency of arc flash accidents prior to 1999, the amount of arc flash incident energy (thermal energy) wasn’t easily predictable, so means of protecting workers (personal protective equipment) wasn’t readily available for arc flash hazards. IEEE Standard 1584, IEEE Guide for Performing Arc Flash Hazard Calculations, was published in 2002. This document contained accurate formulas to calculate prospective incident energy levels based on empirical data from repeatable and accurate laboratory tests that were performed.
The first mention of arc flash hazard in NFPA 70E (Standard for Electrical Safety in the Workplace) appeared in the 1995 edition of that consensus standard. Subsequent editions of NFPA 70E have documented improved safety practices through the use of personal protective equipment (PPE) and other safety equipment and safer work procedures. Since then, considerable work has been done to improve arc flash calculation methods, improvements on PPE design to protect workers, training, and increased awareness of arc flash hazards (Figure 2).
Arc flash — causes and effects
Figure 2. Considerable work has been done to improve arc flash calculation methods, improvements on PPE design to protect workers, training, and increased awareness of arc flash hazards. (Source: Salisbury by Honeywell)
An arc flash results from some condition that compromises the insulation distance between two phase conductors or a phase conductor and a grounded conductor or surface. The cause is rarely the result of equipment failure. Evidence suggests that a very high percentage (more than 95%) of these events that cause injury or fatality to a worker were caused by some unsafe act by the worker.
A tool or other conductive metallic object dropped into energized electrical equipment or a tool that slips and shorts between components is a very common cause of arc flash accidents. Once the arc forms, it heats the air around it very rapidly and the space around the arc becomes more conductive because of ionized metallic particles within the arc plasma. This exacerbates the condition and the arc will continue to develop as long as there’s sufficient voltage and current feeding the arc.
The arc plasma temperature can in some cases exceed 35,000 °F, so the primary mechanism of injury is the thermal release of incident energy causing tissue burns and clothing ignition. A secondary arc hazard is called arc blast. Under certain arc conditions a pressure wave develops reaching a pressure of 2,500 lb/sq ft. This blast pressure wave can cause significant injury, as well.
The amount of the arc flash thermal energy (incident energy) that is released in the arcing event depends on many factors, including voltage, available fault current/arcing current, and, most significantly, the time duration of the arc until it is extinguished. The incident energy released is directly proportional to the time duration of the arc. If the arc duration doubles, the incident energy doubles, as well. Overcurrent protective devices (circuit breakers and fused devices) must be capable of recognizing the fault condition and opening the circuit very quickly to extinguish the arc.
Response time is key
Once an arc initiates, there are only two things that will stop it. Either the circuit protective device ahead of the arc will sense it and open the circuit (fuse or circuit breaker, for example) or the arc will burn enough conductive material away so that the arc gap distance becomes so great the arc can no longer bridge the gap, and the arc collapses. In the first scenario the arc duration is usually expressed in tens of thousandths to a few hundreds of thousandths of a second (milliseconds). In the second scenario the arc might continue for seconds or even minutes, and this is an extremely hazardous situation for the worker and for the equipment in the system. Ideally the arc duration would be less than one electrical cycle. One electrical cycle is about 17 thousandths of a second (17 milliseconds).
Where arc flash hazards exist
In single-phase and three-phase AC systems, lower voltages such as 120 V do not present much propensity to develop a dangerous arcing condition. Even the 208-V phase-to-phase voltage in three-phase-system voltages normally can’t produce arcs that sustain, even under the most favorable test conditions. It has been shown in many laboratory tests that arcs will not sustain at these lower voltages long enough to become a serious arc hazard. However, IEEE 1584 and NFPA 70E both take a very conservative approach on this.
However, all voltages of 240 V and greater can and do produce very dangerous arc flash conditions. In fact there are probably far more serious arc flash injuries in three-phase systems operating at 480 V than in any other system voltage in common use in the United States. The reason for this is 480 V three-phase systems are installed in almost every facility of any size in the United States, and workers are often far too comfortable with them to be fully aware of their hazard potential.
Designing for safety
Today there is a significantly heightened awareness of arc flash hazards, and consulting engineers who design power systems for new facilities are striving to design the safest possible system for their clients. The normal best practices in electrical system design that were commonly used in the past, in some cases, are no longer the best design choices for arc flash potential reduction. This “design for safety” is becoming more common, resulting in safer electrical systems for new construction projects.
The roots of the problem
For existing facilities, there are several conditions that affect the safety of the system. The first of these is the age of the equipment. It’s common to find electrical power and control equipment in service that is more than 50 years old, and most of this equipment is no longer supported by its manufacturer. In some cases the equipment manufacturer — for example, Federal Pacific Electric or Westinghouse — is no longer in the electrical power equipment business. Power equipment is designed to have an effective service life of 25-30 years, with proper maintenance.
Figure 3. Failures are usually the result of normal aging of components, which is exacerbated by failure to perform required regular maintenance and testing. (Source: Fluke)
Electrical power and control equipment can be thought of as having two types of components — passive components and active components. Passive components are such things as equipment steel enclosures and structural framing, busbars, cables, and insulators. Active components are the circuit protective devices such as circuit breakers, protective relays, and fusible devices, as well as some metering devices.
Electrical equipment failures
An unusual characteristic of electrical equipment circuit protective devices is that they will almost always fail in a closed position, giving absolutely no indication of that failure. Even after failure occurs, they will stay closed and continue to provide power. Operation in the failed condition can sometimes extend for years or even decades. The failure will not be discovered until there is a critical fault event that occurs, or someone does performance testing of the device. These profound failures are usually the result of normal aging of the components, which is exacerbated by failure to perform required regular maintenance and testing (Figure 3).
Most everyday consumer devices, such as automobiles, refrigerators, televisions, and air conditioners, do not exhibit this unusual failure-mode characteristic. When these devices fail, the failure “feedback” is immediately obvious to the user, because they no longer perform their intended function.
Why maintenance and testing are important
Electrical circuit protective devices are critical for arc flash safety and system reliability and protection because they are designed to detect a fault condition and operate very quickly to disconnect power to the faulty circuit. Since clearing time is the critical element affecting the release of arc flash incident energy and the subsequent hazard level, poorly maintained circuit protective devices either will not operate as quickly as designed or, in the worst case, will not open under any fault condition. In that case, it’s impossible to predict arc flash hazard levels so workers can sufficiently protect themselves with personal protective equipment when they must work on the equipment while it’s energized.
Most electrical power equipment is so well designed that it will tolerate gross maintenance neglect over a long period of time and continue to provide power. It will usually survive in service much longer than it was designed for, and it will appear to continue to be fully functional, even long after it has failed. Because of these high reliability characteristics, owners of this equipment often don’t consider maintenance to be a very high priority on their financial priority lists. Often the equipment doesn’t get any attention until it explodes or the lights go out and production stops. When that happens, maintenance money seems to come from every direction to get the power restored quickly. Breakdown maintenance is far more expensive, as well as hazardous, than preventive maintenance is.
Electrical equipment maintenance requirements
Figure 4. Maintenance requirements for electrical equipment will vary, depending on factors such as the operating environment, duty cycle, and the type of equipment and its voltage class. (Source: Salisbury by Honeywell)
Maintenance requirements for electrical equipment will vary, depending on factors such as the operating environment, duty cycle, and the type of equipment and its voltage class (Figure 4). Small molded-case circuit breakers, for example, require almost no maintenance. Their internal mechanisms are designed to be relatively maintenance-free over their designed service lives. Maintenance requirements for these devices are limited to operating the breaker on and off two or three times annually. This is called “exercising the breaker,” which helps to distribute lubrication in its pivot/bearing points and wipes the pole contacts to keep them clean. Circuit breaker plastic cases should be examined periodically to ensure the cases aren’t damaged or cracked. Cable lugs should be checked to ensure they’re tightened to factory specifications. Periodic infrared inspections are a good way to easily identify any loose cable lug terminations.
The other end of the electrical maintenance requirement spectrum is large power circuit breakers, either in the low-voltage or medium-voltage categories. These are very complex electromechanical devices and may also contain digital electronic components, typically in their trip units. Power circuit breakers usually control larger blocks of power in the distribution system, so failure can cause very widespread process disruption. Because they’re typically higher-amperage devices with higher available fault currents, a malfunctioning device may have a very high risk factor for arc flash worker safety, as well.
Maintenance requirements for these larger power devices are quite extensive. Maintenance and testing for this kind of equipment should only be performed by qualified electrical maintenance personnel who know the maintenance protocol and have the test and safety equipment to do the work properly and safely. Equipment-specific maintenance and testing requirements can be found in the original equipment manufacturer’s operation and maintenance documentation or in a document titled, “NFPA 70B - Recommended Practice for Electrical Equipment Maintenance.” The current edition of NFPA 70B is the 2010 edition, and it’s available to order at www.nfpa.org.
Organizations often lack the internal resources to do this kind of testing and maintenance, but many suppliers have the expertise to do this work effectively and safely. For liability reasons it’s always a good practice to make certain the company you choose follows the electrical safe work practices outlined in NFPA 70E and enforced by OSHA. Safety-related maintenance requirements for electrical equipment comprise Chapter 2 in NFPA 70E 2012.
Retrofit or replace?
At some point in the life cycle of your electrical distribution system, decisions must be made to replace the equipment. Maintenance and testing costs for very old equipment will eventually reach a point of diminishing return, and replacement becomes the best option. It seems to be a simple decision to tear out the old equipment and put in new equipment, but there are some things to consider that are certainly not widely understood.
First, consider the length of the process or production outage for this strategy. Demolition and replacement can often take weeks to complete, even when carefully planned, staged, and executed. If anything goes wrong, as it inevitably will, things can get out of control very quickly.
Second, consider the footprint of the equipment, that is, the overall dimensions of the existing equipment. For example, low voltage 480-V switchgear that was made before the mid-1980s typically had section widths of 30 in. or 32 in. That means each vertical section was 30 or 32 in. wide. If the switchgear had six vertical sections, then the overall width of the switchgear would be 180-192 in.
For most manufacturers, the current vertical-section-width design for that type of equipment is considerably narrower, perhaps by 8 or 10 in. Of course, smaller seems better, but the problem that most people fail to anticipate is the placement of the conduits in the slab below, or overhead conduits above the equipment. With narrower equipment, if the conduits won’t line up with the new vertical sections incoming conduit spaces, the costs of and the time required for the replacement project can escalate very dramatically.
|Joseph Weigel is owner of Electrical Safety Works in Nashville, Tennessee. Contact him at firstname.lastname@example.org or (615) 628-8736.|
Good options exist today to retrofit the existing equipment in place to a like-new condition and with reasonable cost and minimal outage and process interruption. The reason this is possible is because some equipment suppliers have developed replacement products for the active components called “direct replacement” devices. Versions of these direct replacement devices are available for nearly all equipment regardless of the manufacturer of the existing equipment.
This retrofit option is typically used for low-voltage or medium-voltage switchgear, low-voltage switchboards, and low-voltage motor control centers. Similar options are available for lighting and distribution panel boards, but, in the case of panel boards, the entire panel interior with its circuit breakers is replaced, and a new custom cover is made to fit the existing back box.
With power equipment upgraded in this way, the equipment will be in like-new condition, with an expected service life the same as new equipment. At the same time, both reliability and safety are vastly improved, and, in most cases, future maintenance requirements and costs will be substantially reduced.