Minimizing the impact of electrical fires in power plants requires a combination of prevention, compartmentalization, detection, and suppression strategies. But first, everyone in a plant needs to understand the hazard.
Fires at electric generating stations are rare—but not as rare as one might think. Loss history at hydroelectric facilities, for example, shows that fires involving electrical equipment other than generators and transformers account for the highest likelihood of experiencing a fire.
Although the majority of electrical fires are small and extinguished quickly, some have had severe consequences. Occurrences at Thermalito Power Plant (California) in 2012, Detroit Dam (Oregon) in 2007, and Watts Bar Hydroelectric Plant (Tennessee) in 2002 resulted in major electrical fires. In almost all cases, the fires caused forced shutdowns, some for a year or more. The loss of generation as the result of an electrical fire often outweighs the actual fire damage.
If damaged cables cannot be spliced, large sections of cables must be replaced. If the fire is severe, concrete buildings can be damaged through spalling. Even after a relatively small fire, smoke and soot removal can cost several millions of dollars in facilities with large open spaces. For example, cleanup costs at Thermalito were reported at approximately $90 million, with a total cost of the fire around $200 million.
Electrical equipment carries energy and comprises combustible materials in the form of insulation. Electrical fires typically follow a fault and result in a smoldering slow-growth fire that can eventually become self-sustaining while growing exponentially. Products of combustion usually involve thick dark smoke (Figure 1).
|1. Where there’s smoke… This photo shows a full-scale fire test of a cable tray arrangement typically found in power plants. Courtesy: Mike Mowrer, HSB Professional Loss Control|
Electrical fires are Class C fires; however, the term “electrical fire” is something of a misnomer, as it is the combustible insulation that burns, rather than the conductor or the electrons. Once the energy source is removed, electrical fires become Class B fires.
Unlike flammable/combustible liquid fires, electrical equipment fires are typically slow-spreading. Breakers usually fail suddenly and catastrophically. In many cases, there is no ensuing fire, and the damage is contained to the breaker and its housing. However, when there are nearby combustibles (such as cable trays), the extreme heat can cause a major fire.
Electrical hazards are different than “special hazards” (such as turbines, conveyors, hydro generators, and the like) because electrical wiring, motors, and breakers proliferate throughout the plant. It is feasible and common industry practice to provide fixed fire protection equipment for the special hazards, but not necessarily for common hazards. In other words, it is usually impractical to provide fire suppression systems throughout the entire powerhouse.
Establish Risk Tolerance
It may seem obvious that it is every owner’s goal to prevent and protect against electrical fires, but it is not quite that simple. The reduction of fire risks has a price—both in monetary cost and in equipment accessibility. Owners must therefore establish their risk tolerance before fire protection solutions can be applied. In other words, the lower the risk tolerance, the more elaborate the fire prevention and protection solutions will be.
The nuclear power industry, being at the vanguard of risk assessment, has driven methodologies for determining an owner’s risk tolerance. This tolerance defines fire protection objectives. The four common categories are summarized below:
■ Category I: No electrical faults or cable damage are acceptable.
■ Category II: Limited cable damage such as charring is acceptable, but faulting is not. A fire event would not result in a plant shutdown, and damaged cable will be replaced at the next scheduled outage.
■ Category III: Both cable damage and faulting are acceptable, but fire propagation is not. Cables can be repaired with a minimal plant outage.
■ Category IV: The total loss of all electrical equipment within the fire compartment is acceptable; however, the fire should not spread beyond the compartment.
There is no “right” or “wrong” category; the important thing is that owners realistically determine their risk tolerance. In the author’s experience, most generating stations fall into Category II or III.
The bottom line is that almost all cable insulation burns because it is made of a combustible thermoplastic. There are numerous standards for assessing and limiting the flame spread of cable insulation, and many publications have been written on this subject alone. Although some of the test standards attempt to duplicate what is likely to happen under large-scale conditions, most tests only evaluate the flammability and fire propagation of a single cable. It is important to understand that these tests are conducted in a laboratory under a specific set of conditions, usually with the goal of comparing different cables with each other. The biggest problem with these tests is that they cannot predict the actual configurations in which the cables will be installed.
Recognizing this limitation, the flame spread index can be used as an indicator of fire risk. Generally, FM Global–Approved Group 1 cables, cables with a flame spread index less than 10, or a flame spread distance of 5 feet or less when tested in accordance with National Fire Protection Association (NFPA) 262, are recognized as “non-propagating,” whereas others are considered “propagating.”
Unfortunately, it is very difficult to perform an accurate assessment of the cable types and their flammability ratings in existing facilities, especially at older plants, where documentation may be lacking and the cable layout has been modified over the years. In addition, age can affect the physical properties of the insulation.
The advancement of fire-retardant cables, although positive, is not the alpha and omega of fire protection, and “flame-retardant” cables are not a substitute for proper fire protection methods. The conservative approach is to treat all cables as propagating unless there is satisfactory evidence to the contrary.
Fire Protection Solutions
The most effective strategy to minimize the impact of electrical fires is a combination of prevention, compartmentalization, detection, and suppression.
Obviously, prevention is most desirable, but it has limitations simply because it is not practical to remove all ignition and fuel sources. The idea of compartmentalization is to limit fire spread by using fire barriers. Detection alone alerts personnel to a developing fire, but it does not play an active role in suppressing and extinguishing the fire. Suppression acts by stopping fire growth and reducing fire size. Detection and suppression together not only alert personnel but also involve taking automatic mitigating action, usually without human intervention.
The remainder of this article presents some common fire prevention and protection techniques used against electrical fires. These can be used individually or together, in a systematic approach to fire protection. Some can be performed easily and inexpensively while others are quite costly.
Electrical Equipment Layout. The best opportunity for prevention presents itself during the design stage. Unfortunately, the drive for low installation costs and ignorance are responsible for aggravating electrical fire hazards, which can be difficult and expensive to mitigate once a facility is constructed.
Locating electrical equipment in such a way that it is not unnecessarily exposed by other potential fire hazards should be a key design goal. Also, the geometry of grouped cables should be such that the fire hazard is minimized. Fire likes to travel in a vertical direction, and fire intensity is exponentially proportional to the stacked height of the fuel. For example, flame spread in vertical trays can be three to 10 times faster than in horizontal trays. For this reason, vertical cable shafts and vertical stacking of cable trays present the highest fire risks with grouped electrical cables (Figure 2).
|2. High risk. Numerous vertical levels of wide, stacked trays present a challenging fire protection situation. Courtesy: Dominique Dieken|
Good electrical layout includes keeping control cables and power cables well-separated, avoiding running cable trays directly over other fire hazards (such as hydraulic equipment) or where they are exposed to potential ignition sources, and minimizing vertical stacking. Also, cable trays should not be overloaded. Two lightly loaded cable trays are preferable over one stuffed tray (Figure 3).
|3. Reduced risk. Lightly loaded narrow cable trays have a lower fuel load and, thus, a lower fire risk. Courtesy: Dominique Dieken|
The Nuclear Regulatory Commission has developed free calculation tools, including a spreadsheet that calculates the full-scale heat release rate of a cable tray fire of a given area using various types of insulation. Although this does not apply to stacked trays, it could be useful to designers in configuring layout. A better alternative to cable trays consists of routing cables in metallic conduit, where feasible.
Firestopping. Cables passing through openings in substantial walls, ceilings, or floors should always be sealed against the spread of fire and smoke (Figure 4). This requires a special method using noncombustible materials.
|4. No! This cable tray penetration through a wall lacks firestopping. Courtesy: Dominique Dieken|
Fire codes and the National Electrical Code require that any penetrations through a rated fire barrier be properly sealed in accordance with a listed method that matches the hourly fire resistance rating of the fire barrier/division being penetrated (Figure 5). The idea is to restore the integrity of the fire barrier after making the openings for the penetrations. Even if the barrier being penetrated is not a code-required fire barrier, it is still a good idea to provide firestopping to prevent fire spread from one compartment to another.
|5. Yes! This cable tray penetration through a wall has proper firestopping. Courtesy: Dominique Dieken|
For large and complex penetrations such as cable trays, firestopping is usually performed by a dedicated trade due to the necessity for special knowledge and training. The proper design and installation of firestopping is beyond the scope of this article; thus, if in doubt, an engineer and/or firestopping contractor should be consulted. Relatively easy applications within reach of most plant staff include ceramic wool, firestop foam plugs, and modular firestop pillows. Care should be taken to install the sealing system in strict accordance with the manufacturer’s instructions.
Fire Detection. The purpose of detection is to either provide early warning of a developing fire or as a trigger for an automatic extinguishing system. In the early warning mode, the goal is to have notification of a fire while it is in its incipient stages so that facility personnel can notify the fire department, shut down equipment, and take first-aid suppression actions. It is therefore critical to not only locate detectors in accordance with minimum code requirements but also in a manner that their technological abilities are used to the best possible advantage.
Many methods of fire detection are available, but for most areas, spot-type smoke detectors offer an adequate level of protection. Such detectors should be installed throughout all areas where electrical equipment and cables are present. NFPA 72, National Fire Alarm and Signaling Code, and the detector listing must be adhered to for an adequate and reliable installation. The standard 30-foot maximum detector spacing is reduced significantly if the ceilings comprise beams. There is no minimum detector spacing, and the closer the spacing, the faster the response will be.
Photoelectric smoke detectors are preferred to ionization smoke detectors for electrical fires, as they respond faster to slow-growth smoldering fires. The fastest response smoke detection is an aspirating air-sampling system, which approaches the sensitivity level of the human nose, but such systems also carry a higher price tag and require frequent maintenance. A very effective and relatively inexpensive method of fire detection for cable trays is to install linear fire detection cable (such as Protectowire) in a zigzag arrangement on top of the cables in a tray. In this arrangement, a controller monitors a continuous nominal current flow through the wire. If the controller senses a change in current flow due to increasing temperature, an alarm is initiated.
It should be noted that the presence of smoke detectors in air-handling equipment is not a substitute for area fire detection, because the sole purpose of duct detectors is to shut down air-handling equipment to avoid spreading smoke and fire within the equipment or from the outside.
Fire Protection for Cable Trays. The decision of when to provide fire suppression is less clear-cut than when to provide detection. Although complex methodologies for calculating propagation rates, time-to-ignition, and tray heat release rates have been developed, these offer little practical use to those performing field-based fire hazard analyses.
In very broad terms, if the cable loading is light and/or the cable arrangement is such that a fire is unlikely to propagate along the trays, then suppression is usually not warranted. Unfortunately, the definitions for “light” and “heavy” are subjective. FM Global has developed a more user-friendly method of analyzing horizontal and vertical cable tray spacing necessary to prevent fire spread for propagating cables. The criteria involve physical separation between the trays ranging between 2.5 and 4.5 times the width of the tray in a vertical direction and 1 to 1.5 times in a horizontal direction. For example, to prevent propagation, 24-inch wide trays should be vertically separated from each other by 6 feet and horizontally by 2 feet. Such separation distances are seldom found in actual installations.
The provision of fire suppression should be considered for large concentrations of grouped cable similar to those found in cable spreading rooms, under the floor of computer and/or control rooms, and multiple stacked vertical trays in cable tunnels, as the consequence of the fire damage in these areas is typically too great to accept.
If it is determined that all cables are nonpropagating or have propagation flame spread distance of 5 feet or less when tested in accordance with NFPA 262, water spray protection may not be necessary, as stipulated in FM Global Data Sheet 5-31. If the need for suppression has been determined, automatic sprinkler protection should be considered first. Wet sprinklers are the most reliable and least costly method of fire suppression and have been shown to be well-suited for cable exposures (Figure 6). For complex cable tray arrangements, deluge water spray or gaseous suppression agents (carbon dioxide or clean agents) should be considered.
|6. Standing guard. This cable gallery is protected by sprinklers (red pipe at ceiling). Courtesy: Dominique Dieken|
Manual Fire Suppression. Because electrical fires are slow-growth fires, many have been successfully extinguished in their early stages by plant employees or fire departments using portable extinguishers subsequent to de-energization of the applicable electrical circuit.
As previously noted, the key to successful manual extinguishment is early detection. The preferred extinguishing agent is carbon dioxide, as it is nonconductive, leaves no residue, and is inexpensive. Dry, multipurpose powders also yield adequate extinguishment, but they are corrosive and leave a residue, which can mean additional damage and downtime.
Consideration should also be given to the judicious placement of wheeled fire extinguisher carts in areas with higher concentrations of electrical equipment. Wheeled units are capable of delivering much higher agent flow and stream ranges than standard handheld fire extinguishers and thus furnish increased fire-extinguishing effectiveness. Wheeled units are movable, can safely be used by one person, and fall under the training requirements of portable extinguishers per NFPA 10, Standard for Portable Fire Extinguishers. They are a very effective tool for both facility personnel and the fire department, which should not be overlooked.
Good Housekeeping. Last but not least, electrical equipment areas often tend to be used as storage areas, workshops, and break rooms. Technicians like to keep their tools and supplies close to their work spaces, and others think of these areas as ideal out-of-the-way locations for anything that doesn’t fit in closets. Of course, the problem with this practice is that the ignition and combustibility limitations intended by the designers and codes are defeated.
For example, countertop appliances introduce ignition sources, while paper towels, packaging materials, and furniture increase the fuel load. The likely fire consequence will be drastically different in a space with good housekeeping than in a space with poor housekeeping practices. The bottom line is that electrical rooms should not be used for any other purposes than what they are intended for. ■
— Dominique Dieken, PE, CFPSis a consulting engineer with 26 years of fire protection experience. He specializes in performing risk assessments in the power generation field and other heavy industries.