BY DOUG LEIHBACHER
Manholes provide access to underground concrete vaults and subterranean conduits that run below city streets. Some of these conduits carry storm-water runoff; some carry sewer lines; and others provide passageways for telephone, electric, or gas utilities. The depth of these manholes can range from a few feet to a couple of stories.
Electrical utility company manholes are the focus of this article. They carry high-voltage feeder cables, switches, relays, breakers, batteries, and transformers. Others carry gas lines. The underground vaults on which these utilities are mounted vary in diameter from a few feet to the size of a walk-in closet. Large, high-voltage feeder cables are mounted on the walls of these vaults. These cables can carry voltages in excess of several thousand kilovolts (kV). When things go wrong, powerful explosions can result.
CAUSES OF MANHOLE EXPLOSIONS
Electrical cable failure is at the source of the majority of manhole incidents. Damage to the electrical cables can result from manufacturing defects, failures at splices, or the excessive buildup of sewer gas. However, the corroded power cables caused by water in the manholes is the most frequent cause of manhole incidents. Fires and explosions occur in manholes when CO, sewer gas, or natural gas ignites in the confined space of the conduit belowground. The source of ignition is typically an arc or electrical short circuit in one of the feeder cables. As the heated gases expand within the confined space, they create underground pressure seeking an opportunity to vent. This pressure will blow off at the weakest point, the manhole cover. Smoke from the electric cable’s insulation is often seen before and in the aftermath. Occasionally, fire is showing from the open cover. Once the cover is dislodged, more oxygen may be introduced into the chambers below the street, triggering additional explosions.
 (1) Green flames issue from a blown manhole. The green flame evolves from superheated copper wire below the street. (Photos by author unless otherwise noted.) |
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 (3) Robust three-phase stranded feeder cables enter a vault through a four-inch-diameter pipe. Cables this size carry high voltage; treat them with the utmost caution. |
Older cities such as New York; Boston; and Washington, DC, have a long history of launched manhole covers and other manhole-related mishaps. Manhole problems occur most frequently during cold spells or heat waves; heat and cold strain the infrastructure below the street. In the summer, excessive demand can overload and overheat feeder cables. When the wires overheat, the insulation smolders within the below-grade confined space (where oxygen is in short supply). The ensuing incomplete combustion gives off carbon monoxide (CO) and a whole slew of other flammable gases from the decomposition of the wire. When the CO finds an ignition source, it can explode, dramatically lifting covers. If no ignition is found, the CO can seep into the basements of nearby buildings, causing a significant life hazard. By the same token, in the winter, salty runoff from melting snow carries the residue of rock salt down from the streets into the manholes, where it acts to corrosively degrade pipes and cables belowground, prompting similar results.
OPERATING RULES
The following are some guidelines for maximizing safety when operating near manholes.
1 Do not park over or step on a manhole cover. Keep a safe distance from a smoking manhole cover. Manhole covers can be under tremendous pressure from explosive gases below. Unlike sewer lines, which employ open grates to release pent-up gases, utility manhole covers are solid to seal out rain and runoff as much as possible. Even though the covers can weigh up to 300 pounds, they are the weak link in the system and will pop unexpectedly. These cast-iron covers have been known to fly three to four stories high. (In one case, a cover was found on the roof of a six-story building.) When they blow, they can cause serious injury. Therefore, positioning of the apparatus is very important.
When possible, place apparatus at opposite ends of the block, out of the smoke plume and a safe distance from manhole covers that may be propelled in the air. Members are familiar with the rule of thumb to spot the apparatus one pole length on either side of a downed electrical wire. Apply a similar rule here: When positioning apparatus, be sure to stay clear at least one cover in either direction of the affected manhole (the one pushing smoke or steam). When a manhole cover exhibits smoke, steam, or flame, block off the affected streets with the apparatus. Have the police divert pedestrian and vehicular traffic. Park engines near hydrants in case manhole fires spread to adjacent buildings. If a cover blows, keep an eye on it so you can move away, if necessary. Anticipate that other covers could also be propelled.
Wear your helmet when operating at incidents involving manholes. Stand clear of all covers, and avoid walking next to cars parked over an affected manhole cover. A striking example of what can happen at a routine manhole emergency occurred in 2006, when New York City Firefighter Marc Kroenung was injured when an SUV lifted several feet in the air by an exploding manhole cover landed on him. Kroenung had been operating at a smoking manhole and was returning to his apparatus for a tool when a second manhole blew. The SUV, a Cadillac Escalade, was parked over the cover, and the explosive force was enough to lift the vehicle over his head. The SUV rotated in the air and came crashing down on its driver’s side just as Kroenung, walking alongside, stepped into alignment with the driver’s side window. This happened so fast that he did not have time to react; fortunately, he was in the right place at the right time. Covered in a shower of glass, he found himself vertically encapsulated in the front passenger compartment. Nearby Engine 73 crew members raced to the vehicle, removed the windshield, and freed him. His helmet, gear, and a large allotment of good luck protected him from a life-threatening injury.
2 Call the utility company immediately, and set up a unified command post. Utility companies employ trained emergency responders to handle gas and electrical emergencies. They manage these emergencies frequently, carry specialized equipment, and have maps of the underground grid that include the locations of shutoff switches and valves. Incident mitigation likely will be contingent on isolating the involved feeder cables. Once the utility workers cut the power to the affected area, they bring in the specialized gear needed to repair the damaged equipment. In the majority of cases, the fire department will not be called to assist directly with the involved manhole. The firefighters’ most important responsibilities are to secure the scene; protect exposures; and monitor conditions in nearby occupancies, evacuating occupants if necessary. Getting control of the scene and limiting access to pedestrians are equally important. Nearby metal utility poles and sidewalk grates could be live. Similarly, people sitting in parked vehicles should be removed if they are parked over a cover (see number 1).
3 Do not pull manhole covers to relieve pressure or heat. In keeping with the practice of letting the utility company take the lead in directly handling the manhole, avoid pulling covers. Although ventilation may be a good tactic, firefighters should not pull manhole covers in an attempt to release pressurized gases. Pulling covers without direction from a trained utility company official could trigger additional upsurges and injure firefighters. Let the utility company pull the covers if they need to. In fact, firefighters should not touch anything metallic (such as a light pole or a sidewalk grate) in the vicinity of a smoking manhole; it could be energized. Instead, cordon off the area around the cover with security tape, and keep pedestrians and onlookers a safe distance away. Ventilating manholes with exhaust fans can be effective in removing explosive vapors later in the incident if more than one cover is off. However, undertake this with care and only if requested by utility company officials and after you are certain that the power has been cut. Also, make sure the fans are intrinsically safe so they do not ignite the gases.
4 Check basements with combustible gas detectors and carbon monoxide meters; take continual readings to monitor changing conditions. Assign members to check the basements and upper floors of nearby buildings and take CO readings. CO presents a health and an explosive hazard. Our first priority is to determine that occupants of adjoining structures are not exposed to unsafe levels of CO. The recommended indoor air level for CO is less than 10 parts per million (ppm) for an eight-hour period, according to the EPA’s permissible exposure limit (PEL). Levels in excess of 35 ppm for a one-hour period can cause health risks. Firefighters should follow their departmental protocols concerning when to evacuate occupants but always evacuate if they are experiencing symptoms. Common symptoms of CO poisoning include headache, nausea, vomiting, drowsiness, unconsciousness, and poor coordination, which are caused by a reduction in the blood’s oxygen-carrying capacity.
CO and other explosive gases can find their way into basements through the underground electrical or gas service entrance, where they can accumulate and potentially explode. If elevated levels are found or readings are approaching the lower explosive limits (LELs) anywhere in the building, ventilation and evacuation of occupants are warranted. CO has a wide explosive range of two to 74 percent in air. With a vapor density of .97, it can disperse easily and accumulate anywhere in a building. Sewer gas (hydrogen sulfide) has an explosive range of four to 45 percent and a vapor density of 1.19, slightly heavier than air. If levels are high, do not attempt to shut off the main service switch. Take a reading at the service panel before cutting the power with an instrument that has been zeroed out in clean air. This precaution is necessary because the arc created by throwing the switch could spark an explosion. Vent first, and monitor the levels. Use intrinsically safe exhaust or positive-pressure fans to remove explosive gases from basements if conditions warrant.
Manhole fires can cause electrical fires or outages in nearby structures. It is essential to check adjacent buildings thoroughly, including the service panels for electrical outages. Take readings before the power is turned back on. If the power has been lost and elevators are in the building, be sure to check them for occupants.
5 Do not attempt to extinguish fires in manholes until the power is offand then, only if requested by the utility. Putting water on flames is a firefighter’s natural tendency. However, if water is used to extinguish a manhole fire before the power is cut, there is a danger of destabilizing the situation further and causing greater electrical activity, not to mention endangering firefighters. If a manhole is venting flame, smoke, or steam, leave it alone until after the power has been cut. Manhole fires primarily involve wire insulation and combustible metals. Therefore, once the power is cut, the fire will generally self-extinguish. If it does not, the utility company may request that you extinguish the fire. The copper conductor used in the power cables melts at 1,981°F and will not sustain fire very long once the source of heat is removed. In fact, copper powder is commonly used as a Class D extinguishing agent; it is effective for fires involving lithium and lithium alloys.
Remember that before the power is cut, a manhole fire is a legitimate Class C fire that may include Class D elements. The extinguishing agent of choice would thus be dry chemical or CO2. Aiming a hose stream into a manhole before the power is confirmed to have been cut amounts to putting water on energized power lines, only in a confined space. Water turning to steam in a confined area can erupt violently, perhaps even causing other covers to be propelled. Moreover, it is also like putting water through a window. The stream will rarely penetrate to the seat of the fire. In addition, flooding a manhole could displace CO into surrounding structures, and water or foam placed in a manhole can make the job more perilous for repair crews after the fire is out. Limit the use of water to protecting exposures, and prevent runoff from entering the manhole. If it is necessary to attempt extinguishment before the power is shut off, dry chemical and CO2 are the only safe extinguishing agents.
If the utility company requests that you extinguish a manhole fire, first ascertain that the power has been shut off and then flow water from a distance using an unmanned stream. In his book Responding to Routine Emergencies, Fire Department of New York Battalion Chief Frank Montagna suggests using a secured open hose butt with crews at a safe distance. If a vault is involved and the power is off, an unmanned distributor carefully lowered two to three feet into the manhole can be effective. Cables, switches, and trunk lines are mounted on the interior walls of the vault at various heights; a distributor will scatter the stream in such a way as to envelop the various lines. Because there are often several trunk lines or feeders running through the vault, be sure to confirm that the power has been cut off to all lines before beginning extinguishment with water. Even then, use caution. Even utility supervisors can make mistakes.
When an underground transformer is on fire, the utility company may request that foam be used to extinguish the fire. These transformers can contain up to 400 gallons of cooling oil, and foam can be effective if these Class B liquids are involved. However, keep in mind that foam is more conductive than water, so it is especially important to ascertain that all power has been shut off. It should also be noted that the application of water or foam can fill the vault and displace CO or smoke into adjoining buildings. Take readings in these occupancies before beginning and after completing extinguishment activities.
6 Do not inhale the smoke or gases coming from a manhole. The smoke from burning insulation could be toxic, and PCBs could be present if a transformer is involved. Although asbestos is not present except in very old installations, smoke from burning PVC insulation can be a potent carcinogen and cause extensive lung damage. Although SCBA is not necessary for members operating in the street, use common sense. If you are close enough to inhale smoke from a venting manhole, you are too close. The command post and forward staging areas should be upwind of the involved manhole.
7 Do not enter a manhole before the utility company secures it (i.e., all power is off), you take readings, and you don proper gear. If you have read this far, you probably would not enter a manhole to extinguish a fire; however, if a rescue is involved, you may be inclined to be less cautious. If entering a manhole to effect a rescue, wait for the power to be shut down, and wear full protective gear including SCBA. Take readings to ascertain if explosive gases are present before sending members in. Manholes are below-grade confined spaces; they can be oxygen deficient; be CO rich; and have flammable, explosive, or corrosive atmospheres. Failure to follow confined-space protocols and procedures can result in career-ending respiratory injuries. If rescue or recovery is necessary from a manhole, a properly suited FAST must be ready, decon equipment should be in place, and a safety officer must continuously monitor the atmosphere. A fiberglass utility company ladder, if available, is preferable to a metallic fire department ladder. A tripod and sked are effective in lifting the victim.
FLAMMABLE VAPORS IN STORM SEWERS
Not infrequently, fire departments receive reports of gasoline vapors in sewers. This can occur if spent gasoline is poured down a slop sink or floor drain, an auto with a fuel leak is parked over a sewer grate, or an overturned tanker is spilling product. Petroleum vapors can travel considerable distances underground from where the spill occurs. These vapors can back up into basements adjoining the sewer line and ignite if they come in contact with the pilot light of a water heater or furnace. Gasoline has a narrow explosive range of 1.4 to 7.4 percent, a low flash point of 36°F, and a vapor pressure three to four times that of air. This vapor pressure enables gasoline to accumulate and travel in low places such as sewers and basements.
Responding units should take readings at all sewer grates and check adjacent basements. If vapors are in the explosive range, take steps to mitigate the problem. Find, contain, and control the source of the leak. Once this is accomplished, flush sewers with water to dilute and discharge the fuel. When using this tactic, notify the sewer department and water treatment plant. If this is not effective, use foam to blanket the fuel and inhibit the formation of vapors. If this is ineffective, you may have to evacuate the surrounding structures.
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Manhole incidents are not all that common for most fire departments. However, they can present a significant life hazard. Public safety is our chief responsibility at these incidents. Safe, cautious operations are critical for successful mitigation.
Thanks to Fire Department of New York Battalion Chief Frank Montagna and Anthony J. Natale, Emergency Response Group, Consolidated Edison of New York, for their assistance with this article.
DOUG LEIHBACHER, a 27-year veteran of the fire service, recently retired from the Yonkers (NY) Fire Department as a deputy chief and chief of training. He is a certified fire instructor, a municipal training officer, a hazmat technician, a fire investigator, and an incident safety officer. He has an associate’s degree in fire protection technology and a bachelor’s degree in education. He has been a classroom and hands-on instructor at FDIC and a contributing author to Fire Engineering since 1994.
