DURING EXTREME hot and cold weather, heavy rains, and snowstorms, many fire departments around the country respond to underground manhole and transformer fires and emergencies. Consider the potential for these incidents to escalate from “routine” emergencies to significant events during your initial size-up and maintain situational awareness.
- Manhole Fire Events: Understanding This Energy Hazard
- Managing Manhole Fires
- Pulling Electric Manhole Covers
- A Collaborative Approach to Electrical Vault Fires
Knowing What You’ve Got
The most basic and fundamental consideration with underground manhole and transformers is understanding just what these structures entail. Look at them the same way you look at aboveground telephone/utility poles, with all the same features such as primary and secondary transmission lines and transformers (photo 1). Now, take all those features and add in gas and water utility supply lines, steam lines, and telephone/cable fiber- optics; surround them in a cement vault; and you have a basic understanding of the potential for fires and emergencies.
The covers to these vaults can vary in size and shape, with most weighing 300 pounds. They can be round, square, or rectangular in shape and can also be a solid or a vented design (photos 2-7). What differentiates the vaults from aboveground utility poles is that they are interconnected by ductwork and conduits that allow for the migration of water, air, natural gas, carbon monoxide (CO), and smoke between vaults as well as into homes, buildings, and outbuildings.
In some municipalities, the aboveground and belowground utility systems are both present. Therefore, electrical fires and emergencies that occur aboveground or belowground can spread and affect the other system. Where the systems intersect there will usually be a conduit running up (or down) a utility pole that carries the electric cables into the underground system (photos 8-10).
The Basics
Underground electrical service box fires can cause explosions that are strong enough to blow the manhole covers several stories into the air. They can flip vehicles parked over them onto their sides and can severely injure or kill firefighters and bystanders with flying debris and shrapnel. Even smoldering electrical fires can cause smoke and flammable/toxic gases to travel into homes and buildings hundreds of feet away due to the interconnection of the vault systems.
1. Photos courtesy of author. Video courtesy of fdny24seven on Instagram.
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Transformers are used to either step up or step down electrical voltage from the electrical grid to homes and commercial occupancies. Generally, single phase transformer capabilities range from a few kilovolt-amperes (kVA) to several hundred kVA. One kVA is equivalent to 1,000 volt-amperes.
In commercial applications, transformers carry significantly higher capabilities that in some instances can exceed 1,000 kVA. Typical residential pole and underground transformers carry primary voltages of 7.2 kV to 34.5 kV, which are then stepped down to household 120/240 volt use.
Think of voltage as the potential electricity that could run through an electrical system. Though the number may increase (12 volts, 120 volts, 240 volts), this is just the potential electricity from the source, not necessarily the amount of electricity traveling through the system.
An electrical supply of 1,000 volts is no more deadly than 100 volts, as the danger is determined by the current. Tiny changes in a current’s amperage can mean the difference between life and death when a person receives an electrical shock.1
Let’s compare that to operating a fire attack hoseline with a fog nozzle:
- Voltage = the water pressure from the engine’s pump discharge (100 psi).
- Resistance = the hose with a fog nozzle (aka friction loss).
- Amperage/current = the resulting water flow (gpm) from the nozzle.
Opening the nozzle’s bail partway will decrease the resistance, thus increasing the current (water flow). The initial water pressure at the pump’s discharge (constant voltage) wouldn’t change. However, the increase in the volume of water (increased amperage) due to opening the bail of the nozzle fully (decreased resistance) determines how soaked you get if you are hit with the stream. So, with amps (amperes) vs. volts, the danger is in the amps.
Aboveground and belowground transformers always carry high voltage and therefore potentially high amperes. Additionally, the transformers may be filled with as much as 2,000 gallons of oil (for commercial applications) and between 10 and 125 gallons of oil in smaller residential applications, such as those typically found on utility poles. Modern day transformers use mineral oil as an insulator and a coolant, which is combustible (with a flash point typically around 284°F). Older transformers (prior to 1979 when they were outlawed) may contain polychlorinated biphenyls (PCBs). PCBs are a group of man-made organic chemicals consisting of carbon, hydrogen, and chlorine atoms. This liquid is highly carcinogenic; when it burns, the resulting smoke is deadly to firefighters.
Additional concerns at manhole and transformer fires and emergencies are as follows.
- Older underground primary electric cables may be wrapped in white asbestos coverings as an “arc proofing” measure. All electrical wire insulation is made of rubber and is highly toxic when it burns.
- In underground vaults specifically, arcing from electric cables can damage plastic and PVC gas piping and infrastructure in the vault. This can cause a natural gas leak or produce a gas-fed fire. Although burning electrical cables produce significant amounts of smoke, they usually produce little to no flames. This is the most prevalent type of manhole emergency. However, this same fire in a transformer vault can result in catastrophic failure of the transformer.
The utility company must remove the power to the transformer to mitigate a sustained arcing/transformer fire. Confining the fire area and conducting primary searches and gas readings may be the best actions the fire department can accomplish prior to this power being removed.
Strategy and Tactics
One of the first considerations is apparatus placement. As with any hazmat incident, stage the apparatus (and personnel) uphill and upwind from the affected pole/ vault. Apparatus drivers and officers must ensure that their apparatus is not parked over or near any manhole/transformer covers. Although they may not appear to be part of the emergency, because of the interconnection between these structures’ buildup of flammable/combustible gases, their subsequent ignition/explosion can occur at great distances from the obvious affected manholes/vaults. Be aware that these ignitions/explosions can occur at some later time during mitigation.
Avoid walking on or near manhole/trans- former poles/vaults for the same reasons. If a manhole/vault explosion occurs, rocks, glass, dirt, and projectiles from the cover or cover rim can fly in all directions and for great distances under extreme velocity and force. If you are operating at an intersection with numerous manholes, your safest bet is to stay on sidewalks whenever possible and take the long way around the intersection so as not to become trapped crossing in between the multitude of manholes in that given intersection.
The incident commander (IC) should request early notification and response of the utility company. Communicate conditions found on arrival (seeping smoke, heavy smoke under pressure, visible fire and smoke, and whether a transformer is involved). If the involved utility is underground (manhole or transformer vault), summon both the electric and gas utility companies to respond.
Cordon off an area of safety for responders and civilians with fire line tape for vehicles and pedestrians. If the incident expands, expand the no-access safety zone. A good rule of thumb for this exclusion zone should be at least one pole or one manhole/ transformer cover beyond that which is actively arcing, burning, or smoking.
View traffic control boxes and street light poles with concern. Flammable/ex- plosive gases from the involved manhole/ vault can migrate into their hollow base, filling the poles with gases that can ignite and explode.
Simultaneously with the establishment of the above isolation and safety zones, inspect nearby structures for smoke, heat, and gas extension. Members in teams of at least two should conduct these primary searches wearing all personal protective equipment (PPE) including self-contained breathing apparatus. They should carry CO and multi-gas meters that are zeroed out in a clean air environment.
The best location to start searching and monitoring is where gas, electric, cable, telephone, ethernet, fiberoptics, and sewer utilities enter the structure from the street (the point of entry). These are the areas where the highest potential for gas, smoke, and CO migration will be found as they may run through or be directly tied into the involved manhole/transformer.
Fire department members should avoid turning on or off or attempting to shut down electrical components—even if the lights in the building are flickering. CO in the conduits leading from the affected manhole/transformer could have built up in the electrical panels of the building. Electrical panels themselves may become energized when burning electrical cables arc or fail. The resulting arcing when attempting to turn on or off power could have disastrous and explosive results.
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In photos 11-16, the explosive force and subsequent damage to a cellar area was the result of CO migrating into the commercial bookstore through electrical conduits from an outside transformer vault fire. Firefighters had just arrived on scene and were making their way into the cellar area when the explosion occurred. Thankfully, all interior members were wearing full PPE and they sustained no injuries. However, had they been in the basement at the time of the explosion, the outcome would probably have been different. One firefighter (the first-due ladder company driver) was injured when the sidewalk uplifted and cracked from the explosion in front of the store.
Note: CO has a large flammable/ex- plosive range between 12% and 74% in room air. Although the CO meter measures in parts per million, the displacement of oxygen on the meter may be more telling of an explosive atmosphere in the absence of flammable range detectors.
Stretch a precautionary hoseline with a fog nozzle, but do not operate it into the manhole/transformer unless directed to do so by on-scene electric company personnel. If so directed, the nozzle team should position at least 25 feet from the manhole/trans- former and should use an indirect attack by bouncing the stream off the roadway, curb, or sidewalk rather than play the stream directly into the manhole/transformer vault.
Caution: Remember that transformers may have up to 2,000 gallons of oil in them. Applying water can result in rapid expansion and expulsion of the heated and burning oil,causing the oil to spill out of the container/ vault with explosive fire and force.
Additional engine company members should position to readily advanced attack lines and secure a positive water supply in case of extension into a structure or exposures.
Since the primary and secondary cables are usually attached to the walls of a manhole, you may need to reposition the hoseline to fully extinguish the fire. Remember to maintain a 25-foot distance from the manhole while repositioning.
You can use Purple K (a potassium bicarbonate-based powder), F500 (a multipurpose encapsulating agent), or similar dry chemical/ dry powder agents on transformer fires rather than water. However, follow standard operating procedure regarding their use, as the standoff distances due to the product may be as little as 10 feet compared to the 25-foot standoff distance if using a fog hose stream.
Advise all members when placing the hose stream into operation. Conduct CO and gas monitoring of surrounding properties before, during, and after such operations commence. The placement of water into the manhole/ transformer vault may cause flammable gases to migrate into other manhole/trans- formers and structures.
Wrapping Up
During manhole/transformer fires and emergencies, specifically in underground structures, be aware of the possibility of unexpected explosions in adjoining manholes, service boxes, and surrounding properties. Keep your head on a swivel in anticipation of such events. For an explosion that causes manhole/transformers covers to fly into the air, resist the urge to duck, cover, and run. Instead, attempt to watch the covers so that you can estimate where they will land and avoid being struck by the 300-pound covers rocketing back to earth.
After operating at manhole/transformer emergencies, consider deconning members and equipment as per department protocols.
At these types of fires and emergencies, you must constantly reinspect all exposures including those across the street from theaffected manhole/transformer. Consult with the local utility company representatives on scene before releasing units. Although they are the subject matter experts when it comes to electrical and gas emergencies, WE are the agency responsible for the overall mitigation, extension, and control of the emergency.
ENDNOTE
- Thiele, Timothy, and Kamron Sanders. “Amps vs. Volts: The Dangers of Electrical Shock.” The Spruce, 5 Nov. 2024, bit.ly/41VHhfD.
STEPHEN MARSAR is a 38-year veteran of the emergency services and a battalion chief with the Fire Department of New York (FDNY). A former chief and commissioner of the Bellmore (NY) Volunteer Fire Department, he has a master’s degree in homeland security and defense from the Naval Postgraduate School in Monterey, CA. He has a bachelor’s degree in fire science and emergency services administration; is a graduate, with honors, from the Executive Fire Officer Program at the National Fire Academy; and is a two-time recipient of FEMA’s National Outstanding Research Award. He is an Advisory Board member of FDIC and Fire Engineering.
