Manholes or inspection chambers provide access to underground utilities and date back to the Roman era as part of the introduction of sanitary measures.

Utilities include electrical distribution and transmission conductors, sewage facilities, gas lines, steam pipes and other assets such as communication networks equipment. They allow operatives to undertake inspections, modify/extend networks and carry out cleaning and maintenance. A manhole is normally constructed as a chamber, accessed via a vertical circular entrance pipe or ring. Steps or ladders are fitted to provide safe entry for authorized personnel. Covers may be circular, rectangular or square and are constructed of cast-iron, aluminum, glass-reinforced plastic (GRP) or more recently composite material. Their surfaces have patterns or studs to reduce the chance of pedestrians or cars slipping and may be marked with the foundry of origin or the utility provider. Manhole locations are generally where there is a change of direction, gradient or where access is required for specific maintenance. Typically their position is away from curbs and covers avoiding the wheel line of traffic.

The Environmental Protection Agency (EPA) estimates there are over 20 million manholes in the USA with the NYC Department of Environmental Protection having 350,000 across the five boroughs of New York City alone. One utility company, ConEdison has more than 266,000 manholes and service boxes with 97,000 miles of cable in its underground electrical distribution system serving 2.5 million New York customers. It has approximately 1,000 manhole events per year of which 20 to 30 have serious outcomes. Operational mitigation and repair costs incurred by public utilities can be up to $100,000 per manhole event. Liability litigation and public relations costs escalate this total even further. The response to a single smoking manhole incident can cost between $5,000 and $10,000.

Manhole Events

As with any infrastructure, over time there is degradation of both the manhole structure construction material and assets housed within. In the electrical transmission and distribution industry, failure of underground cabling or other associated equipment may ultimately lead to manhole events. These can begin or remain with smoldering conductor insulation, detected as smoking manhole covers known as ‘smokers’. Pressure build-up due to chemical reactions can lead to a fire if an electrical arc ignites flammable gases, potentially followed by an explosion that displaces or launches the manhole cover into the air with devastating consequences for public safety and damage to vehicles and buildings. Modern secondary cable insulation is typically constructed from filled ethylene propylene rubber (EPR) with an oxidation induction temperature of 200˚C, while older cables are constructed from styrene butadiene rubber with a corresponding oxidation induction temperature of 150˚C, making these much less resistant to becoming brittle and cracking. Other types of insulation include paper insulated lead (PILC), kerite, butyl rubber, neoprene and PVC. Cable insulation is the greatest potential fuel source for an underground fire. Duct construction materials include wood, cellulose-tar, concrete and PVC. All of these materials also produce flammable gases as they decompose.

Causes

Manhole events are usually related to electrical faults and/or accumulated gases being accidentally ignited. Low voltage cabling is the source of >95% of these with less than 5% of events manifesting on the primary transmission side of networks, normally being transformer or primary failure related. Most of the issues are due to the organic materials present in secondary cable insulation and jackets becoming cracked, brittle or decomposing leading to carbonization occurring on the insulation surface. The conductive nature of carbon means that tracking, arcing or stray voltage on street furniture can occur, especially when conductors are exposed to salt water from road surface de-icing activity. Load changes may also lead to cable overheating. Another contributing factor is the constant vibrations from overhead trucks and subway trains which can cause cables to rub against each other, weakening the protective insulation. Rodent damage adds to the overall problem. There are three stages to the formation of catastrophic events, pre-explosion, explosion and post explosion.

A wide range of evolved flammable toxic gases may develop in manholes during an event due to intensive thermal decomposition of polymeric material as a result of combustion or pyrolysis. Decomposition of polymers causes the severest outcomes when the rapid combustion of gases leads to increased pressure and temperature within the manhole, potentially displacing covers. Ignition of combustible gases such as methane, hydrogen (H₂), ethylene, ethane and acetylene may also create fires without a corresponding pressure rise. Large quantities of hydrogen are produced in arcing faults involving aluminum cables. Smoking manholes are a result of incomplete gas combustion, generating toxic gases such as carbon monoxide. Each flammable gas has a lower explosive limit (LEL) and upper explosive limit (UEL) that denotes the range within which the presence of oxygen and a spark will ignite it. Generally, the LELs of these gases and mixtures thereof are about 4-5% by air volume. It should be noted that gas migrates through ducts and may cause problems in adjacent structures, dislodging other manhole covers and spreading fire. Gas combustion can be broken down into two categories, deflagration and detonation. Deflagration is when a flame front propagates by transferring heat and mass (thermal conductivity) to the unburned gas-laden mixture ahead of the front at subsonic speeds. Detonation is a more powerful process of supersonic combustion whereby a shock wave is propagated forward due to ignited material behind the shock releasing energy to generate high pressures supporting the shock propagation.

Manhole events can be categorized into electrical fault-driven or combustion-driven, whereby airflow through the duct results in sustained combustion of polymeric materials within the manhole. Electrical energy resulting from arcing due to a high impedance or short circuit fault rapidly increases the temperature of cables at the fault location, causing melting and vaporization of the copper conductors. Combustible gases then accumulate in the manhole as polymeric materials decompose in the anaerobic environment due to pyrolysis. Typical fault locations are at cable splices, joints and leaking transformers containing combustible oil. Where air flows through ducts and passages, oxygen can sustain the combustion of flammable gases created from pyrolysis. Carbonization of polymeric material then leaves porous surfaces typically leading to smoldering manholes. If combustible gases continue to be generated and oxygen remains available, a fire could then ultimately lead to an explosion lasting for up to three seconds.

Electrical Protection Systems

Several systems can protect cabling sections from thermal overloads or detect arcing, but specialized equipment is required to allow early detection of both electrical and gas-producing faults. Limiters are designed to protect neighboring cables from overloading due to three-phase faults typically in the 1,000 to 5,000A range. Arc detectors typically sense in the 5 to 50A range. Dedicated manhole monitoring systems such as CNIguard’s Sentir can detect cable hot spots using its onboard infrared cameras along with dedicated sensors to detect stray voltage, arcing, rapid temperature rises and carbon monoxide and methane gases at low levels.

Manhole Event Outcomes

The most serious events are those consisting of underground explosions, whereby manhole covers are launched into the air as projectiles, followed by a fireball of ignited gas. Injuries to the public, utility employees, firefighters, vehicles or buildings may result in litigation at great cost to the utility alongside unwanted media attention and loss of public confidence. Damage to the underground infrastructure results in power cuts and asset damage, while pre-event accumulations of carbon monoxide may lead to building explosions. Natural gas facilities may also be affected, as network gas lines are often run close to electrical conductors.

Event Mitigation Strategies

The most important factor to be taken into account when implementing mitigation strategies is the safety of the public and utility crews. Due to the high number of manholes and service boxes, the cost of implementation is also key, with priority given to those manholes serving more densely populated cities. Chosen strategies must be reliable over long periods and not inhibit normal maintenance or servicing routines inside vaults. Basic management of events can be broken down into proactive or reactive approaches.

Proactive approach

In an attempt to prevent scenarios that may lead to explosions, old cables, splices and joints can be replaced as identified during increased routine inspections. Ducts can be sealed to minimize underground airflow and inflatable bladders filled with inert gas are introduced to reduce manhole air capacity. Automated ventilation of manholes upon detection of flammable gases is another option. Increased contact voltage testing can reveal incipient faults. Stray voltage detection can also be conducted locally by a permanently installed system, which can also detect arcing. Sensing of particular gases such as carbon monoxide and methane at set thresholds gives early warning of potential problems, with both visual and especially infrared cameras allowing more detailed analysis of cables suffering from overheating hot spots. Monitoring rapid temperature increase in vaults can determine that an event is on the horizon. Gas detection systems will provide early warning of their presence, but will not prevent explosions due to an electrical fault. Conversely, arc detection systems may prevent explosions due to faults, but not where due to the ignition of gases. A proactive solution should include both detection features. An automated monitoring system such as CNIguard’s Sentir has many of these sensing capabilities, along with monitoring road salt levels that can both indicate the need for salting or negate the requirement if the amounts deployed are already sufficient.

Being Reactive

Reactive approaches to limit the effects of explosions if they occur include tethering manhole covers to limit travel while allowing explosive gases to escape. Flexible energy-absorbing materials are being used in their construction to reduce acceleration forces. One drawback is that manhole access may be impeded by the tether. Covers may be fitted with restraints, springs or hinges to again prevent launching, but bolting down or equipping them with locking systems is not recommended as internal pressure can continue to rise with a potentially catastrophic release when both the cover and frame are ripped from the ground with devastating consequences. Programs have been underway for several years to replace solid cast iron covers with vented ones, which may also be constructed from composite material to reduce weight. Covers from companies including Neenah Foundry and EJ allow gases with lower density than air to escape safely, preventing pressure build-up. The downside of vented covers is that they allow ingress of rainwater or melting snow (salted in Winter), spilt fuel and debris from road surfaces, compounding the problem underground. The size of cover vents is also important, as a balance of effective gas venting while maintaining robustness against traffic weight is required, with the hole sizes not being too small, leading to clogging over time. Vented covers used over smaller secondary boxes are very effective at resisting being dislodged by explosions unless extremely high pressures are involved. A controlled pressure relief system Stabiloc™ fitted to covers was tested with a degree of success by the Electric Power Research Institute (EPRI), consisting of one fixed and one adjustable breakaway latch utilizing two brass shear pins allowing controlled rising of the cover in stages, dependent on the severity of the explosion. Other work in the industry is ongoing to understand explosion dynamics that can allow software modeling to develop new manhole, secondary box and cover construction that mitigate explosions.

The resulting damage from explosions can be both internal and external to the manhole. Internally, the equipment can be displaced with plastic components melting. Vault walls may bulge and natural moisture stripped due to the high temperatures involved. Externally, the road surface can be damaged or even collapse into the manhole. Damage can occur to vehicles, buildings and pedestrians from flying material or covers. Secondary explosions can add to the problem due to air turbulence that ignites the remaining fuel in the structure.

Conclusion

Major manhole events are rare, but cannot be ignored due to their risk to the public, property and utility assets. Various proactive risk management strategies can be implemented to deal with scenarios leading up to explosive events and reactive methods of mitigating destruction and injury following such events. There are advantages and disadvantages to all the approaches, but a combination can be shown to reduce the likelihood of gas build-up due to cable insulation degradation, arcing or overheating as a result and limiting the possibility of manhole covers being ejected from their frames if an explosion does occur. Continued research is ongoing in this area to understand the early warning signs, to modify existing manhole vaults if feasible to contain explosions and re-design new ones to absorb their energy better. Replacement of solid covers with vented types remains as a solution to prevent pressure build-up underground and reduce damage caused if they are launched following an explosion.

Reported Manhole Events

New York, 56^th Street, Manhattan, February 28^th 2021:

The fire department responded to several smoking manholes on 56^th Street between First & Second Avenues in the early hours of Sunday morning. One of the manholes exploded around 7.40am causing multiple power failures on the block. High levels of carbon monoxide were found in surrounding buildings. An emergency medical responder, a ConEdison employee and a firefighter were injured as a result of the blast.

New York, Lexington Avenue, Midtown, November 18^th 2019:

Four manhole covers were launched into the air due to an underground explosion near Lexington Avenue and 44^thStreet. One of the covers smashed through the windshield of a taxi, hitting the driver. He was left with permanent injuries and crippled down one side.

California, Downtown Los Angeles, June 17^th 2019:

An underground explosion popped out four manhole covers in the South Hill Street vicinity following reports of heavy smoke emanating from manholes. Flames rose out of the manholes and 50 residents were without power for a period. No injuries were reported.

New York, Bronx, 10^th November 2018:

A manhole cover launched into the underside of a car near 510 East 156^th Street in the Melrose region of the Bronx. Five people including two NYPD officers were injured.

Michigan, Detroit, Greektown, August 2^nd 2017:

A series of manhole explosions, thought to be associated with an underground transformer fault in Detroit’s Greektown entertainment district, filled the streets with smoke, sparking an evacuation of several businesses. No injuries were reported.

New York, Prospect Park West, Manhattan February 2^nd 2015:

An explosion at Prospect Park West and Fourth Street sent a manhole cover around 25 feet into the air, hitting a man walking his dog and shattering nearby windows. Both the man and his dog suffered serious injuries.

References

Explosions in Electrical Vaults – W.Z. Black, Georgia Institute of Technology.

Manhole Explosions – Mitigation Methods – W.Z. Black, Georgia Institute of Technology.

The Electro-Chemical Basis of Manhole Events – Lily Zhang and Steven A. Boggs Institute of Materials Science, University of Connecticut, Storrs, CT. Stavros Livanos, Graciela Varela, and Aaron Prazan* Consolidated Edison, Inc., New York, NY.

Understanding Manhole Events – Stuart Hanebuth, Vice President Power Survey Company, Kearny, NJ.

Manhole Event Risk Management Strategies – G. Gela, Electric Power Research Institute (EPRI).

Photograph credits:

Smoking Manhole – Gregory P. Mango

Manhole Fire – Fire Engineering (www.fireengineering.com)

Tethered Manhole Cover – Joe Foulk