Why Manholes Flip Their Lids

why-manholes-flip-their-lidsYou may have seen dramatic TV coverage of manhole covers popping off in the street and hurdling through the air. While rare in our service area, manhole covers sometimes become dislodged. The root cause varies; however, these incidents all have two ingredients: combustible fumes and an ignition source.

During the snowy months and spring thaw, the flammable gases are often caused by the salt we put on roads and sidewalks to keep us safe from slippery snow and ice. Other sources of potentially dangerous fumes include sewer lines, gas pipes, decaying organic matter, and the disposal of combustible materials in the streets.

In the late winter and early spring, salty runoff can make its way down a manhole shaft and compromise the insulation around electric cables. The cables begin to smolder, releasing gases like carbon monoxide, methane and hydrogen. In high concentrations, all that’s needed to release this energy is a spark.

 

Manhole covers serve an important purpose. They keep debris from falling into our electric vaults, keep unauthorized people out and ensure we can provide you the safe, reliable electricity service you expect from PSE&G.

We have 28,547 manholes in our system. Underneath their 400-pound cast iron covers are vaults measuring 6 feet high, by 12 feet long and 8 feet wide. The vaults house high-voltage electric cables about the size of your fist in diameter. Multiple cables are bundled together and wrapped in insulation. Those cables feed transformers that step down the high voltage electricity to a level suitable for distribution to homes and businesses.

We have another 11,251 “hand holes” that are covered by the small, rectangular metal covers you most often see in urban areas. These smaller vaults house low-voltage lines that bring electricity to individual customers such as small businesses, apartment buildings, or street and traffic lights. The covers on these hand holes don’t pop often. The lower voltage, combined with a smaller space, make it less likely to trap and ignite gas.

Manhole incidents in our system happen very infrequently. However, if you see something suspicious, like a manhole spewing smoke, it is important to report it. Call 911. Also, be sure to call PSE&G if you see a manhole or hand hole cover ajar or in any way out of place. It could be a sign of a problem lurking underneath.

 

John Latka, Senior Vice President - Electric and Gas Operations John Latka, Senior Vice President Electric & Gas Operations – PSE&G

3 thoughts on “Why Manholes Flip Their Lids

  1. GREAT Information!!!
    Would be nice to share the causes for overhead wires coming down, without a weather event or pole hit taking place.

    Like

  2. Metal fatigue. Although not a reality PSEG has dismissed as never having a remediation. Charged with a solution some seven years ago. The crystalline geometries of conductors truly presented a challenge. Ironically the solution paradigm shift has been exactly that. Construction geometries.

    Like

  3. Con Edison maintains over 250,000 Electric Manholes, and has been faced with this ongoing challenge for many years. They rarely commented on it outside of their inner circle.

    Carbon monoxide generated during these cable burnouts can also be a threat for reaching basements through cracks in a foundation wall.

    Kudos to PSE&G for sharing this type of safety information with the NJ public.

    Like

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