Exploding manholes are one of those weird and interesting
side effects of living in a large city. Imagine walking down
the street when all of a sudden a manhole covers flies 50 feet
in the air!
An 85-pound manhole cover can become a
missile when blasted out of the
In this edition of HowStuffWorks,
you'll learn what causes these blasts and how they compare to
the combustion chamber of a car engine.
Shaking the Ground A cast-iron manhole cover
can weigh between 85 and 300 pounds (35 to 136 kg), and
explosions have propelled these massive discs anywhere from 1
foot to 50 feet (0.3 to 15 m) into the air. The real problem
with these explosions (aside from the risk of injury) is the
loss of power in the
Explosions are typically caused when a spark
from wiring ignites gas inside the
In most cases, these are the events that lead to an
Underground cables become frayed from aging,
corrosive chemicals, overload or rats biting them. These
cables carry on the order of 13,000 volts of
These electrical wires heat up the paper, lead
and rubber insulation.
The insulation smolders and catches on fire,
The pressure from the gas builds up inside the
The electrical wires arc like a bolt of lightning
and ignite the gases, causing a powerful explosion.
Depending on the amount of gas-pressure built up inside the
manhole, the cover may flip over or be launched
several feet in the air. Often, there may not be an explosion,
just a lot of smoke or fire.
Some power companies are in the process of replacing solid
manhole covers with slotted manhole covers. These new
covers allow the gas to be released less violently, and also
give an early warning to possible explosions.
Energy Everywhere We don't often realize how
much untapped energy exists all around us. The
principle behind these manhole explosions is similar to how a car
engine works. Imagine a city-sized engine with pistons as
big as buildings. In this analogy, the manholes are the
combustion chambers of the engine and the electrical lines are
the spark plugs. These powerful manhole explosions could
theoretically power the city if they happened often enough.
There's a very basic principle behind any reciprocating
internal-combustion engine: If you put a tiny amount of
high-energy fuel (like gasoline)
in a small, enclosed space, and ignite it, an incredible
amount of energy is released in the form of expanding
Like in a car engine, the gas
released from a manhole explosion could push on a piston
and turn a
To understand this, you need to understand the basic
process of an internal-combustion engine. Here's what happens
as the engine goes through its cycle:
Intake stroke - The piston starts at the top, the
intake valve opens and the piston moves downward to let air
and gasoline into the cylinder. Only the tiniest drop of
gasoline needs to be mixed into the air for this to work.
Compression stroke - The piston moves back upward
to compress this fuel/air mixture. Compression makes the
explosion more powerful.
Combustion stroke - When the piston reaches the
top of its stroke, the spark plug emits a spark to ignite
the gasoline. The gasoline charge in the cylinder explodes,
driving the piston downward.
Exhaust stroke - Once the piston hits the bottom
of its stroke, the exhaust valve opens and the exhaust
leaves the cylinder to exit through the tail pipe.
The manholes in question are already filled with an
expanding gas, and any spark is going to expand it further. In
a combustion engine, the explosion causes the gas to push up
on a piston. In a manhole, the explosion pops the heavy
manhole cover right out of the ground.
If it were possible to situate a piston over an exploding
manhole, you might be able to use the power to turn a
crankshaft. Each explosion would release enough force to push
the piston. And if you connected the crankshaft to a generator,
and explosions occurred frequently enough, you could tap the
energy of these mishaps to supply power to the residents of an