By: Peter Pomeranz, P.E., and Joseph Faro, P.E.
The Kosciuszko Bridge (pronounced
kosh-’chush-(“)ko) is a steel and concrete structure approximately
4,000 ft in length that connects the boroughs of Queens and Brooklyn. The
bridge carries the Brooklyn-Queens Expressway with six lanes of traffic 140 ft
above Newtown Creek.
The Kosciuszko Bridge is owned by the New York State
Department of Transportation (NYSDOT). As part of its capital program, the
NYSDOT awarded a $14.4 million contract to Ahern Painting Contractors Inc. The
work, which started in November 2000, consists of cleaning, painting and steel
replacement on the Kosciuszko Bridge.
About 10 months after work began, the painters complained
about electrical shocks whenever they touched the steel on the bridge while
standing on a man lift. The first thought was to check for a short circuit
between a utility line or other electric service and the bridge. With the help
of electrical company ConEd and the New York City Department of Transportation,
all electric lines to the bridge were physically disconnected. Yet the shocks
continued.
Shield forced out
A long-standing feature of the Greenpoint, Brooklyn,
landscape was two large gas storage tanks that were demolished in July 2001.
The tanks stood 3,540 ft from the Kosciuszko Bridge and 4,550 ft from the four
vertical antenna towers of local radio station WQEW. WQEW is an AM radio
station that operates on a frequency of 1560 kHz with a power output of 50,000
watts. Its antenna is a series of vertical towers located 5,250 ft from the
Kosciuszko Bridge.
The NYSDOT engineer-in-charge, Joseph Faro, observed that
the electric shocks started as soon as the gas tanks had been demolished. A
hypothesis suggested that the demolition of the gas storage tanks might have
removed a ‘protective shield’ from between the antenna and the
bridge and that the shocks were the result of radio frequency energy.
Measurements were taken that showed a received frequency of 1560 kHz. However,
since the gas storage tanks were not directly in line with the antenna and the
bridge, further investigation found that electric shocks had been reported on
previous contracts dating back a number of years. The conclusion was that the
‘protective shield’ theory was invalid.
The vertical antennas of WQEW radiate an expanding
amplitude-modulated electromagnetic wave. This electromagnetic radiation
consists of interdependent electric and magnetic fields that are perpendicular
to each other and which travel outward at the speed of light. When the magnetic
field, which varies sinusoidally, passes through a given surface it induces a
voltage around the edge of that surface. The relationship between magnetic flux
and the closed integral of the electric field is defined by the Faraday-Maxwell
Law. The bridge was receiving and simultaneously radiating electromagnetic
energy from radio station WQEW.
Based upon an analysis by Steve Jensena, the
contractor’s electromagnetic consultant, the induced voltage for a loop
formed by the bridge and the water below is on the order of 60 volts RMS,
enough to produce a shock
sensation. The two center bridge towers, because of their height, could have an
estimated induced potential of 120 volts.
In order to proceed without the contractor’s workers
being shocked, a short-term solution was sought. However, a long-term fix for
maintenance was desired. There wasn’t concern about anyone on the bridge
receiving an electric shock, since they would be at the same electric potential
as the bridge—and without a difference in potential there can be no
electric shock. The problem arises for anyone contacting the bridge steel from
a man lift. If we assume the man lift is at ground potential because of
capacitive coupling to the ground and that the bridge is at a potential, either
positive or negative with respect to the man lift because of induced voltage,
then a person in contact with the man lift and touching the bridge would
experience a shock.
Another possibility is that the man lift could be at a
higher potential than the bridge at the point of contact. The reason both
scenarios exist simultaneously is due to the standing wave of the induced
voltage on the horizontal members of the bridge and because the man lift is
less than 1⁄4 wavelength long and would have an induced voltage on it
from the electromagnetic field. The induced voltage is approximately equal to
the height of the man lift above ground multiplied by the field strength in
volts per meter. WQEW generates a field strength of 2.15 volts per meter at the
bridge. A man lift extended to 131 ft would have an induced potential of 86
volts.
A frequency of 1560 kHz corresponds to a wavelength of 630
ft. At each half wavelength, 315 ft, the voltage is at a positive or negative
maximum. If a person is at a ground potential or low voltage potential in the
man lift and contacts the bridge steel at a maximum voltage, that person will
be shocked. Alternatively, if a worker contacts a zero potential point on the
bridge steel and is high enough in the man lift, that worker will be shocked.
Prior to the consultant arriving at the site, the contractor
had installed a number of long wires running from the structural steel deck to
a good earth ground. The ground wires failed to ground the bridge because the
distributed inductance of the wire effectively creates a radio frequency choke
making the wire appear to be an open circuit to radio frequency energy.
However, this wire was a convenient test wire that allowed the consultant to
measure radio frequency voltages and attempt to “tune” the bridge.
The initial thought for a long-term solution was to create a series resonant
circuit using the distributed inductance of the long ground wire and a variable
capacitor tuned to resonance at 1560 kHz. Radio frequency energy on the bridge
would follow this resonant path to ground. Although resonance was achieved, the
voltage drop at the connection point on the bridge changed only minimally. Part
of the problem is that while the series circuit tuned the bridge, it did
nothing about the induced potential of the man lift. An additional factor is
that every 96 meters the voltage standing wave on the bridge would be repeated
so that any solution such as running a tuned circuit to ground would have to be
repeated every 96 meters. But even if success was achieved in grounding the
bridge, the shocks would continue.
Today’s answer
After all the tests and analysis, it was realized that only
a short-term solution was possible in order to prevent workers from being
shocked when they come in contact with the bridge. The solution is to clamp one
end of a battery jumper cable, which is no longer than about 18 ft, to the man
lift and then, before touching the bridge, clamp the other end to the bridge.
This forces both points to be at the same potential and eliminates the possibility
of a shock. A battery jumper cable length of 18 ft would have an induced
voltage of up to about 39 volts so there would be no problem from voltage being
induced in the cable itself. If the contractor wanted to use ground-mounted
scaffolding that was in the order of 1⁄4 wavelength long, 157 ft at 1560
kHz, a second jumper cable would have to be installed to maintain the work
platform at the same potential as the bridge. Other short-term measures include
having workers wear insulated gloves when working on the bridge from a man
lift. Alternatively, the contractor could use an insulated bucket which would
keep a worker from being shocked.
Based on the work of the consultant, the long-term solution
of eliminating the difference in potential between the bridge and a man lift is
not possible short of turning off the radio station. Even if completely
grounding the bridge was successful, the man lift would not be grounded and a
worker in the man lift would continue to be shocked because of the difference in
potential between the bridge and man lift. People and vehicles on the bridge
are at a potential voltage with respect to ground, but are in no danger because
they are at the same potential as the bridge.
There is concern about the electrical safety of workers in
man lifts, ground-mounted scaffolding or ground-based cranes. The solution is
to electrically connect the man lift, scaffolding or crane to the bridge or
alternatively to electrically isolate workers on the various platforms from the
bridge. Contract documents should include warnings to potential bidders as to
safe work practices in the vicinity of a radio frequency energy field.
As a final note, the consultant did an analysis of the
hazard of exposure to radio frequency energy at the bridge. OSHA regulation,
29CFR 1910.97 Nonionizing radiation, recommends a power density of 10
milliwatts per square centimeter for periods of .1 hour or more as a safe
standard for human exposure. Based on a measured field strength of 2.15 volts
per meter the consultant calculates a power density at the bridge of 1.2 x 10-6
watts per square centimeter, or about 1,000 times smaller than the recommended
standard.
About The Author: Pomeranz and Faro work for the New York State Department of Transportation, Long Island City, N.Y. Pomeranz is an area construction manager. Faro is an engineer-in-charge.
