tree damage is unusual. The low risk of tree damage and the stricter requirements in NESC for
both wind and ice loads in the design of transmission lines compared to distribution lines means
that damage to the transmission system in moderately severe ice storms is rare. However, lower
voltage transmission lines are often on shorter poles in narrow right-of-ways, or near the edge of
a wider right-of-way and, thus are more susceptible to tree damage.
Galloping of conductors and guys causes large dynamic loads at low ice loads and moderate,
steady winds. The occurrence of galloping was first explained by Den Hartog (1932). Negative
lift on the airfoil shape that is formed by a thin layer of ice accreting on one side of a conductor
or guy leads to a positive-feedback, low-frequency, high-amplitude oscillation that can tear the
attached structure apart. Galloping may continue for days once it begins. A number of devices for
reducing the amplitude of galloping, or discouraging its initiation, have been developed and utili-
ties have installed these devices on spans that have been particularly susceptible to galloping. At
high ice loads, sustained galloping is less likely because of the weight of the ice-covered conduc-
tor and the additional damping from the accreted ice. However, damage to a tower or compo-
nents of a transmission line from galloping in a minor ice storm may cause failures at ice loads
less than the design load in later, more severe storms.
The effect of ice storms on wireless communications systems can be significant (Mulherin 1987).
Wireless communications systems include microwave telephone, cellular telephone, personal commu-
nications service (PCS) paging, commercial radio and television broadcast, and two-way radio link-
age for government agencies, emergency personnel, and businesses. All of these depend upon signal
propagation between antennas mounted on tall towers or masts. Problems in ice storms include loss of
off-site power and signal degradation from ice buildup on transmitting and receiving antennas.
Ice falling from towers is also quite common and can disrupt communications by impact damage
to equipment on the ground or on the tower itself and by puncture damage to the roofs of trans-
mitter buildings at the base of towers, which may cause water damage to the equipment inside.
Although relatively rare, heavy ice accumulation can also cause towers to collapse. Because
these towers are situated in cleared areas and stand well above treetop level, they are not subject
to damage from falling trees. Unlike electrical transmission lines, they are usually single towers with
no physical attachment to adjacent towers, so that the collapse of one does not typically affect
any others. Occasionally, at multi-tower sites, a tower may fall onto and destroy other towers.
Wireless technology is significantly impacted by ice storms but the risk of serious problems can
be reduced in a number of ways. To protect against external power loss, critical tower sites often have
some source of backup power that will automatically maintain operations when commercial power is
interrupted for up to several days. Batteries and diesel- or propane-fueled generators are typical. To
protect against falling ice, transmitter buildings at many sites have reinforced roofs or ice guards over
the roof. Ice guards or steel screens are often used to protect vital components on the tower itself.
The most serious problem caused by ice accreting is structure overload. Towers are typically well
exposed to the weather and are often built on mountaintops. Both mountaintop towers and very tall
towers (1000 to 2000 ft) at low elevations may spend hours or days engulfed in low stratus clouds at
subfreezing temperatures and thus may be subject to severe loads from in-cloud icing. Very tall towers
are engineered considering site-specific loads, including the expected equivalent uniform ice thickness
and the concurrent wind-on-ice load. However, smaller towers are often bought "off the shelf," and
may not be designed to withstand severe icing. Towers sometimes collapse in ice storms, causing
extended periods of off-air time (Mulherin in press). Providers usually restore at least partial service
with alacrity following such a failure because of the highly competitive nature of the communications
Back to contents page