Pole Fires
Pole fires are a
common cause of electrical outages and
remain one of the biggest and most dangerous challenges facing the utility
industry. Wooden pole fires are mostly caused by leakage current flow.
Causes & Conditions
Insulators are used
to attach electric power lines to wood distribution poles. They are typically
made of porcelain and polymer.
Some
types of electrical insulators
Electric
power conductor passing over an insulator
Electric insulator supporting power conductors on a
wooden pole structure
A crossarm is one of the most
important components at the top of an electric transmission and distribution
pole. Although traditionally constructed of wood, crossarms are also made of
steel and fiberglass. In most developing countries wood is mainly used in power
distribution lines mainly because of economic implications. The challenge with
wood, like other biodegradable materials, is that it is susceptible to
contamination and deterioration.
Wooden
pole structure with dual crossarms
The root cause of most pole fires is
electrical tracking across the porcelain insulators and wood crossarms due to
contamination being deposited on the surface. Pole fires are common challenges
among electricity providers where contaminations such as salt, airborne
pollution or dust build up on insulating equipment followed by a weather event.
A light rain, for example, can combine with contaminants on the insulating
hardware and create a bond between the contaminants. Contaminated water
droplets then begin to form and establish paths from the energized overhead
conductor to the crossarm. This provides an electrical path for tracking to
occur. When tracking occurs from the energized conductor to the surface of the
crossarm, small arcs are generated between the wood and the through-bolt that
is used to attach the crossarm to the pole. The crossarm through-bolt is a
common area for a pole fire to initiate because of the minimal surface area
contact and high electrical resistance. Also, this area tends to be dry, which
can cause it to ignite more easily.
Other factors, such
as cracks or lightning damage, may also cause an insulator to fail and cause a
pole fire.
Sometimes the pole
may burn through, leaving the top of the pole, cross-arms and insulators suspended
by the power lines or power lines may break from the weight causing short
circuit between conductors or earth faults. The short circuit trips a switch,
cutting power to the lines, much like the fuses or breakers in a house. The
tripped switch causes an outage, which could be limited to a small area or
could involve a large number of customers.
An
electric pole fire
Research has shown that wood has low electrical resistance at
alternating currents, especially under wet conditions, and also has low
mechanical strength per unit weight. Study results show that the higher the
moisture content of wood, the more its insulating properties decrease resulting
in an increase in the wood's conductivity. The same increased conductivity has
been experienced in wood poles with higher concentrations of salt or dust
contaminants. And the more conductive the material is, the greater the risk of
fire.
How power is restored after an outage.
Customers in the
service area of the damaged pole may have power restored quickly, if power can
been re-routed to them from another location. There may be no other routing
options and in this case consumers must wait while repairs are made.
Poles that have been
damaged by fire usually need to be replaced. In doing so, effort is made to
minimize the length and impact of the outage. Sometimes power is restored to
customers for a short time and then it goes out again; the second outage may be
due to equipment repairs/replacement.
Public and worker
safety are high priorities and maintained throughout any outages and repairs.
These usually prolong the outage period as it involves travelling long
distances to isolation places in the absence of automated isolation options.
The location of the pole determines the difficulty and length of time
required to replace it. The entire process usually takes many hours to
complete.
- A
pole next to a road is generally easier to get to than one in a rural,
less accessible location (in a field, on the other side of a ditch).
- Sometimes
a new pole can be placed next to the damaged one instead of removing and
replacing in the same spot.
- Before
installing a new pole, there is need to obtain underground clearances (electrical,
gas, cable tv, communication).
After the new pole is
placed (or existing pole repaired, when possible), then replacement of any
damaged equipment and wire is carried out. Eventually equipment is attached to
the pole and restoration of electrical service to the affected customers is
completed.
