ELECTRICITY POWER OUTAGES DUE TO WOODEN POLE FIRES

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.