UPDATE: Here’s a statement from Rebecca Hickey in Hydro Ottawa’s media an public affairs department:
The outage on Saturday March 14th affecting Stittsville, Kanata, Richmond and Munster was caused by a Hydro One pole that caught fire. The pole, located along Highway 417 near Huntmar, is the main source of electricity from Hydro One’s provincial high-voltage transmission grid to the local grid managed by Hydro Ottawa.
Pole fires can take place, particularly in late winter, when salt spray from passing cars builds up on the electrical infrastructure. This build up, paired with moisture in the air, can allow the electricity to travel through the insulator and energize the wooden pole, causing it to catch fire.
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The power outage that cut hydro to 12,000 customers on Saturday may have been caused by a pole fire due to road salt.
@renzoroni Cars on 417 cause salt build up. Mixed with moisture in air caused pole fire. Pole belongs to Hydro One, main feed to west end.
— Hydro Ottawa (@hydroottawa) March 14, 2015
If it was indeed a salt build-up plus moisture that caused a fire, how does that happen? Here’s an explanation from PowerStream.ca, a power company in Southern Ontario:
Poles that are most at risk are located in high-traffic, high-speed areas, such as along the 400-series highways…
Over the course of the winter, road salt residue and other contaminants accumulate on the ground, on hydro poles and on the equipment attached to them, such as line insulators, which prevent electricity from traveling from power lines to the pole.
When the temperature hovers around the zero degrees Celsius mark, the buildup of road salt residue, combined with the presence of moisture, creates an environment that can give rise to pole fires.
During spring thaws, accumulated snow and ice begin to melt and the moisture mixed with the road salt allows electricity to be conducted from the power line along the insulator and onto the metal bolt attaching the insulator to the pole.
The metal bolt heats up and if the surrounding wood is dry enough it begins to char and could catch fire. Poles can be severely damaged during fires, and the fire has to be extinguished before crews can repair or replace the pole and restore power to affected customers.