Thousands of Calgarians were plunged into darkness after an underground fire consumed kilometres of wiring last October and the city says they believe the fire was started by a relay used to control street lights.

On October 11, 2014, an electrical fire broke out in a manhole in Calgary’s downtown, knocking out power for 5,000 residents and nearly 2,000 businesses, including the Calgary Courts and McDougall Centres.

Power was restored four days later and 3.5 kilometres of burned wiring was replaced as a result.

Investigators believe a street light relay failure caused the fire but because of extensive damage they were unable to determine what caused the relay to fail.

“The exact cause of the fire is difficult to determine due to the extent of damage caused by the fires,” said Deputy Chief Ken Uzeloc. “Investigators spent a fair amount of time sifting through the debris and reviewing the contents of the manhole. However, they were unable to determine why the relay failed or why the failure resulted in a fire.”

The city’s 90,000 street lights are controlled by about 10,000 relays, which are electronic circuits that use a mercury sensor or a photo-electric sensor to switch street lights on and off.

The city’s manager of transportation says a fire from a relay is unusual.

“In the past, relay failures have resulted in street lights either not coming on or staying on in daylight hours,” said Mac Logan, General Manger of Transportation.

The relays in the downtown are in ENMAX manholes and the city will be replacing all those that are mercury-based, beginning with the oldest first.

They will also be moving them out of the manholes and mounting them in boxes on street light poles like the ones in the outer parts of the city.

“We recognize this incident had an impact on residents and businesses,” said Logan. “We want to thank Calgarians for your cooperation and understanding as first responders worked to ensure everyone’s safety and as crews work over the next several months to enhance our systems.”

The city says relays last an average of 30 years.