Ageing transformer explodes at Belmont substation on hottest day of the year, causing blackout

On 12 June 2017, on what was reported as the hottest day of the year with temperatures above 90 degrees Fahrenheit and electrical demand at a peak with town schools in session, a transformer at the Hittinger substation in Belmont, Massachusetts exploded. The municipal utility, Belmont Light, said the failure was caused by a combination of heat and the age of the equipment. Safety systems acting as a circuit breaker cut the electricity entering the substation, and no fire started. The outage affected between a quarter and a third of the town's residents, with some neighbourhoods without power for nearly 10 hours. Three rented generator trucks were used to supplement the substation until repairs were completed. The high school ended classes early and pupils at two other schools were kept in their buildings for safety and logistical reasons. A 400 amp transfer switch on one school's backup generator malfunctioned during the event despite passing inspection two weeks earlier, and was scheduled for replacement.

The utility attributed the failure to the combined effect of high ambient temperature and the age of the transformer, occurring during a period of peak electrical demand. Officials noted that an oil fire, had one started, would have stretched fire resources and caused months of disruption.

1. Ageing substation equipment can be pushed to failure by combined heat and peak demand, which supports proactive condition assessment and replacement programmes.
2. Protection that isolates a failed transformer quickly can prevent escalation to an oil fire, reducing risk to responders and the length of the outage.
3. Backup generator transfer switches should be exercised regularly rather than only inspected, since infrequent use can cause erratic operation.
4. Pre-arranged access to mobile generation can shorten outage duration after a substation equipment failure.

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