The impact between a car and the utility boxes on the sidewalk at The Mercer Apartments in late June at the corner of 77th Avenue S.E. and S.E. 27th Street has raised questions about the safety and placement of the electrical equipment there.
The accident was caused by the driver of a 1991 Toyota Camry who ran a red light at the intersection, and was hit by another car, then veered onto the sidewalk, hitting the first utility box and pushing it into the second one next to it. The Camry was left straddling the opening in the sidewalk beneath the box. The incident, which began at 12:30 p.m., caused a power outage that lasted an hour or more for thousands of Islanders.
The good news is that no one was injured despite the fact that the equipment inside the big metal can holding electrical equipment was energized. The car that hit the box was scorched by the arcing of the electricity inside.
The driver of the car was safe as she remained inside until she was told it was safe to leave.
Puget Sound Energy and Mercer Island emergency personnel agree, however, that there was a possibility that the ground surrounding the accident site could have been energized and that those near the scene were at risk of receiving an electrical shock.
To get a better understanding of the role of the utility boxes referred to as ‘switching cans’ by PSE, and the sequence of events that caused the outage and why, the Reporter posed several questions to engineers at PSE.
What is the nature of the utility boxes on the corner of S.E. 27th Street and 77th Avenue S.E. and what do they do?
The boxes, called padmount switch cabinets, generally 5 feet wide by 5 feet deep by 3.5 feet high, house connection points for underground power lines serving a neighborhood. They are built to operate at 12,470 volts (12.5 KV), which is PSE’s typical (distribution) voltage.
The cabinets provide protection to segmented groups of homes and businesses. If a problem occurs on the electric system serving the area, the switching equipment in the cabinets isolate the problem to a segmented area and prevent the problem from disrupting electric service to a larger group of customers in the area.
The padmount switch cabinets on 77th Avenue S.E. serve homes and businesses between I-90 to the north, S.E. 27th Street to the south, and between 76th Avenue S.E. and 78th Avenue S.E.
On Mercer Island, there are 17 padmount switch cabinets. Throughout Puget Sound Energy’s nine-county electric-service area, there are 2,746 padmount switch cabinets.
Why did the accident cause such a wide-scale outage and require the shutdown of the substation at S.E. 32nd Street?
When the car struck the switch cabinet, it damaged the electric components housed inside the cabinet. For safety, all responders correctly assumed that the immediate area could be exposed to live voltage.
When PSE’s serviceman arrived, assessed the situation and consulted with Mercer Island’s first responders (police and fire), the decision was to make the situation safe as quickly as possible so that responders could safely reach and help the driver. The serviceman instructed PSE’s operations center to de-energize the substation, which interrupted the flow of electricity that serves downtown and the north part of Mercer Island. Power was disrupted to 6,503 homes and businesses for 54 minutes to accommodate the rescue of the driver. Power was restored to all except for the building served directly from the damaged switches.
Similar to the need to stay clear of live, energized power lines that fall due to storms or other conditions, rescuers needed to stay clear of the damaged, energized cabinet until PSE crews de-energized the equipment.
The driver of the car correctly followed the safety instructions of Mercer Island’s first responders by staying in the car since the car had come into contact with energized electrical equipment.
There was a group of three dozen or so onlookers near the scene; was there a possibility that the ground underneath the switching can or the other box was energized after the collision?
It’s possible there was gradient voltage within close proximity of the accident. For safety, people always should assume damaged electric-system equipment (such as a downed power line) is energized until it is proven otherwise.
Mercer Island’s on-site police and fire personnel safely secured the area by keeping onlookers at least 75 feet away from the site. That distance kept onlookers safe. We are unaware of any onlooker reporting a tingling sensation in their feet or legs, which would indicate the presence of gradient voltage.
Why are the boxes placed at street level rather than underground at that location?
Most switch cabinets are installed above ground so they can be easily accessible to utility crews for safety maintenance, outage repairs and redirecting power.
Utility system equipment like switching cabinets, underground cable, power poles and power lines generally are installed during the early construction development of an area, which may be decades before the area becomes more populated or densely developed.
It’s estimated that moving an existing above-ground cabinet to underground would cost up to $500,000 — an expense that would be passed onto utility customers.
What are the obligations of PSE, the property owner and/or the city for the placement and/or protection of the boxes?
Since the padmount switching cabinets on 77th Avenue S.E. are approximately 15 feet from the street, they are considered well protected for pedestrians and well out of the path of car traffic.
PSE ensures that installation of energy system equipment for both electric and natural gas follows all safety codes and regulatory requirements to protect people and utility workers.
In locations where switching cabinets or other utility equipment would be exposed to physical damage, sturdy, concrete safety posts or bollards are installed to prevent damage.
Editor’s note: The switching cabinets on 77th Avenue S.E. were installed in 1998. They are within the boundaries of an easement established long before the more recent redevelopment of the corner.
