American Electric Power has found the cause of Downtown explosions that sent manhole covers flying in February. It's spending a projected $30 million to prevent similar incidents.

American Electric Power has found the cause of Downtown explosions that sent manhole covers flying in February. It’s spending a projected $30 million to prevent similar incidents.

AEP and an outside consultant tracked the problem to a specific type of insulation used on electricity cables that had broken down because of water, salt and other factors.

The material, called “butyl rubber,” had cracked, exposing metal wires to the air. There was a high level of electricity running through the cables, which produced heat and burned the rubber, releasing a flammable gas, said Ram Sastry, vice president for infrastructure and business continuity for AEP Ohio.

“It’s surprising how much gas can come out of the butyl rubber,” he said.

The gas built up inside a small vault that holds electricity equipment, and then the heat and electricity likely led to a spark that caused the explosion.

The repairs, which have already begun, will not lead to a rate increase, the company says.

“We’re addressing the highest-risk infrastructure,” said Pablo Vegas, president and chief operating officer of AEP Ohio.

Nobody was injured six months ago in the morning explosions, near 4th and Lynn streets Downtown. There were at least two blasts that launched two manhole covers into the air and shattered one of them.

AEP has 259 vaults Downtown, which are accessible through 714 manhole covers. The company is replacing 72,000 feet of cable at locations in Columbus, and 22,000 feet in Canton. This includes all cable insulated with butyl rubber and other types of cable that show evidence of deterioration.

The butyl rubber-covered cables were designed to last 50 to 70 years, Sastry said. Most are less than 50 years old, installed in the late 1960s or early 1970s.

Manhole explosions are a recurring problem for utilities across the country, sometimes with fatal consequences. This is the first such event for AEP Ohio in decades.

The cause of the Columbus explosion is not a surprise to William Z. Black, a retired Georgia Tech engineering professor who advises utilities on how to manage heat in electricity systems. He was not involved with AEP in this case, but he has studied the patterns behind these incidents.

“These old cables suffer an increase in load beyond what was anticipated. They begin to overheat,” he said. “Rubber, you may know, if you’ve ever seen a tire fire, is hard to put out” when it catches fire.

In his experience, this is the most common cause of manhole-cover explosions, but there are few reliable statistics on the subject.

AEP worked with another engineer, Steven Boggs of the University of Connecticut, to help determine the cause. Boggs referred questions back to AEP.

The utility already has begun the work.

The project will cost an estimated $20 million in 2014-16, and $10 million more in 2017-18.

The money will come from AEP’s base-distribution rates, which already are built into customer bills, Sastry said.

To cover the costs, AEP will delay other projects in its maintenance plan that are not tied to immediate safety issues.

“We’re reprioritizing,” Sastry said.