Photographers view the charred traction power cables from the L'Enfant Metro accident in January 2015. (Linda Davidson/The Washington Post)

This Metro shutdown is all about electricity.

This is about the havoc electricity can cause when it gets loose, when it isn’t contained within power cables by tight, reliable insulation — when the juice finds a path out.

This is about the fatal calamity that occurred in a subway tunnel near L’Enfant Plaza station on Jan. 12, 2015, when electrical current escaped from cables along the tracks, generating tremendous heat, which caused melting and fire. The smoke sickened scores of riders on a stalled Yellow Line train, one of whom died of respiratory failure.

Could it happen again? Metro General Manager Paul J. Wiedefeld worries that it could, based on the particulars of a fire that broke out Monday on a track in a tunnel near the McPherson Square station, half an hour before the subway system’s 5 a.m. opening.

A look inside the smoke and fire defects that paralyzed D.C. Metro

“Preliminary findings show commonalities with the cable fire at L’Enfant Plaza,” Wiedefeld, who took charge of Metro on Nov. 30, said Tuesday in announcing that the subway network would close for 24 hours for a system-wide inspection of cables. “While the risk to the public is very low,” he said, “I cannot rule out a potential life-safety issue here.”

The commonalities?

The L’Enfant event, which is still under federal investigation, possibly involved electricity escaping from a type of subway power line called a “jumper cable.” Monday’s fire, Wiedefeld said, also involved a jumper cable.

“And this is why we must take this action immediately,” he said, referring to Wednesday’s inspection of the subway’s 600 or so jumper cables.

More accurately, this is a reinspection. Because last year, not long after the incident at L’Enfant, the National Transportation Safety Board warned that some jumper cables were in poor condition. Metro said it conducted an inspection then and replaced 125 of the cables. Yet look what happened: another jumper-cable fire.

Asked whether he doubted the efficacy of the initial inspections, Wiedefeld referred to Monday’s fire, saying, “I have this in front of me, and I want to do something about it.”

Metro announced that its entire rail system would cease operations for 24 hours beginning midnight Wednesday so crews can inspect cables throughout the system. Here are how some riders feel about the shutdown. (Lee Powell/The Washington Post)

Trains are powered by electricity that flows along third rails. Throughout the subway, there are multi-foot gaps in the third rails, for various reasons. These gaps are bridged by jumper cables, which function somewhat like extension cords, carrying the electrical current across the openings, and keeping the trains running.

Like all subway power lines, the jumper cables are heavily insulated. But if the insulation is compromised — if it wears out or is damaged — there is an excellent chance that trouble will soon follow.

Here is why:

Moisture is commonplace in subway tunnels. So are “particulate contaminants,” including brake dust, rust flakes and metallic shavings from train wheels. There is also a lot of grime in the tunnels. And there is plenty of other gunk, such as oil.

In electrical-speak, these are “conductive substances,” meaning they have a low resistance to electricity. They offer a path for electrical current to jump dangerously all over a tunnel if the electricity escapes from its insulated containment.

All it takes is for a path to be completed — for a trail of moisture, particulates or other conductive material to come in contact with the exposed electrical current.

The phenomenon is called “arcing.”

Early in its investigation of the incident at L’Enfant, the NTSB issued a safety bulletin that described “severe electrical arcing damage” to the jumper cables in the Yellow Line tunnel. That raised the possibility that the insulating material encasing the cable had been compromised and that the cable’s copper strands, juiced with current, were exposed.

When electricity gets loose, following that low-resistance path, it can generate enormous heat as it makes its way to ground. Current flowing into the conductive steel walls of a tunnel, for example, can turn the walls into giant hot plates. And it can set fire to debris in the tunnel. Depending on the severity of the incident, the thermal damage to surrounding infrastructure can create a huge volume of smoke.

That appears to be what happened near L’Enfant station. And Carol Glover, 61, a grandmother from Alexandria who was aboard the stalled train, lost her life.

Wiedefeld offered few details about the McPherson Square tunnel fire, which is being investigated. But he said the trouble there “was very similar to what we had in the L’Enfant Plaza situation.”

The tunnel wall near the McPherson station is lined with metal, as is the tunnel wall near L’Enfant Plaza. “There’s insulation covering the cables,” Wiedefeld noted. And if the insulation is damaged, “it can get open, and you have direct contact” between the current and nearby conductive materials, including the metal wall.

“That’s why we’re doing the assessment of all 600” jumper cables in the subway, Wiedefeld said. “Because I want to know what I’ve got with all 600 of them. This was inspected last year, and basically found to be okay.” Yet the problem recurred Monday. “So I have to find out what’s driving this situation.”

Jumper cables and other power lines are attached to one another, and attached to third rails, by large, elbow-shaped connector assemblies called “boots.” In its investigation of the L’Enfant incident, the NTSB also warned that throughout the subway, “a number” of boots lacked the proper type of “sealing sleeves,” which are designed to keep contaminants away from the electrical current.

Metro acknowledged that about 80 percent of its 6,400 power-cable connector assemblies lacked adequate sealing sleeves. Wiedefeld said Tuesday that about half of the faulty boots have been upgraded, but months of work on that project still remains.

For now, though, the attention is on the jumper cables.

Shutting down the Washington area’s busiest form of public transportation for 24 hours, stranding hundreds of thousands of commuters while Metro crews conduct what amount of maintenance work, is unprecedented in the transit agency’s 40-year history.

But this is about electricity. And electricity can be a killer.

“As the person responsible for the life-safety of this operation,” Wiedefeld said, “this is something I chose to do.”