TOKYO—Nine months before the March earthquake and tsunami cut off power to the Fukushima Daiichi nuclear plant, one of its reactors suffered a loss of electricity under much more mundane circumstances: A maintenance worker from a subcontractor accidentally bumped an electronic switch with his elbow.

The outage triggered a sharp drop in the level of cooling water for the radioactive fuel rods, according to the plant operator.

The little-noticed incident didn't result in any reactor damage or release of radiation. But it highlighted some of the vulnerabilities of the aging facility and raises questions about the precautionary procedures and outmoded equipment that operator Tokyo Electric Power Co. , or Tepco, had in place.

According to Tepco's investigation, at 2:42 p.m. on June 17, 2010, an automatic shutdown set off an alarm in the control room of the No. 2 reactor, one of six at Fukushima Daiichi. Within minutes, control rods were inserted to stop the nuclear fission. But without electricity, the water pumps stopped supplying the reactor core with fresh water. Temperatures and pressure levels rose inside the reactor.

Tepco says no meltdown was imminent, and that normal water levels were restored soon after the power outage. Company officials say that emergency diesel-fuel generators were programmed to start automatically and that if the water inside the reactor had dropped 20 centimeters further, the emergency core cooling system, or ECCS, would have been triggered, preventing dangerous exposure of fuel rods.

But some critics say the consequences could have been more serious if not for quick-thinking control-room operators who manually activated the diesel generators and ECCS. They say the accident revealed weaknesses in blackout situations.

"When I heard about what caused the shutdown last June, I didn't know whether to laugh or cry," said Kazuyoshi Sato, a local assemblyman in Fukushima prefecture's Iwaki city, 25 miles from the plant. He learned of the incident the following day from the local media. Mr. Sato, a longtime Tepco critic who wrote at length about the incident last summer on his blog, added: "If a simple blunder like that can almost cause a meltdown, then what does that say about the plant's safety controls and systemic flaws?"

The company and Japan's Nuclear and Industry Safety Agency blamed human error, in separate reports completed in July 2010. "The subcontractor didn't realize he'd brushed up against the switch while doing other work and said he was completely unaware of its critical function," Tepco spokesman Hiroshi Aizawa, based in Fukushima City, said in a recent telephone interview.

John Lee, professor of nuclear engineering at the University of Michigan, said it isn't unusual for a plant to suffer a brief loss of outside power, as "there could be a power flicker of some sort for a few minutes."

"But a reduction of water in the core is quite unusual, so that's an event of significance in my mind," added Prof. Lee, an expert on the reactor type used at Daiichi No. 2.

Experts say that plant operators don't take lightly unplanned shutdowns, known as a scrams, and that they can prompt structural changes. "This kind of scram is stressful on the reactor," said Robert Albrecht, a University of Washington professor emeritus of electrical engineering, who has a background in nuclear engineering. "It sounds to me like you might want to change the design."

In the wake of the incident, Tepco and NISA called for increased worker diligence, but no fundamental rethinking of the plant's power-supply grid. To prevent a recurrence, Tepco posted warnings signs near critical switches at the plant to make workers aware of their presence.

NISA approved a decadelong extension to the 40-year-old plant's life in February, five weeks before the March 11 earthquake.

The heads of NISA and Tepco were grilled about the incident in parliamentary testimony spurred by issues raised after the earthquake. At a May 1 session, company President Masataka Shimizu confirmed specifics omitted in Tepco's official report. Among them: Water levels inside the No. 2 reactor had dropped by two meters (nearly seven feet) for about 30 minutes before being restored.

The No. 2 reactor was shut down for a month as Tepco determined that it was caused by a technician employed by an affiliate company. While replacing a temperature-recording gauge behind an electrical panel, the subcontractor inadvertently bumped an auxiliary relay, or electronically operated switch, with his elbow.

That led to what NISA and Tepco described in their reports as a "momentary" flutter that was long enough to trip a circuit breaker shutting off the reactor's main power supply, but too short to trigger the normal backup power supply.

As a result, the electric pumps seized up, temporarily halting the flow of water used to cool the reactor's red-hot fuel rods, according to the NISA and Tepco reports.

The switch was part of a power-transmission-line stabilization device installed at Daiichi in 1984 and also in use at other Tepco plants. In 2009, the company determined these devices were no longer needed,with newer transmission lines being added, and plans were made to remove them. "We already had plans to get rid of them because they are outmoded," said Mr. Aizawa.

In 2010, damage to the reactor's core was avoided when control-room operators activated the external diesel generators and vented the reactor vessel to release pressure into a suppression pool. That move, which didn't release any radiation outside the plant, replenished water levels over the following hour, the company said in its report.

The zirconium-coated fuel rods, filled with ceramic pellets containing uranium oxide, can disintegrate if not continuously surrounded by water, according to nuclear-power experts. The No. 2 reactor's fuel rods stood about four meters tall and were normally submerged in water twice as deep. During the June 2010 incident, the water level dipped as low as about six meters.

This year, after the quake-instigated power outage shut off Daiichi's water pumps, the exposed fuel rods inside one of the three active reactors began to break down less than five hours later and had completely melted within 15 hours. The other reactors' fuel rods, including those inside No. 2, melted down within four days.

Tepco said last month that the containment vessels of those damaged reactors likely suffered breaches as a result of the meltdowns which may have led to additional radiation leaks.