The city does not know what caused three radio transmitters to turn off in the early hours Wednesday morning, leading to a 10-hour shutdown of the western half of Ottawa's new LRT system.

"We do not know the root cause, we are continuing to examine the cause of this," John Manconi, the city's general manger of transportation and head of OC Transpo, told reporters late Thursday afternoon.

"That's the one piece everyone is looking into. What caused this to trip? And how did the redundancy systems behave and act?"

On Wednesday morning at 3 a.m., something caused three radio transmitters in the downtown tunnel to trip into the "off" position. The city previously had said the issue may have been related to the lightning storm that occurred around the same time, but can't say for sure.

An hour later, two trains doing the pre-rush hour "sweep" — where trains travel the entire line at slow speed to make sure there are no issues on the system before the daily runs start — were stopped just west of Tunney's Pasture.

The radios communicate the location of moving trains to the central computer, so when those radios are down, trains aren't allowed to move. That's how the $2.1-billion Confederation Line is designed.

But it took workers eight hours to figure out the problem was the tripped radios.

One issue was that technicians examining the alarm data logger, which tracks various alarms on the system, hadn't looked far enough back. When they did, they found that the radios had been tripped at 3 a.m.

Two trains doing the pre-rush-hour run were stopped just west of Rideau station on Wednesday when three radio transmitters went down. (City of Ottawa)

Manconi said that from now on, the data logger will be monitored 24/7.

"We learn and grow from these situations," he told reporters. He also said that, as far as he knew, radio transmitters turning off was not an issue in the testing and commissioning of the LRT system, and is not on the list of outstanding minor deficiencies.

Passengers would have got out

This week's incident will not impact the public launch date of the LRT set for Saturday, Sept. 14.

If something similar occurred with passengers on board, they would not have been stuck on the train for hours. Instead, the trains can be operated manually and moved to the next station.

Announcements would let riders know how long the delay was, and what would happen next.

Residents of Ottawa weigh in on the debut of the Confederation Line, expected to open to the public on Sept. 14. 0:43

In fact, the trains that were stranded Wednesday with no radio communication both eventually moved to opposite ends of the LRT line. Manconi was at pains to point out on Thursday that at no time were the trains "stuck."

Once a train was in a station, passengers would disembark and have to figure out how to get to their destination.

"The reality in one of these situations, if we do have a major outage, is we will have limited bus service," said Manconi.

"We don't have thousands of buses waiting around. And people will do what they do in other major metropolitan cities: they're going to get out, figure out if it's close enough to walk to where they need to get to, grab Uber, bike, walk, taxi, so forth."

Of course, they could also try OC Transpo bus service.

The incident also provided the city's transit staff an opportunity to test their procedures in the instance of an LRT shutdown.

"We used the opportunity to test all our protocols, so we pushed out our customer-facing electronic signals, we pushed out our station announcements, we mobilized our bus-bridging operation," said Manconi. "We were doing our drills."