Why are there power outages when it's hot outside?

• Arizona utilities face heavy power demand in summer
• When equipment fails, utilities try to restore it quickly
• Utilities rely on customers to report and help identify problems on the grid

The first hot weekend of extreme summer temperatures brought power outages to thousands of customers around the state, mostly caused by failing equipment. Electric utilities spend all year preparing for summer, when power demand is highest, but outages are inevitable.

Arizona Public Service Co. and Salt River Project officials explain why the lights, and air-conditioners, go off sometimes in summer.

Why does summer bring power outages?

The power grid is constantly in use, but the demand on the grid is much higher in summer. The peak summer demand in Arizona is about double the strain put on the grid any other time of year.

The extra strain causes some components, such as glass insulators, to fail, even though they have operated normally through winter. So many minor outages are caused by equipment wearing out.

RELATED:Phoenix utilities prepare for hot weather this weekend

"If you have a high load condition, all your electronic devices are hotter and carrying more energy through them," said Jacob Tetlow, APS general manager of transmission, distribution operations and maintenance. "All electrical devices (eventually) fail based on time and temperature. When the ambient temperature is 117, and you pull more current through it, (equipment) is more challenged than normal."

Wayne Wisdom, SRP's senior director of distribution grid services, likens power outages during the first hot weekend to blown tires and overheated engines on a road trip.

"We'll end up with equipment that is weak, for lack of a better term," he said, adding that the number of outages on the first weekend with temperatures greater than 110 degrees was below average. "Maintenance crews have done a lot to get this equipment out of service before it fails."

How long before power comes back on? 

That depends on the problem, but some issues can be solved quickly. For example, if a transformer goes out, a trouble-shooting employee will likely be on site in 30 to 45 minutes to diagnose the problem, Wisdom said. Replacing the transformer can usually be done within about four hours.

If the problem is in an electrical cabinet, employees might be able to isolate the faulty equipment and turn power back on for everyone in the region even faster. That can happen when a power pole is struck by a car, too.

But on top of equipment failures, summer brings wind, rain and lightning with monsoon storms, and wildfires that can disrupt power supplies.

If two pieces of equipment are taken out in a neighborhood, which is often the case during storms, restoring power can take longer because it is not as simple as isolating a single piece of equipment.

"You would be surprised," Wisdom said. "Vehicles hitting poles or equipment is fairly common. (Sometimes) we can open the lines that were attached and we can still serve electricity up to that point, which means nobody is really interrupted for a long duration of time. If we lose another element, then we have lost our contingency."

What happens in an outage? 

First, a utility must be made aware there is a problem. If the outage is caused by a car accident, house fire or other emergency, police and fire departments might be the first to call in the issue to a utility. Often, contractors will call in when they inadvertently cause a problem by digging into power lines. But customers also can play an important role in alerting utilities to outages.

Both APS and SRP allow customers to report outages online (aps.com and srpnet.com) and to call them in. For APS, the number is 855-688-2437, and for SRP, it is (602) 236-8888.

When customers report outages, it helps dispatch crews identify where the problem might be, based on who has power and who does not.

Inside the APS operations center in Phoenix, large computer screen maps cover the walls, and when customers report power problems, their residence is highlighted on the map. This allows workers to identify which power lines, transformers and other equipment serve their neighborhood, and dispatch linemen to inspect and fix the problem.

How good are utilities at avoiding outages? 

In 2015, APS customers had an 82 percent chance of experiencing a power outage longer than 5 minutes. The average customer was out of power for 87 minutes of the year, according to Tetlow.

Those figures are averages. So some customers had no outages, while others had several.

The utility tracks those measures annually, though they don't change very much. The biggest variable on the statistics is storm damage. The summer of 2015 brought the most storm-related outages to the APS system in the past five years.

The American Public Power Association survey of public utilities in 2015 showed nationwide, 91 percent of utility customers experienced an outage, and the average customer was without power for about 63 minutes.

Do utilities have enough power for hot days? 

Absolutely. Utilities are required to project how much electricity will be used on the hottest day of the year and not only have that supply ready, but an additional reserve available to dispatch should demand go higher.

APS and SRP both maintain reserves of about 15 percent should power demand exceed expectations on any given day.

"None of these were related to any shortage of supply," Wisdom said of outages in early June.

What are the main causes of outages? 

According to APS, the top five outage causes in 2015 were storms, which affected 42 percent of customers, foreign interference from vehicles, birds, etc. (14 percent), equipment failure (13 percent), underground-cable failure (10 percent) and substations (7 percent).

What causes most power outages nationally? 

Wildlife causes more power outages than storms and other events, according to the American Public Power Association.

The trade group relies on an annual survey of public power companies, and reports 2.37 outages caused by wildlife in 2015 for every 1,000 customers.

The other leading outage causes were: overhead-equipment failure (2.02 for every 1,000 customers), weather (1.38), vegetation (1.04), "unknown" (0.76), underground-equipment failure (0.52), other (0.06), human (0.03) and vehicles (0.02).