Nearly 4,000 homes and businesses are without electricity or air conditioning Saturday after a power outage zapped parts of Sacramento’s core — including midtown, East Sacramento, Curtis Park and Oak Park — because of a faulty breaker at a substation.

A spokesperson for the Sacramento Municipal Utility District said the outage, first reported at 12:10 p.m., was being fixed at a substation where crews were “repairing a breaker.” At first count, the outage was affecting 3,833 customers, according to SMUD’s outage information webpage.

What caused the breaker to malfunction or fail was not disclosed by SMUD officials, though they said power was expected to be restored by 1:15 p.m. The utility did not say which substation was affected, but one substation at 35th and R streets fell within the outage’s immediate area, according to mapping provided by the California Energy Commission.

Sacramento Regional Transit also reported Gold Line light rail service, which runs from downtown to Folsom, was paused in both directions. RT’s light rail trains are powered by overhead electricity lines and the California Highway Patrol reported minutes into the outage that one RT train had stopped at Alhambra Boulevard and R Street, blocking traffic.

Temperatures in the area were around 93 degrees just before 1 p.m., as the capital region was under a heat advisory until 8 p.m.