Employers in California must give their employees 10 minutes of work-free rest breaks every four hours and can’t require them to remain on call and available for duty, the state Supreme Court ruled Thursday.

The 5-2 ruling upheld $90 million in damages for more than 14,000 private security guards employed by ABM Security Services, which required them to keep their pagers and cell phones switched on during rest periods, remain “vigilant” and respond to calls for assistance.

Reversing an appeals court ruling that overturned the damages, the Supreme Court said that since 1932 California law has required employers to leave workers on their own during the paid 10-minute periods.

“During rest periods employers must relieve employees of all duties and relinquish control over how employees spend their time,” Justice Mariano-Florentino Cuéllar said in the majority decision. “A rest period, in short, must be a period of rest.”

Requiring guards or other employees to remain “on call” during rest breaks creates “a broad and intrusive degree of control,” Cuéllar said, that prevents workers from taking a walk, making a phone call or pumping breast milk for a newborn child.

He noted that state law allows employers to reschedule rest periods when special needs arise and also permits some categories of employers to ask state labor officials for an exemption if the mandatory breaks would cause them “undue hardship.” ABM has been granted two one-year exemptions from the requirement in the past.

The law entitles employees to an hour’s pay if a 10-minute break is canceled. Another state law allows employees to take half-hour meal breaks after five hours of work.

In dissent, Justice Leondra Kruger — like Cuéllar an appointee of Gov. Jerry Brown — agreed that employees must be allowed to take rest breaks but said requiring them to carry a pager or cell phone during that period “does not constitute work.”

On-call employees are free to rest or take a walk during their breaks, said Kruger, joined by Justice Carol Corrigan. Prohibiting an employer from requiring them to carry a communications device serves only to “deprive the employer of any sure means of reaching the employee, even if a truly extraordinary situation requires it,” she said.

The security guards’ lawyer, Drew Pomerance, said the court’s ruling would benefit all sides by providing “a clear standard, so that both employers and employees will know what’s required of them.”

ABM’s lawyer, Theodore Boutrous, said his clients are “incredibly disappointed” and are considering their options for further legal review.

The case is Augustus vs. ABM Security Services, S224853.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @egelko

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View the ruling: http://bit.ly/2hdR6yW