Federal regulators have concluded their investigation of a power outage that led one of Duke Energy's nuclear reactors on Lake Wylie to shut down abruptly in April. A preliminary report from the Nuclear Regulatory Commission says Duke didn't follow proper guidelines in testing upgraded equipment on one of its power generators at the plant. NRC spokesman Roger Hannah says the commission will wait on deciding on whether to issue a violation to Duke. "What we want to do is offer them the opportunity to provide their perspective and to provide the kinds of actions they have taken since then to us," says Hannah. Duke has 30 days to respond to the NRC's findings. The April incident at Duke's Catawba nuclear plant worried regulators because a sudden loss of power can stress a reactor. Duke has back-up generators that worked properly and prevented any danger to the public.