it interrupted a state department briefing and forced the white house onto back-up generators .
outage ` briefly had an impact on the white house complex ' but it was ` back on the regular power source ' an hour and a half later .
white house press secretary says he was in the oval with the president when it happened , and it was barely noticeable .
related to a dip in power in a transmission line at a maryland facility , power company says ; another report says it may have been an explosion .
homeland security says there 's no evidence of malicious activity .