Garage door springs can be safely restrained by running a steel cable through the spring from one end to the other. This is done on garage doors all of the time. Think about a garage door. Say you have a 7' door and a person is 6'6". The spring is mounted about 6" higher than the door and sometimes lower. So you're walking under that loaded spring with your head less than a foot away from the spring. A steel cable through the spring is an acceptable method of securing the spring.
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