I don't know the minimum requirement but that's one thing we don't skimp on. We put them just about anywhere you'll pass on the way out of a problem in the fueling islands and a couple of distant points that you can view them from. We also have a connection to the FACP so that any zone associated with or nearby the fueling will cause an E-Stop on alarm. We can bypass the FACP E-Stop but that will put the FACP into Trouble or Supervisory until enabled. We only use one key reset switch in the Foreman's offices. I sometimes find these reset switches jumpered at which time I tend to light a little fire of my own.