I can't help feeling that whoever modified the door to prevent re-entry is being a tad unrealistic. Wonderful things, fire extinguishers - hold open doors of all sorts out here in the real world! Joking apart, many fire exits are also operational doorways; I can't for the moment think of a reason why they shouldn't be. I think you have made a rod for your own back by trying to prevent re-entry if it is a sensible and valid route for your staff. In the event of an emergency, if your plant shut-down arrangements are robust, your alarm system is clearly audible etc. and staff are properly trained then the risk of re-entry against the flow of people leaving is small. I'd worry more about those within the building who are just finishing their sales call despite that awful background noise and phone security to get it turned off, or those who just know, know, that it's a false alarm. I'm assuming you are not dealing with members of the public, where my assumptions on re-entry are not valid (Summerland etc.).