This was a old hospital, now gone. It had an underground fuel tank, everything around here that I've seen is natural gas now.
Natural gas is great as long as the compressor stations along the transmission pipe lines have power. When the power goes out, the compressors stop and your natural gas stops flowing. This was the cause of very wide spread natural gas outages during and right after hurricane Katrina. Large areas along the Gulf Coast lost natural gas. Results were many hospitals lost their natural gas feed and this caused their generators to go down.
This loss of natural gas is why I am not a big supporter of natural gas emergency power generators. The next bug in the wood pile is the ability to fill propane or LG tanks when the power is out. Other than special tank trucks with pump and a long hose, I don't know of any way to transfer propane from a large tank to the tanks on the ground at your generator. So check with your supplier and make sure they can fill their transport truck if the power is out. Then you only have to worry about trees across the road and flooding.
How many locations do you know of that run their emergency generator for a test, but never pick up the load they are there to support? In my travels around the country, I frequently ask about the backup generator at the dispatch centers. The main question I always ask is if the load is picked up by the transfer switch. If they do that, the following question is how long the test runs. The reason for the second question is to make sure the engine runs long enough to bring the crankcase oil up to operating temperature. If the temperature doesn't get high enough, the moisture that normally collects in the crankcase will not be evaporated from the oil. Left in this condition, the oil can turn to a foamy brownish goop. This goop lacks lubrication and can cause major damage if the engine runs with it for very long.
Another reason for picking up the load on the generator is to exercise the auto transfer switch. These switches, as has already been mentioned, can fail to move from normal power to the emergency mode. Many times they are rusted in the normal position from lack of use. You can beat on them with a sledge hammer when they get like than and you will never move them.
Bottom line here is to run the generator each week for about 20 to 30 minutes and pick up the emergency load on the generator. This tests the entire backup power system. It makes sure the generator will even start. It makes sure that you can pick up the emergency power bus load. It makes sure that the UPS power packages will function on generator power. Plus it lets the facility manager sleep at night during any storm that may roll through.