During UPS maintenance, companies providing mobile power have to service a variety of customers and usually use very good, well regulated equipment. There's not guarantee, of course, and I would voice concerns to the appropriate persons. They may be more likely to ask for assurances from the company providing the service.
The main thing I would suggest is that your Power Feeder is protected with a good Transient Voltage Surge Suppressor (TVSS). This is good practice, anyway. It's likely that TVSS protection might already be installed if the data center space is being leased. If not, a UPS maintenance window would be a good time to add it.
The specific type and size is dependent on the characteristics of the power plant being protected. I suggest consulting an electrical engineer familiar with the specifics of your building power. This is about all you can do, short of buying individual UPS's and putting them on your most critical pieces of equipment. If you do that, however, make sure you use true, "double-conversion" UPS's. The less expensive "line interactive" units can do funny things when connected to generators.
My biggest concern would be how the switchover to generator will occur during the UPS maintenance window. If the central UPS has full wrap-around bypass, this shouldn't be a problem. They will probably just get the generator running and switch the bypass to it. But if the UPS has only internal maintenance bypass, then facilities will need to totally "down" power before switching to the generator. In that case, you would need to do an orderly shutdown of your data center, wait until any other loads are switched over, then start bringing your systems back up.
You would probably need to do the same thing when the UPS is restored. The important thing is that full loads are not suddenly switched to the generator, which would most likely cause it to become a bit unstable for several seconds when the load hits. You want the most critical equipment to be the last things switched over, particularly if this power shutdown will affect non-UPS equipment, such as building elevators or air conditioners. Those should definitely be brought online before any electronic equipment is switched over.
ABOUT THE AUTHOR: Robert McFarlane is a pioneer in the field of building cabling design. He has been asked to speak at countless seminars on building infrastructure for electronic communications, evolving technologies and the requirements of trading floor and data center design. Mr. McFarlane served for twelve years as President of Interport Financial, Inc., a firm specializing in designs for financial trading floors and critical data centers.