It's difficult in practice to transfer load to generators for test especially if there's load shedding involved as is common in larger facilities (higher-power-

It's difficult in practice to transfer load to generators for test especially if there's load shedding involved as is common in larger facilities (higher-power-consumption items like part of air conditioning capacity are cut off when transferred to generators). To help address this a lot of more critical facilities are equipped with a test load (really just a very large outdoor resistor with active cooling) that allows for running the generators at rated load for test and exercise without actually transferring. Of course, these tests do not always detect problems with the transfer equipment, and if things aren't designed and maintained well the generator may experience load under real conditions that behaves differently from the test load.