We work with a number of companies and everyone has their own policy and reasons (or in some cases lack of reasons for it). How you do it takes knowledge of standards but is mostly about using common sence (the least common of the sences) and good judgement.
Things to consider:
I believe that NFPA requires life-safety generators to be exercised every two weeks for 20 minutes, under load. This has been a long standing requirement and I assume has some research behind it. However, its not applicable to a data center as a failure during testing only causes an inconvenience.
My belief is that a generator should be normally tested weekly without load, with at least one week per month under building load. Finally once a year with a full load bank test.
The monthly test under building load should be by opening the main, NOT by exercising the generator from a remote start/transfer control button.
My reasoning is that the ATS needs to be exercised regularly and including your voltage sensing relays and controls.
I've seen too many failures because of not transferiing to building load under consditions that mimic real life.
Could my PM/Testing cause a failure? Probably not, but it could cause a pending failure to show itself at the time of the test rather then when power fails and the generator is required. Some people believe that it's better to wait until the failure occurs rather then instigate a failure. This line of reasoning is naive and fear based. Consider that during testing you have the ability to switch back to untility power. Providing that your UPS should carry your critical load and you can retart the HVAC in a timely manner.
When should we test? If you test during normal business hours you run the risk of causing an outage during prime time. This is true but on the other hand, you likley have staff and other resources available to get you back up and running.
Precautions:
1. All interested parties should be notfiied of when the testing will be performed.
2. Testing schedule should be coordinated with key activities such as data backup, quartery accounting runs, etc.
3. You should have a checklist to run through before you run your generator test (verify that UPS is normal, HVAC chillers & AHUS are running, etc.
4. If you don't have a battery monitor on your UPS system you should check for ripple current with a hand held ammeter (especially if your using VRLA batteries and don't have multiple strings).
4. Arrange your testing such that HVAC is serviced the day before the generator tests.
5. Arrange to have the appropriate service personnel on site or clsoe at hand.
6. After the test you should walk through and check taht all is well.
When you test without load you just need to run the genset long enough to coem to full temperature and allow the retransfer relay to shut it down. When testing under building load 20 minutes is probably more apprpriate. For the annual load bank test, a lot of people want 8 hours but I prefer either two or four hours. Two hours is sufficient to undcover any problems.
If you have redundant generators then you should rotate allowing the pair to carry the load and then one with minimal load sharing then the other.
Ken Baudry
www.kjbaudry.com