What are the pros and cons of having your own UPS attached to your own server hosted in a data center?
Pros:
- None
Cons:
It interrupts the flow of the Emergency Power Off (EPO) in the datacenter. If there is a life or death emergency in a datacenter, that EPO might be triggered to save someone's life. If your rack has its own UPS, it will violate that EPO order.
You will not get extended runtime. Chances are as soon as your upstream UPS switches into battery mode, your UPS will detect the change in sine wave and drop into backup mode as well.
You'll violate your warranty and potentially your datacenter's UPS warranty. UPSs are warrantied to be installed in very specific scenarios and power sources. Your UPS-on-UPS is not going to be a supported configuration.
(from rexkogitans) You have to care about your UPS while there is a staff caring for a UPS that may be provided to you nonetheless. So it adds administrating a UPS to your work really unnecessarily.
Apart from low-probability scenarios, the single most important reason an unnecessary UPS really cheeses people off is that a year or two from now, it will start beeping.
An UPS needs regular battery replacements, and it usually tells about it by making a really annoying beeping noise that goes on forever. In a locked-up rack. Right next to the other rack you are trying to get some work done on. And it will keep making that noise for days (or months), because whoever installed it and didn't maintain it, also couldn't be bothered to configure any remote alerts on it.
And adding insult to injury, all this happened, because someone wouldn't trust the much better UPS system already in place. Yes, use an UPS when you have to place your rack in a utility closet of an 18th century farm house. But don't do it, when you have way more reliable power readily available on the outer side of the UPS.
You may need to run your own UPS when your datacenter doesn't quite deserve the name and it does not provide centralised UPS for you and other customers.
On the other hand, if that is the case there will probably not be a rule preventing you from installing your own though.
If you only get a couple of units in the top of a cabinet, good luck hoisting your UPS that high and not tipping over the cabinet, those things can be heavy!
And maybe stating the obvious: an implied consequence when you need to run your own UPS, is that everything that is not connected to it will not be protected. That may include your connectivity and other services the datacenter provides.
Don't forget the Electrical Code. Most have something like NEC 110.3b, "equipment must be used according to its labeling and instructions".*
That gives the instructions the same force of law as the Code itself.
If your UPS says "do not feed from a UPS" or if the datacenter's UPS says "do not supply UPS's", then you place the datacenter in dutch with the electrical codes. You don't get written up - they do.
* Because that is the scope of testing that was done when it was UL listed. This section is partner to 110.2, which requires only approved equipment be used. All this is on the first page of NEC. You can't miss it.