"If there is ever a con line
The most common use of a UPS is to warn a computer user that there has in fact been a power failure (including that the power is too low to run the computer satisfactorily) and will allow them time to save any work in progress and then shut the computer down in an orderly manner. The next most common is when a site is fitted with a standby generator. A UPS is used to power a piece of power-critical equipment while the generator starts after a mains failure.
In both of the above examples the function is simply to maintain the integrity of any data and/or operating system files. This is the only claim to fame a UPS can even come close to! Beyond that, the UPS offers no guarantee of protection against damage from transients, surges, sags, etc.
Before we continue to slate the wild claim made by UPS builders, we need to define the 3 basic models. The first is known as a TOPS (Take Over Power Supply) and, as its name suggests, is only brought on-line when the mains supply is no longer deemed sufficient to keep the load happy.
Then comes the standard "battery charger / inverter" model. This usually has a common Earth and Neutral so as to keep any bypass circuit simple. The final model is the fully isolated version of the previous, allowing the Earth to be connected as best suits the installation.
Let's get the TOPS out of the way as this would be the one not likely to be employed in large data orientated concerns, but still employed in many homes and small businesses (and sold with the same con line). TOPS manufacturers don't like the real term and prefer to have a TOPS called a 'passive UPS' thus still making customers believe they provide uninterrupted power! Included in this definition are standby generators of larger systems.
By the pure fact that these systems reside alongside the power source, waiting to take over the function of supplying the power at a moments notice, means that while they are in "standby" mode the load is strapped directly across the mains supply. This means surges go "straight through" uninhibited and wave their unwielding energy in the face of the power supply of your precious PC.
The charger-inverter type is not that much better as they have the Earth and Neutral straight through. Yes, agreed, L-N transients don't (in theory) make their way through, but transients don't only exist between L-N. Not only are N-E transients still able to appear on the load side, N-E noise is also found there as well as being further aggravated by the UPS's unclean switched-mode generation.
The only UPS that may class itself as some form of protection against the outside world is the fully isolated type where the output has absolutely no reference to either the incoming mains or the earthing system. This allows the installation of the UPS to be guided by the location of the earthing star-point thus reducing as much of the LN-E noise as possible (that is, of course, if the system is not going to be correctly installed as balanced).
The reason even the most well built fully isolating UPS cannot offer full protection is multifold. It has, in order to keep the voltage stable, a regulator of its own. Serious transients could destabilise this causing the output voltage to either sag or surge. Incoming sags or surges, even if within limits, could cause the 'mains fail' detection circuit to trigger as half cycle dips or swells move the zero crossing point causing an apparent phase shift.
Even if fitted with transient protection, the UPS must never be deemed as able to stop high-speed impulses. If the UPS is installed at a low impedance node then only a few hundred volts of impulse at the UPS could translate into many thousands of volts within a few metres.
Although this is not an exhaustive list of reasons to not put faith in the protective ability of a UPS, it is hopefully enough to prove the myth. But the con line is still alive and well.....
The extent of how far the con line has propagated is evident by the fact many facility managers of "server hotels" are still refraining from installing decent power quality monitoring recorders. Sadly, it appears as if there is only one way these guys are likely to learn to stop hedging bets.... and that is the hard way. Even reading a page like this is not likely to convert them.
Let me end this myth this way; If I were intending to spend many thousands of Pounds housing a bank of expensive servers in a data facility, one of the first questions I would ask is "have you got full-disclosure monitors on both your incoming supply as well as what is fed to the data racks?". I can almost vouch the answer will be "No".
..... and yet your advertising says "we pride ourselves on top quality, highly reliable data centres that strive for 100% up-time".