Pros: No switching glitches when the power goes out; versatile mounting
Cons: Slightly less efficient than a line-interactive type
The Liebert GXT2-2000RT120 is an "on-line" type of uninterruptible power supply capable of supplying up to 2,000VA of power (1,400 watts). It is an excellent choice for protecting extremely critical or sensitive equipment.
I work for a regional television control facility where we have literally hundreds of UPSs of various types and sizes in service. For the last year or so, I have been installing GXT2-series UPSs in the most critical areas of our plant; last week I installed six of this model to protect our main video server system.
To appreciate what makes this model (and the whole GXT2 series) stand out takes a bit of explanation.
Background: "on-line" versus "line interactive"
There are two basic approaches to building a UPS, and each has its pluses and minuses.
A "line interactive" UPS contains an electromagnetic switch called a relay that steers utility power to the load as long as the utility power is within proper limits; if the utility fails, the relay switches the load to the inverter, which creates AC from the internal batteries. When utility power returns, the relay switches the load back. Line interactive UPSs can be quite efficient: the only additional power they consume goes for keeping the battery charged and for running the control circuitry. The downside is, the relay does not make a seamless switch between the utility feed and the inverter, so there will be a brief glitch every time it switches in or out. Most computers have sufficient filtering in their power supplies that this is not generally a problem... but if you are protecting particularly sensitive equipment, switching glitches are something to be avoided.
An "on-line" UPS uses utility power solely to keep the batteries charged; the load is always powered by the inverter. As a result, even nasty utility power failures with repeated up-down-up-down recycles never have an effect on the load. Another benefit to on-line systems is that the utility voltage and frequency can vary over a wide range and keep the battery fully charged (this is very important if you have a standby generator). The downside is that since the charger and inverter aren't 100% efficient, it will take slightly more power to run the same load compared to a line interactive UPS.
While a line-interactive model can be a cost-effective solution for less critical applications, an on-line UPS like the Liebert GXT2 provides the best protection for sensitive electronics, especially under adverse conditions.
Installing the GXT2
As a class, uninterruptible power supplies are heavy and take considerable muscle power to install. The 2000VA GXT2 weighs in at 54 pounds, which sounds like a lot... but a comparably sized APC SmartUPS is nearly twice the weight. Much of the difference is because an on-line UPS like the Liebert doesn't need the elaborate filtering and voltage correction circuitry that a line-interactive unit like the APC requires.
The GXT2 comes with a pair of plastic floor stands, or you can purchase an optional rack mount kit for about $70. The mounting versatility is nice feature: most stand-alone UPSs can't be rack mounted, and most rack-mount UPSs don't lend themselves to stand-alone installation.
This particular 2000VA model comes with a 20-amp plug at the end of a ten-foot cord, so the outlet you plug it into must have a 20-amp receptacle (it looks like an ordinary socket, except that instead of two parallel slots, one slot is "T" shaped).
Once the UPS is in position and plugged into power, you just hit the "on" button -- the batteries are already connected, and there's nothing else you have to do to make it go.
The UPS comes with a data cable and software that lets your PC communicate with the UPS; among other things, it lets your PC automatically do a safe shutdown before the batteries run down. It also lets you monitor the condition of the UPS and get alerts to potential problems, like aging batteries. You do not need to install this, and generally we do not... but it is available if you want that functionality.
On the output side, the GXT2 has four 20-amp T-slot outlets. The voltage comes set to 120 volts at 60 Hertz, but you can change both if your application requires it. The power is a pure, low distortion sine wave, so any equipment will run very happily from it.
The front panel has several indicator lights, a bar scale meter, and buttons to control the basic on/off, self test, maintenance bypass, and alarm silence functions. The indicator lights tell you when utility power is present, when the unit is running, when power is coming solely from the battery, and when the system is in bypass. When utility power is present, the bar scale shows how much load is connected; when utility power fails, it shows how much battery life remains.
Our broadcast facility includes a large diesel generator with an automatic transfer switch; transitions between the utility power and the generator (and back) are sometimes rather dirty. But the output of the Liebert is unfailingly clean and stable, free from any hint of disturbance.
Maintenance for the GXT2 is minimal: keep it clean. You can expect the batteries to last at least 3-5 years, so long as you keep the unit at room temperature -- heating will shorten battery life. Replacing batteries is particularly simple with the GXT2: the entire assembly is self-contained and can slide out even with the UPS powered and feeding the load. By comparison, replacing batteries in APC SmartUPS units requires some care, as battery wiring is exposed. Also, it is not unusual for aged batteries to swell, and removing swollen batteries from certain APC rack-mount models is especially difficult.
The most obvious option for a UPS is extended run-time. At the full 2000VA load, this model will run without utility power for about six minutes; at half power the run-time increases to 14 minutes. If you need additional run-time, you can plug an unlimited number of add-on battery packs that daisy-chain to each other, and finally to the back of the UPS.
Liebert also offers a plug-in module that lets the UPS connect to your network, so that it can perform clean shutdowns on multiple machines -- even running different operating systems.
The Liebert GXT2-2000RT120 typically sells somewhere in the $1,150 range, about $50 more than a comparable rack-mount APC SmartUPS XL. But the difference in weight means that the cost difference will be largely offset by lower shipping charges... so the final cost should be nearly even.
Our experience with this particular model goes back about a year; with 15 on line to date, we have yet to experience a problem. We have a similarly unblemished record with our older Lieberts, going back more than eight years. Of particular note is what happens when the batteries need replacement: the system keeps right on running properly (though obviously, run-time is reduced). By contrast, we have had a number of APCs completely shut down as a result of battery failure.
A UPS of this size is not a small investment, and anyone considering such a unit is most likely doing so to protect expensive or mission-critical equipment. While the nature of the Liebert on-line system makes it slightly less efficient that some competing models, the total isolation between the load and the utility supply provides greatly superior protection. The GXT2 series has become our standard for replacement and new installation, and should be at the top of your list.