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If you have any role in dealing with the power provided to the systems you manage, it's useful to be familiar with the terminology used to describe power and power devices such as UPS systems. Let's examine some of the terminology that you are likely to hear when people are talking about power provided to computer systems.
UPS - Uninterruptable power supplies are primarily meant to provide continuous power to systems through periods in which building power is unavailable. Some come in the form of small boxes meant to protect the availability of a single system. Others power entire buildings. Some keep systems immune to the minor spikes and sags (defined later in this column). Others are able to keep systems up for days. UPS devices make use of batteries to store power that can then be used when no other source of power is available.
generators - Generators convert one form of power into another. If power to my old farmhouse goes out, for example, I can use my diesel-powered generator to turn diesel into electricity. This would provide me with enough power for lights, clocks and small appliances for as long as the power is out and I have a good supply of diesel fuel.
blackout - A blackout is a complete loss of power and can seriously damage computers. A blackout is equivalent to yanking the power cords from your systems except that, in the case of a blackout, you would also need to concern yourself with whether there might be troublesome power spikes when power returns and your systems attempt to reboot en masse. You don't have this problem if you're plugging in power cords one at a time.
brownout - Brownouts are partial power losses. When voltage levels fall below 102 volts on 110 volt service lines or 204 volts on 220 volt circuits, you have a brownout. Brownouts can happen when too many devices are pulling power from a circuit at the same time. Anything below 105 volts can be damaging to electric motors (e.g., power tools), but power supplies on computers are generally able to condition power effectively, avoiding this problem.
NOTE: You will sometimes hear 110 volt power referred to as 120 or 110/120 and 220 volt power referred to as 240 or 220/240.
sags - A power sag is similar to a brownout, but involves less severe power losses -- between 102 and 105 volts on a 110 volt circuit and between 204 and 210 on a 220 volt circuit.
spikes and surges - Spikes and surges are both sudden, large increases in voltage. They can be caused by anything from lightning to the act of turning everything back on after a power outage.
surge suppression - Surge suppression involves removing power surges from a circuit so that the voltage fluctuation is minimized.
noise - Noise represents flaws in electric circuitry that affect power quality. Noisy power, for example, might involve major departures from the sine wave ideal of alternating current. Imagine a fuzzy sine wave and you'll get the picture.
volt amps - Volt amps or "VA" are a measure of power usage calculated by multiplying volts times amps. While volt amps are equivalent to watts, a lot of UPS systems these days are rated both in volt amps and watts and the measurements may not match. This is because the measurements are used for different things. The volt amp ratings are used to represent the apparent power that a UPS can produce while watts are used to represent the real power. You will often see power measurements in terms of kVA (1,000 VA) and MVA (1,000,000 VA). If you know the current draw of your equipment, you will be able to estimate the size of UPS you will need in kVA or MVA
watts - As just explained, watts are similar to volt amps, but are often used to represent an average of power flow over time. Since computers fluctuate in how much power they draw -- more on startup and when disk usage is heavy than at other times, wattage is always less than the volt amps measurement for the same system.
AC - Alternating current, or "AC", is the kind of power that power companies send to our homes and offices.
DC - Direct current, or "DC", is the kind of power that comes from car batteries. UPS batteries and the power supply feeding the electronics inside your computers. Transistors do not work on AC.
rectifiers and battery chargers - Rectifiers and battery chargers turn wall power into DC current (i.e., AC into DC). Rectifiers and battery chargers are components of UPS devices.
inverters - Inverters turn DC power into regulated AC power, useful for powering computers. Inverters are components of UPS devices. Low quality invertors make square waves where high quality invertors synthesize sign waves. A third, unusual kind of inverter uses a rotating DC motor connected to a rotating alternator that makes perfect sign waves.
voltage regulators - Voltage regulators clean dirty power, making it safer for use with computers and other electronics.
storage batteries - Storage batteries are chemical devices which convert electrical energy to chemical energy and vice versa. Most contain corrosive acids and/or toxic metals. They are used in UPS devices and are recharged as needed. The storage batteries in your UPS should have enough stored charge to keep systems up past small power outages and as long as is required to shut the systems down gracefully.
NOTE: Large storage batteries are not only heavy, but are potentially very dangerous when a shunt (i.e., a short circuit) occurs between the terminals. For ordinary truck and golf cart batteries, a metal wrench can easily touch both terminals simultaneously. When this happens, don't be surprised if you suddenly see glowing wrenches, flying bodies and exploding batteries due to the massive discharge of direct current. Warning signs should be clearly displayed where large storage batteries are within reach of workmen and sysadmins.
bypass mode - In bypass mode, a UPS is out of the loop with respect to funneling power to systems. UPS devices are usually put in bypass mode for maintenance.
alarms and events - Alarms and events are signals emitted by UPS devices to alert managers and dependent systems to changes in the state of the UPS. For example, when a UPS is within range of losing battery power, it may issue an alarm to inform systems running on UPS power that they should shut down. A UPS may also issue an alarm if its batteries are failing to hold a charge.
offline UPS - An offline UPS sends AC power directly to the attached computers while slowly charging its batteries. When a power loss occurs, the UPS switches from AC to battery. During the switch, there is a very quick power loss called a "glitch" which can cause problems for some systems, such as dropping connections.
online UPS - An online UPS always sends power from the battery to the computers (except in bypass mode). They continuously filter AC power to DC power, providing clean power to the systems they support.
