Dream Machine

[gotmilk]

Know your enemy

Irregularities come in four flavours: surges, sags, spikes and outages

A surge is a lengthy (2.5-second or longer) increase in the supply voltage.

A sag is a similarly lengthy decrease.

The closer to maximum capacity a power supply is, the less likely it is to handle a given surge or sag.

Outages are plain old blackouts, which are the Russian roulette of computing, as you’ll probably get away with no damage or only minor system corruption if the power drops out, but if you’re writing to the only copy of an important file at the magic moment, you can kiss it goodbye.

Spikes are the real nasties. A spike is a brief increase in the supply voltage, less than 2.5 seconds, and often a lot less. For a fraction of a second, a spike can easily subject your equipment to several hundred volts. If this doesn’t blow something up outright, it can progressively damage power supply and other components. So, after a few (or a few hundred) more spikes and surges, your PC dies, for no obvious reason. You may lose a power supply, motherboard, or your hard drive and everything on it.

If lightning directly strikes the power lines near your house, you will have a very exciting time and probably lose some gear, unless everything is unplugged. Fortunately, direct strikes to power lines are rare, because, by definition, a power line is well isolated from earth, and the lightning is looking for an earth. Buried power and telephone lines are a different story, though; even if lightning strikes a long way away from you, it can create large induced spikes on these sorts of cables that go all the way to your house.

Some people say that lightning surges in power or phone cables can be arrested by simply tying a series of ordinary overhand knots in the cable. Allegedly, the knots do a nifty induction trick when the cable tries to pass super-high voltage, and burn out, saving the equipment.

The physics behind this is not nonsense, but there is still considerable controversy over whether or not knots do any good in the real world. They certainly don’t stop ordinary lower-voltage spikes, and they’re ugly.

The chief surge-clamping component in a basic filter-board is a Metal-Oxide Varistor (MOV). MOVs pass current only when the voltage across them is above a set value, and they react very quickly (in a matter of microseconds, against the tens of milliseconds a circuit breaker takes). That’s the good news.

The bad news is that MOVs wear out. They’re only good for a few uses, and the bigger the spike, the more damage is done.

Cheap power filters seldom give you any indication whether the MOV is alive or not. If the powerboard has an illuminated power switch, the switch light often goes off when the MOV has died. The switch lights themselves generally last for decades, so no light almost definitely means no MOV - but since the light only actually shows the status of a fuse, and the fuse won’t blow if the MOV has been killed by lots of smaller surges, the light can keep glowing merrily when the MOV has long since kicked the bucket.

Anti-spike gear may also include gas arrestor tubes, which are far more durable than MOVs but too slow for computer applications, or silicon avalanche diodes, which give much of the robustness of gas tubes with the speed of a MOV. The best spike suppressors have all three components.

The next step up from a basic surge suppressor, and the cheapest choice if you actually want reliable protection, is a line conditioner. Line conditioners do everything a surge suppressor does, but do it better, and often also boost the output voltage during sags. They’re considerably more expensive than a simple suppressor, because they often include a honking great transformer which makes them satisfyingly heavy, and some chunky capacitors.

These components provide some degree of outage protection - a brief power cut, enough to put the lights out for a moment, will be filled in by the conditioner and leave your PC un-reset. It’s even possible to accidentally unplug the computer, jam the plug back in and keep on trucking.

If you’ve got a laser printer, don’t plug it into your SPS or UPS. Laser printers draw a lot of power when they’re warming up and, when on, they keep themselves warm with periodic reheats. This periodic heavy drain can easily overload an inverter.

A line conditioner is all you need for your printer, anyway. If you’re not printing money, you can probably stand to mess up a page if there's a blackout.

In a Standby Power Supply (SPS), incoming power is used to charge the onboard battery, but the inverter only kicks in during an outage. A true Uninterruptible Power Supply (UPS) charges its battery with incoming power, and permanently runs its inverter to provide connected equipment with AC. In the event of an outage, there is no "cutover" delay.

above info from: http://www.dansdata.com/sbs9.htm
Originally published in Australian Personal Computer magazine, July 1998.
Last modified 26-Aug-2008.