Step By Step 9 - Power Conditioning
Originally published in Australian Personal Computer magazine, July 1998. Last modified 01-Apr-2008.
Power conditioning is one of those unsexy computing topics, like backups, that people generally only become enthusiastic about after a horrible experience. It’s a lot less painful, though, to learn from someone else's mistakes.
Thanks to lightning strikes, industrial equipment, high current household appliances and a variety of other events, the mains supply (240 volts here in Australia; other voltages in other countries) can vary a great deal from its nominal value. Most household equipment doesn’t care much about its incoming power, and at worst hiccups a bit when the power drops out or surges. Computers, however, can corrupt data or just plain stop working when subjected to a power fluctuation that your microwave, freezer, TV and washing machine laughed off. If you value your hardware and your data, read on.
Know your enemy
Mains 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. By and large, computer power supplies deal with both of these quite well, though it of course depends on the severity of the irregularity, not to mention the quality of the power supply, and how much of its capacity is being used by the computer. The closer to maximum capacity a power supply is, the less likely it is to handle a given surge or sag. For this reason, a computer with a 300 watt (W) Power Supply Unit (PSU) is likely to deal better with line irregularities than one with a 235W PSU, although it may not ever need more than 200W of the PSU's possible output.
Outages are plain old blackouts, which are the Russian roulette of computing - 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, 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 or modem; you may lose your motherboard; you may even lose 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; lightning strikes a long way away can result in large induced spikes on these sorts of cables.
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. Do it if you like. It doesn’t hurt. But don’t tie knots instead of getting proper power conditioning gear.
The bargain basement
The plain surge/spike filter powerboards you can buy at various electronics, electrical and hardware stores are, arguably, worse than nothing. This is because they give you the impression you’re protected, when you probably aren’t - well, not for long, anyway.
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 your 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 generally last for decades, so no light almost definitely means no MOV - but since the light only 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, but you won’t find those at the hardware store. Standard MOV-equipped powerboard suppressors sell for around $50 (Australian dollars).
Doing it properly
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 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 - but I wouldn’t bet on it.
Since a line conditioner costs a couple of hundred dollars Australian, though, you might as well save up a little longer and get the next step up, a Standby Power Supply (SPS).
Laugh at blackouts!
If a line conditioner just isn’t heavy enough for you, it’s time for an SPS. SPSes are frequently sold as Uninterruptible Power Supplies (UPSes), but technically, they’re not. They act as a regular surge suppressor or line filter when all is well, but they also contain a backup battery (usually a lead acid "gel cell", hence the weight) and an "inverter", a device which makes alternating current (AC) from the battery’s direct current (DC).
The inverter only kicks in when there’s a power outage - before the computer’s power supply capacitors run out of juice, the SPS "cuts over" to battery power. Small models have enough battery power for only a few minutes of operation, but that’s long enough to save your files and shut down elegantly, or just ride out a brief outage, if you’re brave. Bigger models have huge battery banks that can keep important equipment running for much longer - but these devices are more likely to be proper UPSes.
The only use a true UPS has for mains power is to charge its battery. Its inverter runs all the time, so there’s no switchover time if the incoming power drops out. Proper UPSes have to have high quality inverters, which produce an output waveform as close as possible to a sine wave. Cheap "square wave" inverters are unsuitable for many applications and can, over time, damage PC power supplies, and the "modified square wave" versions that have become popular recently aren’t a whole lot better, for continuous use.
Incidentally, the same goes for cheap generators, whose output waveform is legendarily horrible. A small generator’s output fed through a good line conditioner is probably good enough to power a computer, provided the generator sticks reasonably close to the mains frequency - 50 hertz (Hz, cycles per second) here in Australia and some other countries; 60Hz in many others. But without the filter, forget it.
Provided the filter components are good, an SPS will do for domestic purposes. Since it isn’t running its inverter all the time, but only when the power goes down, it can afford to use a lower quality and much less expensive inverter; realistically, the computer’s unlikely to be running from the inverter for more than half an hour in its whole lifetime. As long as the computer power supply is good enough that the SPS’s cutover time is not too long - even if the blackout hits in the middle of a big disk operation - then an SPS is more than adequate. Again, going for an "overpowered" computer PSU will give your PC more tolerance of brief power cuts, including the brief cut while an SPS cuts over.
SPSes start at about $300 Australian, with a $450 400 volt-amp (VA) model adequate for a stacked PC.
Many SPSes and UPSes provide some form of data reporting, so the PC knows when power has gone down and can, for example, alert the user or automatically shut down.
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 stand to mess up a page. Incidentally, the same caveat goes for refrigerators, whose brief start-up current surge is enormous.
Things Not To Do
If you’ve got a UPS or SPS, don’t plug it into some other power filter and plug that into the wall. A quality UPS or SPS should be a perfectly good power filter by itself, and may not appreciate the other filter shorting spikes to ground for it. If you've got a cheapo SPS that doesn't filter power very well (or at all...), then another inline filter may be OK, but you'd do better to retire that SPS to the important task of keeping the VCR running through blackouts, and buy a better one for your computer.
Similarly, don’t use another filter between your SPS/UPS and the computer - that’s just unnecessary.
While I’m on the subject, bear in mind that computer cabling makes it possible to do dangerous electrical things like connecting together the grounds of two separate buildings. Say you decide to run, for example, a 10Base2 (thin Ethernet coaxial; see my networking guide here) network cable between two buildings. If the network is earthed somewhere, the shield of the network cable will be at the potential of that earth. If the network then gets earthed in the other building, and if those two earths have different potentials because of poor earthing in one or both buildings, miswiring or power surges, the unfused, unprotected Ethernet cable will deliver the higher potential to the lower potential end.
If you’re lucky, this will just stuff up your network. If you’re not, it can fry computers, start fires (Ethernet cable braid is not designed to carry high currents…) and shock anyone unfortunate enough to unplug the cable.
By and large, though, electrical mishaps with computer gear are dangerous only to the equipment and your data. A few hundred bucks worth of power filter can keep that three thousand dollar PC alive for a lot longer.
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