UPSes protect devices from more that just power outages. I've got a Schneider SURTA2200XL UPSes (manual here). It protects against all of the following:

  1. undervoltage (aka brownouts)
  2. overvoltage
  3. power surges (e.g. lightning strikes, transformer shorts, etc.)
  4. power outages

I think it might also protect against badly distorted mains voltage waveforms but I'm not sure.

For devices that can tolerate power failure, how can I determine whether they can tolerate overvoltage and brownouts?

Are there ways to protect against undervoltage, overvoltage, and power surges without the expense of a full-blown UPS?


Background: I manage two sites in a town with overhead powerlines, winter storms, people with guns, and the occasional lightning strikes. These circumstances combine to create some rather hostile seasonal power events. We typically lose a small but significant number of devices to mains power events each year. The devices on online UPSes seem to be fine. It is the devices connected directly to mains power that seem to be vulnerable. One of our sites happens to be in the same building as a master electrician whose company also happens to be, among other things, the power company for some remote villages. So, for better or worse, this solution is mainly informed by an electrician rather than an IT professional.

The Threats

There seems to be three common threats to devices directly connected to mains power:

  1. Undervoltage or Brownout
  2. Harmonics and transients
  3. High Voltage (as in several thousand volts)

Undervoltage or Brownout

In my area there are two common causes of undervoltage or brownout:

  1. a large dynamic load in the neighborhood
  2. loss of a phase

A large dynamic load in your neighborhood can cause the voltage feeding your site to fluctuate. The voltage you see at your site will be maximum when the load is switched off, and minimum when that load is switched on. The dynamic load in the neighborhood of one of our sites (the electrician thinks it might be a deep fryer or water heater) causes the voltage on one of the two phases to fluctuate between 106 and 142 volts. Occasionally and momentarily, that same phase inexplicably drops to 98 volts.

When only one phase of a multi-phase service is interrupted, the interrupted phase's voltage does not go to zero. This is because many multi-phase devices (like furnaces, water heaters, motors) will backfeed some voltage back to the interrupted phase. So, other single-phase devices directly connected to mains power (like, say, access switches and monitors) will see some low AC voltage. I measured between 20 and 80 volts during a recent phase interruption.

Based on my experience so far, the 106-142 volts doesn't seem to cause harm as long as the voltage is stable. However, the 20 to 80 volts experienced during that recent phase interruption proved harmful. During that event, an unmanaged switch and the power supplies of two monitors were damaged and had to be replaced. This is roughly consistent with the 90-240 volt input power specifications of many universal power supplies. In other words, when one phase is interrupted, you are probably subjecting all of the devices directly connected to that circuit to power that is out-of-specification.

Harmonics and Transients

Some industrial devices (think big motors, servos, heaters) backfeed harmonics and/or transients back onto the power grid. Once on the power grid, the whole neighborhood's power can be polluted by these transients and harmonics. Europe requires that some devices be immune to mains power harmonics and transients. North America has no such requirement. In a previous career I designed electronic devices that underwent immunity testing for Europe. When subjected to mains power transient and harmonics, the output voltage of power supplies occasionally went wildly of specification. Those problematic power supplies I tested are blackboxes indistinguishable from the handful I see powering switches, routers, monitors, and laptops at our sites.

The moral of the story is this: If you have enough transients and/or harmonics on your mains power, it is certainly possible that some of your adapters will occasionally produce an output voltage that is out-of-specification.

High Voltage

This is the fun one. By high voltage, I mean thousands of volts. There are two common reasons you might see high voltage at your site:

  1. short-circuiting of primary and secondary transformer windings
  2. lightning strike

In our area, the voltage of the conductors at the top of power poles is commonly 14,400 volts. The grey cylindrical transformers you see on poles and those ground level metal boxes you see around subdivisions transform that power down to the hundreds of volts that is typically fed to your site. Apparently people like to use the pole-top transformers for target practice. The problem with that is that the transformers are oil-cooled. When the oil-bath is penetrated by, say, a bullet, the cooling oil leaks out. Without oil, the windings eventually heat up enough that the insulation between the primary and secondary windings break down. When that happens, the full 14,400 volts or so is applied to your site until one breaker or another trips. This all usually happens faster than the eye can see, but normal circuit breakers, fuses, and surge protectors don't handle this event correctly or quickly enough to prevent damage to your devices.

In our area, at least, lightning strikes that cause problems with electrical equipment are extremely rare so I don't have any experience with them. As best I can tell, however, the measure for protecting equipment are similar for both shorting transformers and lightning strikes.

Power Companies' Tolerances

You might think that the threats I enumerated above are somehow the responsibility of the power company to mitigate. As far as I have been able to establish, in my area of BC at least, it is not. BC Hydro's "System Voltage Control" policy, for example, explicitly permits voltage deviations of +/-20% for minutes at a time. I have yet to even find a policy that contemplates transients and harmonics. And, of course, the power company is not likely to be responsible or liable for vandalism-by-bullet of their transformers.

Mitigation Strategies

First of all, online UPSes seem to offer the best protection against all of these threats. They also support graceful shutdown of equipment. On my sites, at least, it is impractical both physically and cost-wise to have all vulnerable devices on an entire site powered by online UPSes. So, what to do about the devices that are necessarily connected to mains power? I am using two separate strategies to deal with the threats:

  1. power monitoring and contactor site disconnect
  2. HRC fuse and surge protection device

Power Monitoring and Contactor Site Disconnect

The concept here is that your entire site is connected to mains power if, and only if, the mains power meets certain power quality criteria. When those criteria are not met, power to the entire site is disconnected. Set up correctly, this will eliminate completely the damage caused by undervoltage and brownouts -- if one phase is interrupted, both phases are immediately disconnected completely from mains power.

For reference, I'm using the Bender VME420 along with a contactor in line with the panel feeder to protect my most problematic site. You can program it to fit your needs. It basically just opens an electromagnetic contactor when the power goes out of my specifications -- which it occasionally does. I've got it set to re-close after mains power has returned to normal for a while.

I'd like to also monitor for harmonics and transients, but I haven't found a device that does that yet.

HRC Fuse and Surge Protection Device

The strategy here is that when high voltage is applied to your feeder, two devices work in tandem. First, a purpose-built Surge Protection Device (SPD) acts as an electrical shunt and crowbar to limit the voltage reaching your site to what is typically hundreds of volts. It does this by passing very high current very soon after the appearance of the high voltage. By itself an SPD isn't very useful. It will quickly overheat and fail open thereafter providing no further protection.

The other parts of the puzzle are high-rupture capacity (HRC) fuses upstream of the SPD. When the SPD shunts the high current required to keep the voltage at a safe value, the fuse upstream of the SPD will blow. This has to happen fast enough that the SPD does not overheat and fail. The high rupture capacity of the fuses is required because the voltages we are dealing with are high enough to arc across plain-old fuses even after they have blown.

For reference, I'm using this Siemens SPD at my most problematic site.

This strategy should protect your site against both lightning strikes and transformer shorts.

You'll Need an Electrician

Installing any of these devices requires a licensed electrical contractor in my jurisdiction and most other jurisdictions as well.

Budget and Business Case

All told, the parts and labor to install this protection was about $2500 CAD. I estimated that brownouts were costing $1000 to $2000 per year in lost productivity and parts. So I expect a payback in two or three years. If our transformer shorts or we have a lightning strike, it should pay for itself immediately. On the other hand, if you have rather clean and reliable mains power, all of this stuff is probably not worth it.

  • I have to say this is nonsense. The modern PC power supply ensures clean DC power to the components and is fairly tolerant of mains over/under voltage. When the mains gets too bad, they fail gracefully, in other words, they shut down rather than feed bad DC to the components. – psusi Jan 22 '14 at 2:06
  • @psusi I did not make any claim about PC power supplies. Nor did I make any claim about DC output voltage as a result of mains over/undervoltage. The only claim I made about out-of-spec DC power is this: I have seen, firsthand, power supplies similar to those typically powering switches, routers, monitors, and laptops that output out-of-spec DC when subjected to mains power containing harmonics and transients. – alx9r Jan 22 '14 at 2:27

I think it might also protect against badly distorted mains voltage waveforms but I'm not sure.

This is "On-line UPS", this kind of UPS has double conversion i.e. two main parts: one is a power supply/rectifier, which charges the battery, and second is inverter. So, this is ideal protection. It has nothing to do with waveform if input voltage - inverter is independent as long as battery capacity is above minimal.

Are there ways to protect against undervoltage, overvoltage, and power surges without the expense of a full-blown UPS?

There are voltage regulators and surge protectors, but it just can't provide isolation as specified above, because there is a need at least in capacity (battery does the job well). There are two main types of regulators: Mechanical and Electronic/Circuit. Mechanical is not effective for stability: there's a range of input voltage and if it's out of it - then logical controller operates the motor drive and changes the position on the transformer, these are used for general stabilization, not for surge protection and it's good for high currents, because it's also cheap - small logical part + transformer + drive, that's all. In case of circuit one, everything depends on capacitors, it can partially protect you from some issues, but still, for very unstable situation it just cannot control output voltage. None of these two methods will solve the problem with input voltage frequencies i.e. when running on diesel generators.

On-line UPS usually don't care about input frequencies because of rectifiers on the input.

How should mains power be conditioned for devices not protected by a UPS?

Well, use surge protectors. And where input voltage is varying too much - add voltage stabilizers.


It should go without saying that every mission critical piece of equipment should never, under any circumstances receive power that has not been "conditioned" as you said.

Now, as for the question of the UPS, only you can decide if you really need everything to have auxiliary battery power. You should ask yourself: "Can the apps running on this server wait for the power to come back?" This might take hours, or even days depending on the nature of the outage.

There is a wide range of devices that "condition" power, like surge protectors, voltage regulators and filters that fix the harmonics (the waveform) of the power. For REFERENCE you can check out the range of APC for this kinds of devices and decide for yourself.

If in the end you conclude that you can live with a few minutes of downtime after each outage, you could always go with the tried and true (though decidedly low tech) approach of the venerable gas powered generator. Connect the noisy devil in parallel with the mains line, BEFORE the conditioning hardware, and when everything goes black just pull the cord.

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