UPS plugged into a surge protector, bad?

There is no problem putting a surge-protector ahead of the UPS.

Most surge-protectors simply clamp any voltage above 120 volts AC to protect electronics.

But it is sometimes a bad idea to plug a analog device (like a audio amp or video display) into a UPS because it can affect the output voltage a bit. This is not a problem for a computer where any rough spots on the input power is ignored by the digital nature of the electronics. But the output of a UPS can look like a "sawtooth" pattern that may affect analog electronics.

Make sure the UPS documentation says it can be used for audio/video/Home Theater equipment. Some of the newer units are designed to NOT affect the power passed through for these devices.

Plugging an UPS into a surge protector is fine. What you shouldn't do is plug a surge protector into an UPS. I've never seen a great explanation as to why, but what I have read is that the dirty output of the UPS operating on battery will look like many small surges to the surge protector. This in turn will cause the surge protector to shunt power to the ground wire, quickly draining the UPS's battery and destroying the surge protector (most surge protectors are the MOV type, which are degraded every time they activate).

As for stepped sine output being a problem for HT electronics, I have a feeling that's not as big of a deal as some people make it out to be. In most UPS's (read any one you're ever likely to see) power is normally sent straight through from the wall unchanged. It's only when the switch to battery (brownout/blackout/overvoltage situations) that they produce their own power, and that's when you'll get the stepped sine wave approximation. I don't know about you, but when the power just went out I'm not very concerned about my soundstage collapsing, I'm more concerned with getting a flashlight out.
UPS manufacturers might caution against using them for an HT simply because HT's are so power hungry they will very quickly drain the battery. Same reason they say not to plug a laser printer into an UPS.
On the other hand, I don't particularly care about powering my HT for another 10 minutes when the power goes out either, so I don't see much point to an UPS on the HT. A good surge protector seems like a better deal anyways.
BTW, there are UPS's which are always on battery power (and use the wall just to keep the battery continuously charged), but those tend to be very expensive, and probably produce good sine wave output as well.

-- Dave

Matt said...
Some manufacturers don't recommend it (APC for example) but you'd have to check with them what their reasoning is. There doesn't seem to be a clear consensus on this which suggests to me that you'd be OK with this scenario.

Your explanation David is entirely reasonable and correct especially if the UPS is dirty. You'd have to check with the manufacturer to find out if it's a sine wave, quasi sine wave or something else. THD numbers ought to give you an indication of how good that power coming out is.

My understanding is that a UPS with a true sine wave output (such as an "online" or double conversion topology), the batteries are always connected to the inverter, so that no power transfer switching is necessary, therefore at no point are connected components exposed to the mains (wall) AC at any point, so the load (whatever is connected) experiences no disturbances.

The other (read: less expensive) topologies or types of UPSes only generate an approximated or "stepped" sine wave, sometimes also called a square wave or square sine wave, that can be harmful to some sensitive equipment that rely on a more "pure" sinusoidal waveform, such as power amplifiers for speakers etc. Think of an approximated sine wave vs. a "real" sine wave as the difference between the sampling rate of a compressed mp3 file, vs. a pressed music Compact Disc. (one sounds much better than the other).

So with an online UPS, the AC from your wall is constantly charging the batteries, and an inverter then converts the DC from the batteries back to a true sine wave AC output again, which is what your connected equipment "sees".

A really good article is here:

http://www.pcguide.com/ref/power/ext/ups/typesOnLine-c.html

also,

http://en.wikipedia.org/wiki/Uninterruptible_power_supply

As far as connecting a UPS (any type) to a surge protector that is connected to your wall mains AC, I can't see how this would be a problem. The only effect this might have is that your UPS is better protected from dangerous AC overvoltages, or "surges" that your UPS is not designed to handle. In fact, if you look at the joule rating of most UPSes vs. dedicated surge protectors, the surge protectors almost (if not always) have a much, much higher energy dissipation rating (measured in Joules or kilo-Joules) than the UPS does.

I guess the question becomes how the benefits of the surge protector being in the chain carries over to your connected components with the UPS in between the two.

I'm not an electrical engineer, but intuitively, if one thinks of a surge protector as a sort of "firewall", than placing it anywhere "upstream" or before your components is probably better than not having one at all.

I have been looking to do a setup like this myself, and I couldn't really find any concise, definitive answers anywhere. Maybe someone can chime in with something more technical.

SuckRaven, I'm not sure if you'll have many people chiming in on the subject, as the last post in this thread before yours was over 6 years ago 

Years ago I'd heard I shouldn't plug a UPS into a surge protector. Never could understand why not. Today I read in reviews that some of the UPS's I have for the business are not adequate as surge protectors and equipment is at risk, so I'm in the position of researching it again.

I came across an official answer from APC and thought it might help someone looking for this in the future. Here is the link to their site.... and in case they remove it in the future, the pertinent text:

http://emea-en.apc.com/app/answers/detail/a_id/1372/~/using-surge-strips-with-apc's-back-ups-and-smart-ups-products.

Using surge strips with APC's Back-UPS and Smart-UPS products.
Published 01/07/2002 08:00 AM | Updated 07/13/2010 02:19 PM | Answer ID 1372
This document will explain why APC recommends against the use of any surge protector, power strip or extension cord being plugged into the output of any APC Back-UPS and Smart-UPS products
APC recommends against the use of any surge protector, power strip or extension cord being plugged into the output of any APC Back-UPS and Smart-UPS products. This document will explain why.

Plugging a surge protector into your UPS: Surge protectors filter the power for surges and offer EMI/RFI filtering but do not efficiently distribute the power, meaning that some equipment may be deprived of the necessary amperage it requires to run properly causing your attached equipment (computer, monitor, etc) to shutdown or reboot. If you need to supply additional receptacles on the output of your UPS, we recommend using Power Distribution Units (PDU's). PDUs evenly distribute the amperage among the outlets, while the UPS will filter the power and provide surge protection. PDUs use and distribute the available amperage more efficiently, allowing your equipment to receive the best available power to maintain operation.

However, please note that the UPS is designed to handle a limited amount of equipment. Please be cautious about plugging too much equipment into the UPS to avoid an overload condition. To understand the load limit of your particular model UPS please consult the User's Manual, or visit APC's Product Page at www.apcc.com/products.

Plugging your UPS into a surge protector: In order for your UPS to get the best power available, you should plug your UPS directly into the wall receptacle. Plugging your UPS into a surge protector may cause the UPS to go to battery often when it normally should remain online. This is because other, more powerful equipment may draw necessary voltage away from the UPS which it requires to remain online.

We had a problem with our computer workstation which was plugged into a UPS. The UPS was plugged directly into the wall outlet along with a laser printer. Whenever the laser printer fired up, the UPS was triggered. After about two years the UPS failed (whenever the laser printer fired up the UPS would trigger but the computer would lose power). A new UPS solved the problem with the computer turning off, but the lase printer was causing surges that killed the UPS. We put a surge suppressor between the wall outlet and UPS to protect it from triggering when the laser printer fired up. As long as other equipment is not plugged into the suppressor the problem APC warns against should not happen. Maybe we should put the suppressor between the wall outlet and the laser printer? Don't know but we have three dead UPSs what we have collected over the years and replacing their batteries didn't revive them.