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UPS plugged into a surge protector, bad?

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9 replies to this topic

#1 of 10 OFFLINE   Matt_Smi


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Posted May 25 2005 - 02:09 PM

I was wondering if it is bad to have a UPS (which provides surge protection in itself) plugged into another surge protector? I think I remember reading this that this should not be done but am not sure. The reason I ask is because I have an outlet mounted surge protector http://www.newegg.co....82E16812120214 on the outlet that I want to plug my UPS into. I have three other things plugged into that outlet as well so I need more than two plugs. So I was wondering if I should just get rid of that protector and get a traditional strip style one for the other things I need plugged in and then just plug the UPS directly into the wall, or if it is ok to plug it into my existing protector. Thanks.

#2 of 10 OFFLINE   Bob McElfresh

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Posted May 25 2005 - 08:00 PM

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.

#3 of 10 OFFLINE   Matt_Smi


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Posted May 25 2005 - 08:05 PM

Thanks for the reply Bob, actually I am using this UPS on my computer. I was originally going to get one for my HT gear as well, but then I started reading about sine waves and how only very expensive ones put out a pure sine, where as the rest are step sine. Anyway good to know there is no problem in just plugging it into the protector, if anything I figure it gives me an extra layer of surge protection.

#4 of 10 OFFLINE   David_Rivshin


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Posted May 26 2005 - 05:58 AM

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

#5 of 10 OFFLINE   Chu Gai

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Posted May 27 2005 - 10:16 AM

Matt said...


I was wondering if it is bad to have a UPS (which provides surge protection in itself) plugged into another surge protector?

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.

#6 of 10 OFFLINE   SuckRaven



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Posted August 25 2011 - 01:25 PM

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.c...esOnLine-c.html also, http://en.wikipedia....le_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.

#7 of 10 OFFLINE   JoeLouie



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Posted August 30 2011 - 03:24 PM

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 

#8 of 10 OFFLINE   JustOnePost



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Posted September 02 2011 - 07:56 AM

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:


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.

#9 of 10 OFFLINE   santaregina



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Posted January 17 2012 - 05:10 PM

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.

#10 of 10 OFFLINE   UPS user

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Posted December 23 2012 - 11:53 PM

Generally, the previous post has the correct idea. Never plug a surge protector into an output port of a UPS; a "dumb" power strip, without any surge protection, should be fine for multiple low power needs - but don't overload your UPS or risk a fire hazard. If you don't have enough existing UPS ports, you may want a bigger UPS unit. As long as the surge protector exceeds the amp rating of the UPS, you theoretically could plug the input of a UPS in a surge suppressor strip - but it would be a bit redundant because a UPS usually does some kind of surge suppression. Buy a better UPS with better surge suppression if you have a concern about that. Never plug your laser printer into a UPS battery protected port; also, be careful about plugging a laser printer into the same UPS surge protector port - you might exceed the total combined current capacity. If you have a Laser, the current draw on the heating element can approach 10 Amps when printing; that in turn can cause a surge. The real problem can be inadequate AC wiring that can't supply enough current and that will trigger an overload or drop in voltage. If you are seeing light sources dim when the laser printer is running, you have this problem. I tried a power conditioner and it didn't help resolve this problem. So, a connected UPS will sense the line voltage drop and short-cycle the UPS to battery mode to compensate; this in turn, will shorten your UPS battery life earlier than usual. The only solution that makes sense is first to put the laser printer on a separate circuit - I did this and it solved the issue in my household. The next steps I'm planning, after replacing the dead batteries in my UPS units, is to put my big screen HDTV on a power conditioner (or quality surge power strip) and put my satellite receiver on a UPS (it is a Linux computer after all). If I want UPS power for the TV or audio equipment, I'll just cough up the extra money for a UPS with a sine wave power output. If your UPS is still dead, after battery replacement, check for the obvious blown/reset fuse or not as obvious internal fuse - or even battery wiring. I hope this helps, even though spending more money may be required. Do your own research, for your particular issues. Usually, you can depend on the bigger outfits like APC or Tripp Lite to assist you in your power issues - they hope that you will buy there equipment afterwards. Don't expect a "brick store" to give you the same level of qualified help.

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