Devices Using MOVs In Series - Is It a Bad Idea?

My question really stems from some research I am doing on surge protection devices. (SPD) Most consumer level uninterruptible power supplies / battery backups (UPS) provide some level of surge protection, but according to what I have read, it is not the best protection. It would then seem best to plug a UPS into a quality surge protector. However, some individuals say that that can cause one or both to fail. The stated reason is that " you cannot put MOV's in a series, and that is what you are essentially doing when you plug a UPS into a SPD. This would be the same as plugging one surge protector into another. "

...any overflow of juice on the power line will be attracted to the MOVs (generally three MOVs to a protector) and drawn to a ground area. The rate at which electricity flows is enough that this can bounce between the two surge protectors, negating any and all protection and even cause some models to shut down.

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There seems to be some misinformation about this, as a bulletin from Eaton Corp. recommends putting a surge protector upstream of your UPS. The following sums it up.

...... So why
not fully protect the equipment connected to the UPS,
and the UPS itself, from the damaging effect of voltage
transients by installing the most effective surge protec-
tion system possible?

Is the surge protection provided by a UPS adequate?
The answer is NO! The surge protection provided by
most UPS units is limited, and often does not contain
EMI/RFI filtering, which lowers the let-through voltage
provided by MOVs alone.

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some individuals

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Who are some individuals and what are their sources?
I'm a little confused because of them being likened to MOV's in series, if two devices with MOV protection on all three lines of a single phases system were connected to each other they'd be in parallel not series, I could understand the possibility of some strange oscilation issues right at the threshold of conduction but I'd like to know more about why these people are saying this is bad.

1. The OP's post is totally wierd. A UPS plugged into an SPD is not in series it is in Parallel. There are no issues.

2. There are three major UPS topologies:
2a. always on
2b. on when there is a failure

And two different output types
3a. true sine wave
3b square waveish inverter

An SPD before the UPS will protect the UPS. An SPD downstream from the UPS probably isn't needed, but it would protect the equipment from damage from the UPS. If I had a choice the SPD would be before.

In reality, I would use a power conditioner first, then a surge suppressor followed by a UPS. I've used ONEAC devices on computerized systems and only had mechanical failures after 15 years using the technique just described.

The ISOBAR series surge suppressors are preferred because of the warranty offered for connected equipment, just save you receipts.

Generally suppression is lots of stuff in parallel.

For example, gas discharge tubes can be used for a first shot, then MOV's for a second shot then an RF filter to get rid of the RF junk. These devices are internally in parallel and is effectively the same as plugging one SPD into another. It just increases the energy that can be absorbed.

Attached is a circuit depicting how the Marvelous MOV is generally used.

I agree with Sceadwian as to the terminology used as to "series". There is no series that I can see. MOVs are configured in a network as shown in the cartoon. If I plug one MOV device into another I have a parallel network of MOVs as I see it. Also when anyone begins to use the word "juice" in an electrical circuit I pretty much discount whatever follows. Here nor there.

While the MOV is a useful device I have seen the results of flames escaping MOVs on numerous occasions. They have their place in circuit protection but their use only extends so far. If I place a 150 volt MOV across my 120 volt line in theory when the voltage begins to exceed 150 volts the MOV will begin to conduct and subsequently a fuse or circuit breaker will trip. However should lightening strike the utility pole outside my house I wish me good luck with my MOV surge protection. A MOV just like a man has limitations and needs to know those limitations. This is also true of people employing MOVs as a protection device.

If you really want to gain an understanding of surge protection devices I would suggest you get a good curve tracer like the old Tektronix 575 and run curves on several MOVs. Note the curves and note the curves from several MOVs of the same voltage rating and lots. Then just for fun take a few 150 volt or so MOVs and hit then instantaneously with 240 or 480 volts. Note the flame and the time to breakdown. Also note many do not break down as a short as immediately following short they blow open. Note the times from applied voltage to a fuse or circuit breaker opening. It gets interesting as to what the MOV does or does not actually protect.

Ron

The MOV entry on Wikipedia covers this, MOVs are excellent for transients, extended over voltage will cause them to fail in flames readily. I would watch out for curve traces on MOV's they're pulsed power devices NOT anything for repetitive over voltage.

Sceadwian said:

The MOV entry on Wikipedia covers this, MOVs are excellent for transients, extended over voltage will cause them to fail in flames readily. I would watch out for curve traces on MOV's they're pulsed power devices NOT anything for repetitive over voltage.

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I agree, the merit to curve traces is the ability to actually watch how the device works. I figure it this way, if you are doing research on surge protection then as part of the learning process it will not hurt to generate and look at actual response of assorted MOVs. While MOVs are excellent for transients in the case of a transient being an increase of say 120 to 150 volts over a slow period they are as useless as teats on a bull for a lightening strike in close proximity. To say transient really doesn't say much unless we define transient to a period of time and magnitude.

Ron

Thanks guys! You have been great help.

@ Sceadwian - As for the "individuals" who say such things, I just found various posts here and there after doing a few search queries on the net. None of them give any sources for their information which is one reason why I was suspicious. (I would have given some links but since I was a new member, I did not want my post to have to go through moderation. ) Here are a few. Give special attention to the third ink as this guy writes as if he wants you to believe he knows what he is talking about, but even with my limited knowledge of electronics, things he said did not quite seem right.

https://honda-tech.com/showpost.php?p=19948235&postcount=11
https://www.codinghorror.com/blog/2006/07/power-surge-protection-pcs-and-you.html (second to last paragraph at the bottom)
https://reviewsurgeprotector.com/can-you-plug-one-surge-protector-into-another.html

I assume it is the same individual from the last link that wrote the following article as well. Of note is the 3rd 4th and 5th paragraph. I will quote a few select statements from them below. https://reviewsurgeprotector.com/ups-surge-protectors.html

As for (UPS) units that don't have surge protection, you won't be able to plug it in to a surge protector and reap any benefits. The surge protector will disrupt the performance of the UPS by blocking information filtered by the protector. This loopback will keep the UPS from diagnosing problems and identifying power outages in time.....As for clean power, the UPS has power conditioning built into it. This helps the battery supply power the equipment hooked into it. One thing to note, the UPS will not offer surge protection if the battery is down

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Maybe he did not really mean to say batteries function as surge protectors, but that is what he wrote and I really wonder about how well he understands what he is talking about.

@ Reloadron -- I would love to be able to do the testing / experimenting you suggest. As a child I had a powered breadboard or two, a nice digital multimeter, and an old analog O-scope, which I played around with a lot. I learned quite a bit of stuff from tinkering, but unfortunately that is far behind me now. I still retain enough of what I learned back then to get myself into trouble, but I got into computing, mainly programing, and CAD, and have not pursued those early interests further which I often regret. So I must rely on brilliant minds I find in places like this one.

I have been searching for the source of the misinformation. It seems it came from Belkin technical support. (They would benefit greatly from a spell checker too I might add)

A UPS uses extremely sensitive electrical components, which monitor incommin power and provide battery backup when necessary. Plugging a UPS into a surge protector will cause the UPS to receive pre-filtered power and will also cause MOV’s used in both a UPS and Surge protector to be in series, as ciscussed above this will cause the MOV’s to malfunction..

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Link

Then there is this guys response which is nearly identical to what you all wrote.

I don't quite see where they get "pre filtered power" from a surge protector. Unless the surge protector has a few caps tossed in with the MOVs.

On a side MOV note a few months ago I had a facilities engineer in our department approach me and ask if I would have a tech look at a recently reworked Blue Wave ultra sonic cleaner machine he claimed was dead. Few hours later the assigned tech comes to me with "Ron, you may want to look at this". The system uses 4 power driver boards that run off of 220 VAC. Each board has a full wave bridge, a few filter caps and the oscillator with I think six large power transistor drivers. Each board had caps literally blown apart, and toasted rectifiers as well as each MOV across the 220 VAC input literally split and toasted. Yeah a mains 20 amp fuse blew too but not before the smoke and fire. The boards and entire drive system as well as ultrasonic transducers were all new. Part of a rework.

Here is the *****, this unit is in the basement and I use a 480 volt / 240 volt 7.5 KVA transformer to get the power. The voltage was normal! That same 480 volt 800 amp bus feeds countless other electronics and nothing but nothing experienced any problems. Go figure as it beats the hell out of me? I'll have to ask Chuck if he took any pictures of the boards before we sent them to Blue Wave. Blue Wave swears it looks like a huge voltage surge which I would agree with but why did nothing else cook? What made me think about the MOVs was the boards came back today. If Chuck took pictures I'll post a few if they show the blown and split MOV. Yeah, the $10 fuse went right after about $4,000 in boards.

Ron

The best, on a side note, issue that I ran into was for a Tracor-Northern Multi-channel analyzer which we had a service manual for. It died a few times right after one another.

After looking at it, I found absolutely no line protection. I'm not even sure it had an RF filter.

Their comment: We specify that the unit requires 120 V 60 Hz +-x%.

Moral: You don't provide clean power, our equipment isn't guaranteed.

I like companies that when you tell them of really bad design issues they offer a free fix or even a free replacement with current technology.

One time we evaluated a product that was supposed to control light intensity to less than 1%. They used a photomultiplier tube. We sent it back. Later they designed a light pipe based, thermoelectric cooled detector which did finally work. later, I discovered that they used a 10 V bridge for a 40 A supply. Fixing was a little harder on their part. They changed the transformer and went to a 1/2 wave bridge. They did the upgrade for free. It really was an evolving process for them, we in esscence evaluated their equipment and lenses. It was a really nice relationship.

The only filtering that most surge protectors do, is of EMI/RFI. Of course most power conditioners also provide surge protection, but I would think that clean power would be beneficial and would lengthen the life of a UPS device.

Anyway after several more hours of digging into this issue, I am ready to lay it to rest, with what the experts in this thread have said as well as what I have found in two of the major UPS and SPD manufacture's statements. I will post them below for anyone who is interested. The long and short of it is that it is permissible to put a surge protection device before a UPS, as long as you plug one of their own UPSes into one of their own SPDs. Using some other manufactures SPD may void your warranty. Note that main caution is that care must be taken not to overload the surge protector, which the UPS is connected to, by plugging other "powerful" energy demanding devices in to the surge protectors remaining empty slots. By other "powerful" devices, I presume they would mean high wattage devices such as, electric heaters, microwaves ovens, refrigerators, etc. Something like an inkjet printer, a scanner, or some mobile computing devices should be perfectly fine.

From Tripp Lite http://www.tripplite.com/en/reseller-center/learning/reseller-faq.cfmn (see under section: What are Tripp Lite's UPS connection and equipment connection warnings?)

Tripp Lite recommends connecting a UPS system directly to a properly grounded AC power outlet. Do not plug the UPS into itself; this will damage the UPS. Do not modify the UPS plug, and do not use an adapter that would eliminate the ground connection. Do not use extension cords to connect the UPS to an AC outlet. The warranty will be void if anything other than a Tripp Lite surge suppressors is used to connect the UPS to an outlet. If the UPS receives power from a motor-powered AC generator, the generator must provide clean, filtered, computer-grade output. Do not use Tripp Lite UPS Systems for life-support applications in which a malfunction or failure of a Tripp Lite UPS System could cause failure or significantly alter the performance of a life-support device. Do not connect surge suppressors or extension cords to the output of the UPS. This may damage the UPS and will void the surge suppressor and UPS warranties.

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From Tripp Lites limited warranty. "The warranties of all TRIPP LITE surge suppressors are null and void if they have been connected to the output of any UPS system.
The warranties of all TRIPP LITE UPS Systems are null and void if a surge suppressor has been connected to its output receptacles."

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From APC **broken link removed**.

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.


Maintaining EPP and Warranty: Plugging any non-APC surge protector, power strip, or extension cord into the output of an APC brand UPS could void your Equipment Protection Policy (EPP). However, the standard 3 year product warranty is maintained. If, after taking into consideration this knowledge base document, you choose to use an APC brand surge protector in conjunction with your APC brand UPS, your warranty and Equipment Protection Policy will be maintained.

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Notice that only Tripp Lite forbids plugging a surge protector into a UPS, APC even alows for that as long as it is one of their own units.

Interesting. Seems more like a marketing issue. APC and Trippe-Lite have some nice warranties on connected equipment.

A reminder that if you need to use a UPS with a generator, the UPS must be generator compatible which generally means relaxed frequency rules. It's not uncommon to have to have some equipment 100% on all the time before the generator starts.