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Old December 15th, 2015, 06:04 PM   #1
OkiePC
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OT: UPS neutrals

What are the ramifications of tying the neutral connections together on the input and output of an uninterruptible power supply?

This is for an offline type for 120vac. I'd like to do this to simplify wiring but I am not sure if I would be creating a problem.
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Old December 15th, 2015, 06:35 PM   #2
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Are you talking about connecting the "dirty" neutral and "clean" neutrals together?
I have never done that but I can see some issues walk ally if you have large loads on the dirty neutral bus.
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Old December 15th, 2015, 06:36 PM   #3
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On most UPS's, except the big brown 4-wheeled ones, the neutrals are connected together internally to keep it a grounded neutral.
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Old December 15th, 2015, 08:57 PM   #4
James Mcquade
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That would depend on several things in my opinion.

1. What is the output waveform of the UPS. Square wave, simulated ac wave or true sine wave.
2. What is the long term frequency response they will need to match, if they get out of phase 180 degrees, what is the result?
3. Will the batteries compliment each other, (drain at the same rate).
4. Most importantly, will the above factors create a motor / generator type situation. One acting as a generator, and the other acting as the motor.

I'm sorry, but its been quiet a while since I had motor / generators in college.

regards,
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Old December 15th, 2015, 11:10 PM   #5
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This is for powering a remote telemetry panel with a small amount of hardware in it. The amp draw when running is probably around 0.5 amp. I tested it with a temporary connection in the shop and saw no ill effects.

Apparently this UPS does not have the neutrals tied together internally because if I measure from the output "hot" to the input neutral it is a floating value that I read. I will have to get some real numbers tomorrow for current and voltages.

I am sure that the output is not a sine wave, probably simulated ac wave. I don't have a scope. The literature that came with it is sparse.

I am not sure I understand point 4, James. Point 3 is one that I had not thought about though. I suppose I need to call the maker and ask their opinion. And it is not a big deal to label the 2nd neutral and keep the few wires that are on the input neutral separate. I just didn't want to end up with a panel where someday I get a phone call "Hey I got a 64 volts from hot to neutral so I replaced the UPS and it is still doing it."
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Old December 16th, 2015, 05:13 AM   #6
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It entirely depends on the actual design of the UPS, which you can't know without a schematic...
BUT...
If they are not bonded already, then I wouldn't bond them. There is PROBABLY and output transformer isolating things, but no guarantee, and I wouldn't take the short, or long term, risk of damaging the unit.
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Old December 16th, 2015, 06:23 AM   #7
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I also recently installed a UPS in my PLC panel and was trying to determine the same thing. It is a SOLA din rail mounted unit which is replacing a desktop type unit in which both neutrals were tied together. After a bit of research online I came to the conclusion that unless you have very sensitive loads that are supplied by the UPS the neutrals should and probably will be tied together. In most cases with 120V, you are going to want to ground your neutral for safety, so the neutrals will be bonded through the ground anyway. The only time you wouldn't tie the neutrals together is if the equipment being supplied is extremely sensitive to noise (such as some lab equipment). In my experience anything that could run off straight power with little to no filtering/surge protection can run off the UPS with neutrals from input and output tied together. This includes PLC power supplies, Enet switches, DC power supplies, PCs, etc...
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Old December 16th, 2015, 10:55 AM   #8
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In regards to point #4.

When you tie 2 or more generators together, they must be in sync as far as the frequency. If one of the generators is so far out of sync with the others it essentially will become a motor, which is not good at all.

If your ups units are in sync ok, but since they have their own electronics to generate the output waveform independent of each other, I would have a concern with that.

james
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Old December 16th, 2015, 12:50 PM   #9
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Unless I'm mistaken, Okie is talking about tying the two neutrals together in one UPS, not tying the neutral on two different UPSes together.
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Old December 16th, 2015, 01:26 PM   #10
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Oo-v-oO,

Thanks for the correction.

Having said that, tying the neutrals of the input and output together of a ups is not a good idea in my opinion.

When the input power is lost, the ups unit will kick in and provide a live neutral
for the input and to anything the unit is plugged into. Maintenance will in most cases fail to check the hot and neutral and assume everything is dead not knowing there is a potential voltage at the ups unit itself.

Please correct me if I am wrong.

James
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Old December 16th, 2015, 02:26 PM   #11
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James, in most cases the neutral will be grounded, therefore the neutral will not be "live" as it is at the same potential as ground.Only in rare cases will the neutrals not be grounded as I posted above.
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Old December 16th, 2015, 03:06 PM   #12
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Quote:
Originally Posted by Oo-v-oO View Post
Unless I'm mistaken, Okie is talking about tying the two neutrals together in one UPS, not tying the neutral on two different UPSes together.
Yes, this is correct. This is a single small panel with a SOLA SDU500 in it.

I am still undecided and have not had time to call SOLA yet.

I did fire up the panel today and found out that the neutrals do measure zero volts. With no power applied and no wires on the neutrals I can ohm between them and read open leads... So if there is a device connecting them, it is an active one that drops out when power is lost.
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Old December 16th, 2015, 08:47 PM   #13
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Quote:
Originally Posted by OkiePC View Post
...With no power applied and no wires on the neutrals I can ohm between them and read open leads... So if there is a device connecting them, it is an active one that drops out when power is lost.
I just checked both of my UPS's & measured the same voltage from hot before & after and neutral before & after & all 4 measurements were 120.8 volts, not even a decimal place off. Then I checked neutral to neutral & got 0.0 volts. Then I ohmed out the 2 neutrals & got a fraction of an ohm, so they are electrically connected internally . This was repeated for the other UPS too. I didn't try shutting down one (too much stuff running) to ohm out the neutrals when off.
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Old December 16th, 2015, 10:48 PM   #14
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I have followed this post with interest and there have been a lot of very good replies with good explanations.
But I keep thinking something is missing. And that is that the definition of a neutral conductor is misunderstood.
I know I am going to get a lot of flack about this but it is important understand grounding and neutrals and the differences.
By definition a neutral conductor [White] is a conductor that is connected at to the ground [Earthed, Grounded, Bonded ] at the source. That is not to be confused with the Grounding conductor [Green] while they are the same potential they are not the same and caution should be used when discussing or using them.

For uses of the term "ground" or "earth" in electricity but not in the context of mains wiring, see ground (electricity).
As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground
Ground or earth in a mains (AC power) electrical wiring system is a conductor that provides a low-impedance path back to the source to prevent hazardous voltages from appearing on equipment. (The terms "ground" and "earth" are used synonymously here. "Ground" is more common in North American English, and "earth" is more common in British English.) Under normal conditions, a grounding conductor does not carry current.
Neutral is a circuit conductor that normally carries current, and is connected to ground (earth) at the main electrical panel.
In the electrical trade, the conductor of a 2-wire circuit connected to the supply neutral point and earth ground is referred to as the "neutral. [1]
In a polyphase (usually three-phase) AC system, the neutral conductor is intended to have similar voltages to each of the other circuit conductors, but may carry very little current if the phases are balanced.
The United States' National Electrical Code and Canadian electrical code only define neutral as the grounded, not the polyphase common connection. In North American use, the polyphase definition is used in less formal language but not in official specifications. In the United Kingdom the Institution of Engineering and Technology defines a neutral conductor as one connected to the supply system neutral point, which includes both these uses.
All neutral wires of the same earthed electrical system should have the same electrical potential, because they are all connected through the system ground. Neutral conductors are usually insulated for the same voltage as the line conductors, with interesting exceptions
If you are using a UPS with a Neutral then it is bonded to the chassis and would be the same electrical potential as the Ground connection that not to say they you can't have or use an isolated UPS or transformer but they would not have a Neutral but they would have a Ground connection. They must be clearly marked so there is no confusion as to the potential.
Excuse me for stressing the difference but I have seen people get severely shocked by not knowing and understanding the difference. You can spend many hours trouble shooting a system with a broken neutral the symptoms will keep changing depending on what's powered at the moment
NFPA require that all power sources have one leg bonded to the ground or chassis at the source and only at the source. While grounds or bonds connects at every point that it is exposed box, device whatever.
If you follow the code 24VDC power supplies need to have one leg connected to ground. Not that this is always followed I have seen a lot of problems with systems with multiple power supplies that were not bonded together. Spent many hours chasing down intermittent problems because of it.
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Old December 17th, 2015, 11:08 AM   #15
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My two cents on this issue based on my UPS experience:

Neutral is an "unbalanced load carrier". When wiring up distribution, utility runs Line (which is 2-3 depending on type of service) and a ground.

Your main panel/transformer/whatever it may be will run in the lines and the neutral will be bonded to your ground.

Because of this, at ALL further panels, neutral will feed back to the neutral line of your distribution, but ground will feed directly back to the ground of your main distribution system.

That being said, a UPS will monitor this circuit, keeping its battery charged, and use an inverter to simulate the sinusoidal waveform when power goes below a certain level. It will also protect against over-voltage, but that is another discussion.

This means that in the internal electronics, the line and neutral are tied together, but the circuit is COMPLETED at the outfeed side of the UPS. That being said, if you tie the infeed and outfeed neutral together, you are, in effect, crossing neutrals and creating a loopback situation that can give you all kinds of headaches.

You really shouldn't do this, especially to "simplify wiring".

Hope this helps.
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