Voltage Question

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Voltage Question

Postposted on Wed Jul 18, 2012 12:21 am

Hey all,

I realize that I'm probably being neurotic, but I wanted to make sure that I'm not damaging my computer inadvertently. I recently installed a window air conditioning unit in my office. It's a long story, but there's no central air in the house that I rent (ridiculous, I know), and that room can get pretty hot. However, when the compressor of the AC unit turns on, the lights dim in the office and the adjacent bathroom. The computer is plugged into an outlet in the same room. It runs fine when this happens, even when gaming - no lockups or random hardware freezes occur. The monitors don't dim either, AFAIK. Even though I don't notice it, I was just wondering if I'm shortening the life of my hardware each time the AC compressor turns on. I just recently upgraded my system, and it'd be a shame to burn through all these parts too fast.

Some background information: Right now, I have my computer (including monitors and all peripherals) plugged into a UPS. It's a 750 W Cyberpower unit that I bought new about 2 years ago. I'm not sure if it has automatic voltage regulation, but the software that is installed with the UPS is programmed to go on battery power if voltage gets too low or high.

Thanks for all your help!
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 12:31 am

The UPS should protect you from anything the PSU can't handle. Lack of AVR (if you don't have it) isn't the end of the world, all it means is that you'll be dipping into the battery power whenever there are voltage fluctuations; as long as the voltage fluctuations aren't happening constantly (causing the battery to drain), it shouldn't be much of an issue.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 12:35 am

Erazor GTX wrote:Some background information: Right now, I have my computer (including monitors and all peripherals) plugged into a UPS. It's a 750 W Cyberpower unit that I bought new about 2 years ago. I'm not sure if it has automatic voltage regulation, but the software that is installed with the UPS is programmed to go on battery power if voltage gets too low or high.


Does the UPS actually switch to battery power when the lights dim during AC unit switching? If it never switches to battery power - then the voltage drop is probably not dangerously low and I wouldn't worry about whole thing. If it does switch to battery power each time the light dims - first try to set the UPS sensitivity (through software or using "hardware" buttons) to "low", if it doesn't help - then I guess try getting a new UPS with AVR (if your current one doesn't have this function already)...
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 5:03 am

The UPS should protect you from anything the PSU can't handle. Lack of AVR (if you don't have it) isn't the end of the world, all it means is that you'll be dipping into the battery power whenever there are voltage fluctuations; as long as the voltage fluctuations aren't happening constantly (causing the battery to drain), it shouldn't be much of an issue.


Thanks for the speedy replies! Sometimes, I try to borrow trouble when I don't really have anything else to worry about. While the voltage fluctuations do not cause the UPS to switch to battery power AFAIK, they do happen constantly. I guess this was one reason I was a bit concerned - because if there was damage happening, it would accumulate fairly quickly.

Does the UPS actually switch to battery power when the lights dim during AC unit switching? If it never switches to battery power - then the voltage drop is probably not dangerously low and I wouldn't worry about whole thing.


No, I don't think it ever switches to battery power when the air conditioner kicks on. I checked the software and the UPS will switch to battery power when the voltage drops to 90 V or less. Would that be too low to protect my computer in this case? To be a bit more specific, it's more like the lights in the room flicker when the AC kicks on initially, then it goes back to normal. Another concern I had was that this transient drop in power was just too quick for the UPS to detect.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 11:25 am

The UPS is about the best you can do in a situation like this. If you owned the house, I would suggest installing a dedicated circuit for the A/C unit direct from the service panel, but unless you're renting from a very good friend who is open to that idea, it's not going to happen in this case.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 1:43 pm

What is your starting voltage? The places I have lived, have been from 115 to 125. If you’re on the high end the voltage drop shouldn't make any difference. If you were on the low end, you would possibly be experiencing problems already. I'm guessing you are somewhere in the middle and it's never going beyond allowable tolerance. You could always get a Kill-a-Watt Meter for ~$30 and see for yourself. As an example only, I can try to give you an idea of the voltage drop, but I will have to make wild guesses at a few things. Since it's an apartment, I would guess it's a relativly short run from the panel and using 14 awg wire. I'll guess again that you may have a 10,000 BTU HVAC and on average it pulls 10 amps. At 120 volts a 15amp inrush would drop the voltage to 116 volts. This really is a wild guess though, if the run was 75 feet the voltage would be 114.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 1:51 pm

Here at work they occasionally run a big electric floor polishing machine in the hallway outside my office. When they do that the line voltage drops below 90 (and the UPS kicks in). When the floor polisher isn't running the line voltage at my desk is 118.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 1:52 pm

First of all, APC has a transformer box called the Line-R, iirc. It ups the voltage in brownouts and lowers the voltage in overvoltages. It's about 60 bucks I think. Secondly, if your lights are dimming like that when your A/C kicks on, you may have a potentially dangerous wiring problem in the building, and should probably have it checked by a reputable electrician.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 2:05 pm

Sorry to hi-jack the thread a bit but I've got a similar situation. Would a good surge protector help with this?

Also, I have a good power supply (Seasonic x650), but if the surge protector won't help, will the PSU take any damage from this behavior?
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 2:23 pm

Ryhadar wrote:Sorry to hi-jack the thread a bit but I've got a similar situation. Would a good surge protector help with this?

Also, I have a good power supply (Seasonic x650), but if the surge protector won't help, will the PSU take any damage from this behavior?

A surge protector won't do anything to compensate for voltage sags.

If the sag isn't too bad the PSU will be able to compensate. If it happens really frequently and the PSU is heavily loaded, you probably ought to consider a UPS even if the PSU seems able to ride it out. When the voltage sags, the PSU needs to draw more current to provide the same amount of power to the system (since power = voltage x current); so you're potentially stressing the circuitry on the input (line) side of the PSU.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 2:40 pm

Ryhadar wrote:Sorry to hi-jack the thread a bit but I've got a similar situation. Would a good surge protector help with this?

Also, I have a good power supply (Seasonic x650), but if the surge protector won't help, will the PSU take any damage from this behavior?


Like the dude above said - surge protector is really worthless for power sags/blackouts/brownouts. You should get a proper UPS, with built-in AVR (for longer battery life). Personally I'd suggest something like Cyberpower's new PFC Sinewave series - I've purchased plenty of them over the last year, they are relatively inexpensive, pretty well-built, with pretty stable and user-friendly power management software, and they have "pretty" front status displays (for people who are into such things :wink: ) which can show any relevant information about input/output voltage, VA, power, frequency, load, battery status and so on. The only drawback of these models is that changing the batteries is a little bit less convenient than on APC units, but you shouldn't be doing that anyway for at least couple of years.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 2:40 pm

A surge protector attempts to supress surges. It can do nothing for voltage sag, although having one nearby is not a bad idea, since the voltage recovery after the initial sag may overshoot slightly before settling back at nominal.

The problem does not require bad wiring to occur, either. Induction motors usually have a locked-rotor inrush current which (as a rule of thumb) is around six times the full-load current. A typical $300, 12000 BTU window air conditioning unit might have a running current of 10A, which is well within the range of a normal 15A or 20A residential wiring circuit, but when first turned on it pulls more like 60A for maybe a tenth of a second.

The OP indicated that a bathroom is also affected by the flicker, which means a 20A circuit. Per NEC, this would be wired with AWG #12 conductor, which has copper resistance of about 1.6 Ohms per 1000 feet. If we assume the wire run from the panel is a bit on the long side, plus some daisy-chaining between junction boxes, there might be 75 feet of parallel conductor from the panel to the AC outlet, or 150 feet of copper, having a total resistance of:

R = 1.6 * ( 150 / 1000 ) = 0.24 Ohm

The computer and the lights might be pulling down another 3A on the same ciruit, so the voltage drop from the panel to the outlet with everything running normally is approximately

V = I * R = 13A * 0.24 Ohm = 3.12 V

If the voltage at the panel is 120 V then the voltage seen at the outlet is about 117 V, which is acceptable (any standard 120V appliance should function normally to around 112V, below that all bets are off). However, during that tenth of a second when the A/C unit pulls 60A while the computer and lights are still pulling their own 3A share, the following happens:

V = I * R = 63A * 0.24 Ohm = 15.12 V

Which means the voltage seen at the outlet is about 105V for that same tenth of a second, which is more than enough to create a visible flicker for all devices connected on that branch. I've simplified by ignoring the inductive reactance of the wire, which will not affect a steady-state load very much, but will temporarily add even more resistance when that 60A surge is briefly demanded. So the actual voltage at the outlet might be down below 100V when the A/C unit starts.

A bad connection in a junction box somewhere could contribute to the problem, but if that were the case the voltage sag would probably be far enough to trip the UPS, and the A/C unit would probably have a hard time starting at all.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 2:50 pm

Hmmmm... that's good to know.

I think my lights dim a bit when the air is turned on in the office but I'll have to double check when I get home. The A/C just plugs in to the other outlets in the room so, of what little I know of wiring, it's probably not on it's own circuit.

That being said, before I freak out over anything, I'll take a multimeter to the wall socket and measure the voltage drop (if any) with the A/C on. Hopefully my multimeter will be able to measure a drop if there is one.

Otherwise, this has been very informative. Thanks, and I'll be looking into one of the AVRs (as I don't need my PC to be uninterruptable) if I need one.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 3:04 pm

So the UPS I have my computer connected to reads an output voltage of 117-118V, and the compressor is on right now. In general, it sits around 118-120V when the AC is on, but the compressor is not running.

Which means the voltage seen at the outlet is about 105V for that same tenth of a second, which is more than enough to create a visible flicker for all devices connected on that branch. I've simplified by ignoring the inductive reactance of the wire, which will not affect a steady-state load very much, but will temporarily add even more resistance when that 60A surge is briefly demanded. So the actual voltage at the outlet might be down below 100V when the A/C unit starts.


Just to make sure that I understand correctly: In my case, I'm still OK because the UPS that I have modulates the incoming voltage and makes it more constant. If it were to drop below 90V, the battery would kick in for that little bit of time, as set by the UPS software. Is this the right way to think about this?
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 6:02 pm

Just to make sure that I understand correctly: In my case, I'm still OK because the UPS that I have modulates the incoming voltage and makes it more constant. If it were to drop below 90V, the battery would kick in for that little bit of time, as set by the UPS software. Is this the right way to think about this?

Not likely, unless your UPS cost somewhere north of $1k. What it does do is wait for the source to fall out of a specified range and then throw over a high-speed relay that disconnects the load from the mains and reconnects it to the backup supply, then over to the mains again once power is restored. The disruption is brief enough that most devices will not be interrupted, particularly devices with switching power supplies.

The first thing a switching power supply does is take the AC source and change it to a DC supply using diodes and a big, fat filter capacitor. The capacitor is necessary to buffer the incoming pulses from the AC source, but it holds a substantial amount of charge and can also keep the supply stable through quite a bit of source variance (although the briefer, the better, of course).
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 6:03 pm

ludi wrote:Not likely, unless your UPS cost somewhere north of $1k.

And no one wants to be in the same room with a dual-conversion UPS because the inverter whine will loosen your molars.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 6:23 pm

Most modern power supplies can happily work with input even as low as 100v.

http://www.hardocp.com/image.html?image ... 8yX2wuanBn

HardOCP includes a test of power supplies at their lowest rated input voltage.

http://www.hardocp.com/article/2011/03/ ... y_review/5
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 6:45 pm

*Phew*

Even with every turned on in the room my computer is in I never saw my multimeter read anything below 115V.

I'm glad I learned about this though, but looks like I'm going to go without buying an AVR. Though, it might not be a bad investment for my router, ATA, modem, etc...

Thanks again for the help,
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 7:13 pm

An ATX supply has to be spec'ed to operate anywhere in the world. This includes the east coast of Japan, where nominal service is 100 volts 50 Hz (pretty much worst case for a mains-attached supply). The manufacturer also has to account for low voltage conditions, so a good ATX supply is spec'ed (with universal input) for 90 volts (100 volts -10%) on the low end and 264 volts (240 + 10%) on the high end.

While it's not a great idea to run the supply at 90 volts on a continuous basis, a temporary sag to that level should not damage the supply and most supplies should provide at least 80% of their rated output at low line. A good UPS, while nice to have, is really only needed for a serious brownout lasting a minute or more.

Maybe there's another wall outlet available for either the computer or the AC unit? Sometimes you'll have separate circuits in the same room.
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 7:23 pm

Ryhadar wrote:*Phew*

Even with every turned on in the room my computer is in I never saw my multimeter read anything below 115V.

I'm glad I learned about this though, but looks like I'm going to go without buying an AVR. Though, it might not be a bad investment for my router, ATA, modem, etc...

Thanks again for the help,


The multimeter won't really tell you anything useful - it may not be sensitive enough to register momentary voltage drops or may not have the function to display the lowest/highest voltage registered over certain period of time (like during the time you were not looking at its display). Also dedicated consumer-grade AVRs are really a waste of $$$ - if the voltage will drop below certain point or raise above certain point the AVR won't do anything useful and your equipment WILL turn off. If you don't care about your components turning off during total power loss or a very low voltage drop then you may as well save some $$$ and buy a good quality surge protector :wink:
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Re: Voltage Question

Postposted on Wed Jul 18, 2012 9:24 pm

So, I guess the question becomes: what do I do, if anything? Do you all recommend that I upgrade to a better UPS? If so, which one would be a good investment? This is the one I have right now: http://www.ebay.com/itm/GEEK-SQUAD-875VA-GS-875U-UPS-BATTERY-BACK-UP-SYSTEM-USED-/150720341604. It's getting a little bit long in the tooth, but it still works really well.

Edit:
I found this power supply: http://www.cyberpowersystems.com/products/ups-systems/adaptive-sinewave-series/CP1000PFCLCD.html

It's a line-interactive UPS (for frequent undervoltages) that's compatible with Active PFC PSUs. Would this be a good upgrade, or am I better off going with something more high-end?
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 12:52 pm

Erazor GTX wrote:So, I guess the question becomes: what do I do, if anything? Do you all recommend that I upgrade to a better UPS?

I am confused by your concern. Due to how it is and must be designed, then electronics work just fine even when an incandescent bulb dims to less than 50% intensity. The UPS is for something different- a blackout. If lights are dimming that much as to cause problems for any electronics, then an appliance at greatest risk is the air conditioner. Low voltge does not cause electronics damage - except in myths. It is harmful to motors.

Dimming should not happen. Assuming you have calculated the load of everything on that circuit, then serious dimming implies a wiring problem. Most are only problematic for the air conditioner. Some can be an indication of a major human safety threat.

A UPS does not clean AC power. It connects AC power directly to the appliance. No problem. No matter how clean that incoming AC power, the computer will first make that voltage higher and 'dirtier'. Because 'cleaning' functions inside a computer eliminate its intentionally dirtier power. Same circuits also clean trivial AC mains anomalies. What is the 'dirtiest' power incoming to a computer? When a UPS disconnectsd from AC mains. And then switches to battery power.

What is most at risk when a UPS is powered from a battery? Electric motors and power strip protectors. Due to superior designs in electronics, that same 'dirty' UPS power is also ideal perfect.

Do two things. Add up consumption numbers found on each appliance (where power cord connects). To confirm you have not created an overload. Then inspect wire connections in each relevant box back the breaker box.
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 1:12 pm

Dimming should not happen. Assuming you have calculated the load of everything on that circuit, then serious dimming implies a wiring problem.

Incorrect. Flicker can occur on a properly-sized circuit for any device that generates a large inrush current. Continued flicker during operation, or a sustained dimming, would indicate a serious circuit problem. The OP has indicated that the flicker only occurs when the AC unit starts, and that the circuit voltage with the AC unit running is only a couple volts lower than without that load. Both behaviors are within normal operating conditions.
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 3:25 pm

Erazor GTX wrote:So, I guess the question becomes: what do I do, if anything? Do you all recommend that I upgrade to a better UPS? If so, which one would be a good investment? This is the one I have right now: http://www.ebay.com/itm/GEEK-SQUAD-875VA-GS-875U-UPS-BATTERY-BACK-UP-SYSTEM-USED-/150720341604. It's getting a little bit long in the tooth, but it still works really well.

Edit:
I found this power supply: http://www.cyberpowersystems.com/products/ups-systems/adaptive-sinewave-series/CP1000PFCLCD.html

It's a line-interactive UPS (for frequent undervoltages) that's compatible with Active PFC PSUs. Would this be a good upgrade, or am I better off going with something more high-end?


If your current UPS still works and if it doesn't go on battery each time you see lights "flickering" - I'd say you should just continue using it, until its batteries will stop holding necessary charge. After its batteries will become useless - throw the whole thing away and get a newer model. And yes, that Cyberpower unit you linked to is a pretty good model.
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 4:55 pm

ludi wrote:Incorrect. Flicker can occur on a properly-sized circuit for any device that generates a large inrush current. Continued flicker during operation, or a sustained dimming, would indicate a serious circuit problem. The OP has indicated that the flicker only occurs when the AC unit starts, and that the circuit voltage with the AC unit running is only a couple volts lower than without that load. Both behaviors are within normal operating conditions.

If voltage drop is only a few volts, then dimming is near zero. Irrelevant.

Meanwhile, dimming even to 50% intensity remains sufficient and acceptable votlage for all electronics. Why would anyone then recommend an expensive solution to a problem that does not exist?
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 5:34 pm

westom wrote:If voltage drop is only a few volts, then dimming is near zero.

Once again, incorrect. Flicker is usually observed in unregulated electric lights, i.e., those having incandescent filaments or unregulated ballasts. These typically output light in proportion to the power they dissipate, and that power fluctuates as a square of the voltage. A 60W lamp, for example, has a filament resistance of about 240 Ohms. By V^2/R, it dissipates 60W at 120V nominal. If the voltage on a branch circuit briefly dips to 105V during a compressor start, the power dissipation of that 60W bulb will correspondingly drop to 45W.

In the particular case of incandescent lamps the effect can be more dramatic than just the change in brightness because the filament design is optimized for a target color spectrum output at its nominal power, so when a dip occurs, it doesn't just reduce in brightness; it also color shifts.
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 6:02 pm

ludi wrote: If the voltage on a branch circuit briefly dips to 105V during a compressor start, ...

A 12 amp air conditioner on a 20 amp circuit consuming a massive *40* amps on startup means voltage might drop by a whopping 4.5 volts. Where did you come up with a 15 volt drop? Well, many see that dimming. Then assume it is acceptable. Informed consumers learn where a problem more likely lies.

An example: a wire connected to receptacles using the back stab method. It is acceptable for code only because it does not threaten human life. Meanwhile, better electricians always wrap that wire about side mount screws so that 120 volts does not drop to 105.

Why would anyone cure a symptom? Because many have been told dimming is acceptable. Yes, when poor workmanship is acceptable. Unacceptable when one learns actual reasons for dimming. Wiring that is just barely code compliant.

Again, why is dimming (even 120 volts dropping to 105 volts) harmful to any electronics? It is not. Why were his computers working just fine? Why was a monitor not dimming? Because ideal voltage for electronics is even when incandescent lights dim to 50% intensity. Those low voltages are bad for motorized appliances (ie air conditioner). And not harmful to any electronics. What might need the UPS? The air conditioner. But that creates other potentially destructive problems.

What was the OP's original concern? That an air conditioner was harming electronics. Worry more about low voltage harming motorized appliances. Serious dimming means fixing poor workmanship in the wiring. Or worse, a rare human safety threat.
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 6:28 pm

I had this problem recently, I got this to fix it:
http://www.amazon.com/Tripp-Lite-LC2400 ... 0000514OG/

As others have said, measure the voltage first to see what's going on; then you can look at conditioners that meet you load needs. Using a UPS to fix brownouts/low voltage, especially when it's frequent, isn't a great idea unless the UPS has been designed for it.

Part of my review for that conditioner:

"tested this on a circuit that's always been susceptible to voltage drops due to load...

When I'm running a single computer on the circuit:
118 volts measured at wall outlet.
Line conditioner led reads 'normal'.
118 volts measured at conditioner's outlet. (seems to be just passing through the voltage when it's operating in the 'normal' range.)

When I add ~150 watts of load (lights):
116 volts measured at wall outlet
Line conditioner led reads 'low'. (audible relay click accompanies this change)
126 volts measured at conditioner's outlet.

Adding 800 watts of load (space heater):
102 volts measured at wall outlet
Line conditioner led reads 'very low'. (audible relay click accompanies this change)
120 volts measured at conditioner's outlet."

As to why one would fix a symptom?
It might be cheaper.
The landlord might not want to fix the actual problem.
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Re: Voltage Question

Postposted on Thu Jul 19, 2012 9:33 pm

Westom, you are arguing against a phantom. The consensus here is that there is no risk to the equipment from the brief low voltage created by the A/C unit starting. This was already established before you entered the conversation, so your attempt to correct people who are not disagreeing with you is mildly amusing, but that act is a one trick pony and you already had your turn riding him.

And yes, I always wrap my wires around the screws. I've seen those push-in connectors pull right back out of 1970s-vintage receptacles when removed for refurbishing, and have no respect for them whatsoever. Not sure how that's relevant since the OP does not have a wiring problem, but it came up somehow.
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Re: Voltage Question

Postposted on Fri Jul 20, 2012 7:54 am

ludi wrote:Westom, you are arguing against a phantom.

Even numbers say that voltage drop should not be 15 volts. That your only number is only speculation. Or that you ignored wiring defects. An air conditioner causing a significant voltage drop (dimming lights) means wiring problems. Your dimming (a 4.5 volts drop - not a 15 volt drop) is near zero. The OP does not really say how much dimming he has.

1) Dimming even to 50% is ideal voltage for all electronics. The many concerned about damage to electronics are worrying about a strawman. A UPS for those electronics solves nothing. Money wasted on a threat that does not exist - an unjustified fear.

2) An air conditioner causing 120 volts to drop to 105 volts or 102 volts implies a wiring problem. His air conditioner is causing serious dimming only if a wiring problem exists. A problem easily solved even by a layman. But that unfortunately requires knowledge and inspection. Dimming is caused by poor wiring. Otherwise dimming is near zero - hardly noticed - a threat to nothing.

3) If dimming is serious, the air conditioner (not electronics) is suffering most strain. And another reason why that UPS solves nothing.

UPS solves phatom fears. Dimming is an easily solved wiring problem. Or dimming is near zero - irrelevant. Significant dimming means only an air conditioner is suffering.

If that building has aluminim wires, then dimming may be a major human safety threat. Just another reason why informed layman fix the problem; not cure symptoms with a UPS.
westom
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