# AFCI Failure



## NolaTigaBait (Oct 19, 2008)

So he removed the neutral to the subpanel hot?... I'm trying to understand exactly what he did to cause this...


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## 480sparky (Sep 20, 2007)

Why is he using *20*a AFCIs? What 20-amp circuits require them?


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## wildleg (Apr 12, 2009)

what's wrong with 20A afci's ?


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## 480sparky (Sep 20, 2007)

wildleg said:


> what's wrong with 20A afci's ?



Nothing. I was just inquiring as to _why_ 20s are installed.


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## I_get_shocked (Apr 6, 2009)

480sparky said:


> Nothing. I was just inquiring as to _why_ 20s are installed.


Why not? Maybe he got a steal on 12/2 or just felt like it


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## reddog552 (Oct 11, 2007)

*20 amp Ckts.*

I live in Belleville Il. Our AHJ specifies all wiring must be #12 0r larger. We use 20 amp breakers 20 amp rec.


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## Shorty Circuit (Jun 26, 2010)

NolaTigaBait said:


> So he removed the neutral to the subpanel hot?... I'm trying to understand exactly what he did to cause this...


See my comments on MWBCs and why I don't allow them in the thread called "Interesting Lighting Problem." Some people think it can't happen, won't happen, never happens. Were these all MWBC pairs or did he just open the neutral feeder to the whole panel? Either would explain why the voltage across the breakers exceeded 136 volts and destroyed them.


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## millerdrr (Jun 26, 2009)

Shorty Circuit said:


> Were these all MWBC pairs or did he just open the neutral feeder to the whole panel? Either would explain why the voltage across the breakers exceeded 136 volts and destroyed them.


Educate me, if you don't mind. I've read the other thread, and I understand why you want zero amperage on the neutral by turning off a single circuit. But....how does that make the voltage jump? Isn't voltage already fluctuating by a wide margin, 60 cycles per second? 

My electrical theory/engineering knowledge leaves a lot to be desired.


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## Shorty Circuit (Jun 26, 2010)

millerdrr said:


> Educate me, if you don't mind. I've read the other thread, and I understand why you want zero amperage on the neutral by turning off a single circuit. But....how does that make the voltage jump? Isn't voltage already fluctuating by a wide margin, 60 cycles per second?
> 
> My electrical theory/engineering knowledge leaves a lot to be desired.


I'll try to make it as simple as I can. I'll use the residential single phase transformer as an example. The secondary voltage of this transformer is 240 volts and has a center tap, that is a wire connected right at the middle of the secondary winding. That is the grounded conductor, the source for the neutral that is grounded where it enters the house. The voltage between either leg and the neutral is 120 volts. When you have two circuits, one from each winding on a different circuit breaker and each one has its own neutral wire back to the panel, if one neutral opens up, its circuit is broken and no current flows. The other circuit remains unaffected. But if you have both circuits sharing the same neutral wire back to the panel and it opens up between the point where they join and the panel, you now have a circuit where 240 volts is applied. It consists of one bus in the panel from the transformer, one circuit breaker, any loads on that circuit, any loads on the other circuit, the second breaker, and the second bus connected to the other winding. How this 240 volts divides depends on the ratio of the connected loads so one set of loads may see much more than 120 volts. This can be sufficent to damage the load or destroy it without tripping the breaker.


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## Mr. Sparkle (Jan 27, 2009)

Shorty Circuit said:


> See my comments on MWBCs and why I don't allow them in the thread called "Interesting Lighting Problem." Some people think it can't happen, won't happen, never happens. Were these all MWBC pairs or did he just open the neutral feeder to the whole panel? Either would explain why the voltage across the breakers exceeded 136 volts and destroyed them.


Listen man I cant compete with your ability to spew overly engineered legalese in regards to wiring methods and to some degree I kinda think I respect some of your technical jargon maybe but I've had more that a handful of drinks this evening and even I know there is nothing in the OP about using MWBC's, nonetheless the fact that there is I think only one manufacturer at this time that actually makes a 2-pole combo AFCI breaker, so.......where were you going with this?


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## millerdrr (Jun 26, 2009)

Thanks for your time. 

I found a cool link that helped me quite a bit, if anyone's interested.

http://www.code-elec.com/userimages/Lost%20Neutral.ppt


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## Mike in Canada (Jun 27, 2010)

Shorty Circuit said:


> I'll try to make it as simple as I can.


 It took me a minute to be certain of just what exactly was being referred to as a multi-wire branch circuit. I've got it, now, and I've certainly seen a lot of them. Many guys seem to feel that the savings of using one strand of 12/3 BX instead of 2 strands of 14/2 BX is worth the misery they inflict in the future maintainers of the system. Let me see if I can make the situation even more simple. Those who have the situation well in hand can ignore the following.

Take a strand of 12/3. The black goes to one single-pole breaker. The red goes to another single-pole breaker. The white goes to the neutral bus. You now have two circuits (red to white and black to white) on one cable. That's a multi-wire branch circuit. You have devices on both circuits. Now, while the power is on cut the white and what happens? Since both circuits are connected to the white they are essentially shorted out together. It's now black to stub-of-white to red. It's a 240V circuit. The devices on what used to be two 120V circuits are now on a 240V circuit, and that voltage is shared according to the resistance on each circuit. You might have 150V going through one device and 90V through another device. The device getting 150V might not make it.
The danger is in the connection to the same white... you're creating a potential short circuit.


Now, that having all been said, I'm not convinced that there was a multi-wire branch circuit involved in the OPs post. Surely the guy didn't have one neutral shared by four circuits? If he did then he'd get a cascade failure which would certainly kill 3 out of the four breakers, but I would expect one of them to survive.

Mike


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## alpha3236 (May 30, 2010)

the 20A, SP, breakers in question feed 4 large bedrooms and are required by NEC. the breaker feeding the sub panel was off when the ground wires were moved to their own bus bar. the breakers were operating for some time before failing. I am assuming a power surge caused the failure. HO lives in a very rural area and at the end of a long overhead line. power is supplied by an old REA and surges are common.
I thought 120/240 breakers were rated at 250V. apparently not!!!


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## 480sparky (Sep 20, 2007)

Mike in Canada said:


> ................The danger is in the connection to the same white... you're creating a potential short circuit...........



I fail to see how opening a neutral in a MWBC can cause a short circuit.


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## Big John (May 23, 2010)

alpha3236 said:


> ...The breaker feeding the sub panel was off when the ground wires were moved to their own bus bar....


 Something here doesn't add up.

These are regular branch circuits, each with it's own neutral.
The HO had landed both neutrals and grounds on the same bus.
The HO shut off power to separate the grounds an neutrals.
The HO restored power, after which all the AFCI's tripped and would not reset.

Is that a correct synopsis? Either this was simply a freak coincidence that the house happened to receive an over-voltage right after the HO did that work, or else there's something else going on. Nothing about separating grounds and neutrals should have caused an over-voltage....

-John


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## MF Dagger (Dec 24, 2007)

480sparky said:


> Why is he using *20*a AFCIs? What 20-amp circuits require them?


Dining room receptacles, 12o volt window a/c circuits etc...


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## 480sparky (Sep 20, 2007)

MF Dagger said:


> ...........12o volt window a/c circuits etc...


They still make those ancient things?


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## MF Dagger (Dec 24, 2007)

Unfortunately yes, and it's very hard to rough one in when the homeowner doesn't make a decision


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## Shockdoc (Mar 4, 2010)

I would check the stability of the neutral connection at the main panel, service and grounding. A loose neutral even on the POCo side can cause this since the residence most likely depends on ground rods instead of city water for supplemental grounding.


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## jwjrw (Jan 14, 2010)

Shockdoc said:


> I would check the stability of the neutral connection at the main panel, service and grounding. A loose neutral even on the POCo side can cause this since the residence most likely depends on ground rods instead of city water for supplemental grounding.


I agree the service neutral could be a problem but....
The "city" water pipe is a grounding electrode. Not a supplemental electrode like a rod or a plate.


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## tates1882 (Sep 3, 2010)

alpha3236 said:


> Had a wierd one yesterday. Guy wired his own house (and did a good job for a homeowner), sub panel had 4 20A AFCI breakers. During the rough-in inspection the AHJ had him seperate the neutrals & ground in the sub panel. Power is on so the guy can work. He goes to insulate and hears 4 pops. All 4 AFCI's tripped & won't reset, so he calls me. Even without any wiring to the breakers they won't hold. Meg the wiring, h-n, n-g, h-g & its all good. I could read 1600 ohms from the hot to the neutral screw on the breakers, so I called Tech Support. The breakers were new, and from 2 different batches. The manufacturer agreed the breakers were bad and assigned a case no. for warranty. The Tech guy told me the breakers will fail if voltage exceeds 130v to 135v, and the guys on the end of a rural line, I read 126v to ground.
> Sounds like he'll have problem in the future. Anybody else ever hear of this? I do mostly commercial & industrial so have only installed maybe a hundred of these things.


I would take the breakers up to the main panel and test them on another good afci circuit. I would think if it was a surge it would have affected some other equipment such as maybe a weak temporary light bulb. I have had afci's fail when they are stacked on top of each other, i try to seperate them with a regular cb. Being combonation afci cb its possible that maybe they are sense a fault in the feeder suppling the subpanel , i would ohm the nuetral and ground at the subpanel feeder cb, where the subpanel is unbonded they shouldn't ohm out. Sorta how I would go about troubleshooting the problem.


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## Shockdoc (Mar 4, 2010)

I said it before and I'll say it again, JUNK.


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## Mike in Canada (Jun 27, 2010)

480sparky said:


> I fail to see how opening a neutral in a MWBC can cause a short circuit.


 It's a short circuit in the sense that the two hots are connected... when the connection is complete to the neutral bus then it all works out, but if that connection is interrupted then it's just two hots forming a 240V circuit.
At least, that's the way I see it.

Mike


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## NolaTigaBait (Oct 19, 2008)

Its not a short...its a series circuit


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## 480sparky (Sep 20, 2007)

Mike in Canada said:


> It's a short circuit in the sense that the two hots are connected... when the connection is complete to the neutral bus then it all works out, but if that connection is interrupted then it's just two hots forming a 240V circuit.
> At least, that's the way I see it.
> 
> Mike


And that same 240v circuit has two loads in series.

Here's what happens when you open a neutral: Two lamps in series, 100w each, 240 circuit (NO neutral):








​
Left to right the meters are reading :Voltage on left lamp, voltage applied to entire circuit, amps of entire circuit, voltage on right lamp.

Show me the short circuit.


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## Mike in Canada (Jun 27, 2010)

This is becoming a matter of semantics, now. If you had two 'proper' circuits, each with a hot and a neutral, and you cut both neutrals, then what would happen? No power flows. Hang on... look at it this way.... if you had two circuits and the hots got connected by *accident* then what would you call the result?

By the way, I appreciate the effort in doing the test. If you're willing to go a step farther, then do the test with one 100W bulb and one 40W bulb.

Mike


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## 480sparky (Sep 20, 2007)

Mike in Canada said:


> ........ Hang on... look at it this way.... if you had two circuits and the hots got connected by *accident* then what would you call the result?.......


I would call the result of that changing the parameters of the circuit. You could easily do that with ANY circuit and cause a short.




Mike in Canada said:


> ..............
> 
> By the way, I appreciate the effort in doing the test. If you're willing to go a step farther, then do the test with one 100W bulb and one 40W bulb.
> 
> Mike


No problemo. (I just used a 60 instead.)











As you can see (and which someone named Ohm predicted), the voltages change, the amperage goes down a bit, the 100watter on the right gets dimmer: but they both light..... no big explosion! I kept the power on for about 5 minutes, and nothing earth-shattering happened.​
I still don't see any short.​


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## Mike in Canada (Jun 27, 2010)

There's nothing necessarily catastrophic about a short, but on reflection it was a poor choice of words simply because the term 'short' implies something inadvertent. I retract the word 'short' and will instead say 'connection'. The test you did showed the reason why the multi-wire branch circuits are not a good idea. I'm actually surprised that the bulb handled 190V okay. I expect its lifetime would be limited. It makes you wonder, though, about those 'rural service' bulbs you buy.... if a regular one can handle 190V..?

Mike


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## 480sparky (Sep 20, 2007)

Mike in Canada said:


> There's nothing necessarily catastrophic about a short, but on reflection it was a poor choice of words simply because the term 'short' implies something inadvertent. I retract the word 'short' and will instead say 'connection'. The test you did showed the reason why the multi-wire branch circuits are not a good idea. I'm actually surprised that the bulb handled 190V okay. I expect its lifetime would be limited. It makes you wonder, though, about those 'rural service' bulbs you buy.... if a regular one can handle 190V..?
> 
> Mike


"rural service" bulbs? Must be a Canadian thing. :laughing: 

Or 'rough service' as we call 'em here. They simply have more supports for the filaments. Sometimes rated for 130 volts.

But no doubt, at 190 v, the life will be shortened. But once it burns out, it opens the circuit and there is no circuit possible, therefore no short circuit.


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## Mike in Canada (Jun 27, 2010)

480sparky said:


> "rural service" bulbs? Must be a Canadian thing. :laughing:
> 
> Or 'rough service' as we call 'em here. They simply have more supports for the filaments. Sometimes rated for 130 volts.


 We have 'rough service' bulbs as well (for drop-lights and whatnot) but rural service bulbs are different. They're rated for higher voltage to handle fluctuating power. Not very common.



> But no doubt, at 190 v, the life will be shortened. But once it burns out, it opens the circuit and there is no circuit possible, therefore no short circuit.


 If you wired it in series then yes, but normally those things are wired in parallel so you don't lose all of your lights when a bulb goes.

Mike


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## alpha3236 (May 30, 2010)

Thanks for the ideas. I've taught apprenticeship & code classes, so I'm aware of Grounding/bonding. First thing I checked is connection location & tightness. Has to be an overvoltage situation. I was seriously shocked when Tech Support said the breakers were only good to 10% over circuit voltage and would fail at 130-135v. I think the whole AFCI thing is a load of crap anyway. Idaho has opted out of the "whole house" thing in the 08' NEC.


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## Shockdoc (Mar 4, 2010)

alpha3236 said:


> Thanks for the ideas. I've taught apprenticeship & code classes, so I'm aware of Grounding/bonding. First thing I checked is connection location & tightness. Has to be an overvoltage situation. I was seriously shocked when Tech Support said the breakers were only good to 10% over circuit voltage and would fail at 130-135v. I think the whole AFCI thing is a load of crap anyway. Idaho has opted out of the "whole house" thing in the 08' NEC.


These breakers will prove to be a problem in generator panels, especially those fed by smaller portable ones to a manual transfer switch. JUNK!


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## mikewillnot (Apr 2, 2013)

OP: What does this mean, he goes to insulate?



> He goes to insulate and hears 4 pops.


and what does this mean:



> Power is on so the guy can work.


does this mean he was separating grounds and neutrals in the sub-panel while it was energized? And if so, were the branch circuit breakers in the on position?


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## A Little Short (Nov 11, 2010)

mikewillnot said:


> OP: What does this mean, he goes to insulate?
> 
> 
> 
> ...


You do realize you just jumped into a 4 year old thread don't you?:whistling2:


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## mikewillnot (Apr 2, 2013)

no. missed that.  
the original post still doesn't make sense though, and nobody seemed to catch it, or untangle it. 
nevermind.


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## 3DDesign (Oct 25, 2014)

Open Neutral damaged the breakers like many here have described.
He's lucky that no appliances were damaged by the fluctuating voltage which could have been anywhere from 0 to 240 volts depending on the unbalanced load.
Have him buy four new AFCI breakers, chalk it up to experience and next time, turn off the power.


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## Meadow (Jan 14, 2011)

That is probably the 8th time Im hearing these breakers fail at around 130 volts. Seriously? Whats the point?


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## MTW (Aug 28, 2013)

AFCI's should be confined to the trash can where they belong.


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## RIVETER (Sep 26, 2009)

3DDesign said:


> Open Neutral damaged the breakers like many here have described.
> He's lucky that no appliances were damaged by the fluctuating voltage which could have been anywhere from 0 to 240 volts depending on the unbalanced load.
> Have him buy four new AFCI breakers, chalk it up to experience and next time, turn off the power.


I am old but I still can learn. If you supply two hots of opposite phases to the load which consists of, let's say a toaster on A phase and a mixer on B phase and the common neutral opens feeding the branch how does either get fried?


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## A Little Short (Nov 11, 2010)

RIVETER said:


> I am old but I still can learn. If you supply two hots of opposite phases to the load which consists of, let's say a toaster on A phase and a mixer on B phase and the common neutral opens feeding the branch how does either get fried?


Because it changes the circuit from a parallel to a series circuit. If the 2 loads are not the same you will have a voltage drop that is unequal.

In a series circuit the Rs add. So if the toaster was 12 ohms and the mixer was 24 ohms, you would have 36 ohms of R in that circuit.

Ohms law says that I (current) is E/R
so 240V/(12 ohms + 24 ohms)
=240/36
I=6.7A

toaster has 6.7A x 12 ohms = 80v
mixer has 6.7A x 24 ohms = 160v

So the mixer will have 160V and probably fry.
It would be worse on something with sensitive electronics such as a TV or computer.


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## 3DDesign (Oct 25, 2014)

A Little Short said:


> Because it changes the circuit from a parallel to a series circuit. If the 2 loads are not the same you will have a voltage drop that is unequal.
> 
> In a series circuit the Rs add. So if the toaster was 12 ohms and the mixer was 24 ohms, you would have 36 ohms of R in that circuit.
> 
> ...


YES, well explained! Please let me add a few details. 

If the Neutral opens on a single 120v circuit, the circuit simply doesn't work.
When a Neutral opens on the Main Service or in this case a sub panel, that is when you can fry everything.

It affects the entire panel of phase a and phase b. Let's say phase a is drawing the most amperage. It will have the highest voltage and will fry equipment on that leg. As you notice the problem and turn things off on phase a, phase b now has the highest amperage and equipment on that leg now gets fried.

The worst I've seen is 177v phase a, 63v phase b. It was caused by the homeowner running 4/0 to a 100A sub panel. The wire didn't fit so he cut down the 4/0 to fit.


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## The_kid (Nov 4, 2014)

A Little Short said:


> Because it changes the circuit from a parallel to a series circuit. If the 2 loads are not the same you will have a voltage drop that is unequal.
> 
> In a series circuit the Rs add. So if the toaster was 12 ohms and the mixer was 24 ohms, you would have 36 ohms of R in that circuit.
> 
> ...



Wow. That was well explained. Thats important info.


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## coolbreeze22 (Nov 12, 2014)

Breakers could be fine. What could be growing off the current flow through he wires and make the breaker "pop" is the AWG wire size of the insulator. If he is operating at 100 amps or less he should have used 60 degrees Celsius wire TW or THHn copper only. He probably used the wrong calculations when determining a pacify load through wires. It's a huge important involvement when understanding how current flows through wires and how insulation supports them.. You should of checked where he wired the ungrounded and neutral conductors at the connection points of his devices. That could also send a high voltage spike and cause a breaker to pop


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## bei (Jan 21, 2009)

Went to a house today expecting to find a dead phase. POCO had recently re-hung a triplex from pole to house that had been sagging a few feet off the ground, cause of droop unknown, but in a wooded area.
QO panel with 21 CAFI breakers as well as assorted 1- & 2-pole breakers. Every other AFCI breaker is tripped (all on same phase). Pretty cool to see this black/orange/black/orange pattern. Checked voltage with solenoid tester & had 120/240.
None of the tripped AFCI breakers would reset, & I'm assuming they are fried from an overvoltage on one phase that happened when the triplex went down. going back tomorrow with 10 new QO arc fault breakers.


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## Chris1971 (Dec 27, 2010)

bei said:


> Went to a house today expecting to find a dead phase. POCO had recently re-hung a triplex from pole to house that had been sagging a few feet off the ground, cause of droop unknown, but in a wooded area.
> QO panel with 21 CAFI breakers as well as assorted 1- & 2-pole breakers. Every other AFCI breaker is tripped (all on same phase). Pretty cool to see this black/orange/black/orange pattern. Checked voltage with solenoid tester & had 120/240.
> None of the tripped AFCI breakers would reset, & I'm assuming they are fried from an overvoltage on one phase that happened when the triplex went down. going back tomorrow with 10 new QO arc fault breakers.


Did you check the buss bar? See if it's damaged?


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## bei (Jan 21, 2009)

Yes, bus was clean, Chris.


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## Chris1971 (Dec 27, 2010)

bei said:


> Yes, bus was clean, Chris.


What was the outcome of the service call?


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## RIVETER (Sep 26, 2009)

A Little Short said:


> Because it changes the circuit from a parallel to a series circuit. If the 2 loads are not the same you will have a voltage drop that is unequal.
> 
> In a series circuit the Rs add. So if the toaster was 12 ohms and the mixer was 24 ohms, you would have 36 ohms of R in that circuit.
> 
> ...


If both loads are 120 volt then when in series such as two lamps why would they not dissipate the applied energy as if they were a 240 volt load? I am still learning.:thumbsup:


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## Big John (May 23, 2010)

RIVETER said:


> If both loads are 120 volt then when in series such as two lamps why would they not dissipate the applied energy as if they were a 240 volt load...?


You're right, they work just like that: If both are the same wattage, they both have the same voltage drop across them, and no problems.

But issues arise when the two lamps (or any loads) consume different amounts of power, and they have different voltage drops. This means one gets a larger share of the 240V than the other. 

If that imbalance is severe enough, that's when appliances get smoked.


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## RIVETER (Sep 26, 2009)

Big John said:


> You're right, they work just like that: If both are the same wattage, they both have the same voltage drop across them, and no problems.
> 
> But issues arise when the two lamps (or any loads) consume different amounts of power, and they have different voltage drops. This means one gets a larger share of the 240V than the other.
> 
> If that imbalance is severe enough, that's when appliances get smoked.


If that is the case let's say you have a 40 watt device and a 100 watt device now in series because of the lost neutral. As the power is available to the 40 watt, and it uses it, then the 100 watt will utilize the remaining. Is that wrong? The system should see the circuit and supplies the power necessary to operate it. The grid doesn't know the neutral is not there. All works off of a difference of potential and if the voltages are correct for the combined devices why do things burn up?


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## Big John (May 23, 2010)

Because wattage is a function of device impedance, so for things to have different wattages they have to have different impedances. And in a series circuit, each impedance only consumes its own percentage of the voltage drop. 

So if a series circuit has a total resistance of 10Ω and one device is 7Ω and the other is 3Ω, then that means one device is consuming 70% of the voltage drop (168V) and the other is consuming 30% of the VD (72V).

When you put the neutral in between series appliances, it forces the voltage to zero exactly in the middle of the circuit, so each device has no choice but to use the 120V drop applied across it.


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## RIVETER (Sep 26, 2009)

Big John said:


> Because wattage is a function of device impedance, so for things to have different wattages they have to have different impedances. And in a series circuit, each impedance only consumes its own percentage of the voltage drop.
> 
> So if a series circuit has a total resistance of 10Ω and one device is 7Ω and the other is 3Ω, then that means one device is consuming 70% of the voltage drop (168V) and the other is consuming 30% of the VD (72V).
> 
> When you put the neutral in between series appliances, it forces the voltage to zero exactly in the middle of the circuit, so each device has no choice but to use the 120V drop applied across it.


I may be old but you are kind of making sense.:thumbsup: It gives me something to think about.


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## A Little Short (Nov 11, 2010)

RIVETER said:


> I may be old but you are kind of making sense.:thumbsup: It gives me something to think about.


*So this didn't "kinda make sense"?*:blink:



A Little Short said:


> Because it changes the circuit from a parallel to a series circuit. If the 2 loads are not the same you will have a voltage drop that is unequal.
> 
> In a series circuit the Rs add. So if the toaster was 12 ohms and the mixer was 24 ohms, you would have 36 ohms of R in that circuit.
> 
> ...


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## bei (Jan 21, 2009)

The outcome, Chris, was that we changed the 10 bad breakers. Next day got a call from owner/landlord that the 3rd floor was without power (tenant was away for a week, so this was not discovered right away). I went back and changed out another 8 QO1xxCAFI breakers. Bus looked clean in the subpanel, just like in the main.

I had told owner to check all appliances, but she is not very technical, so I went thorough with a plug tester, flipped every switch & turned on everything else. Found 2 incandescent bulbs in a bathroom that would not come on even though they tested as having continuity, and one wall oven and one clothes washer that would not work (both with electronic controls). My guess remains an overvoltage from POCO ruined the breakers.

2 interesting asides- first, the 3rd floor panel had bad QO AFCI breakers on both phases, although the bad QO AFCI's in the main panel were all on the same phase.

Second, the manager at the Square D Authorized Distributor supply house said he had never heard of any similar problem. This guy has been a manager there for a long time and seems pretty well-versed in SQD products. I wonder how he could not know of this kind of problem, since overvoltage is not a rare condition.

Once the owner gets over the shock of this invoice, I will probably install a whole house surge protector.


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