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Old 04-02-2010, 03:17 PM   #1
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MWBC bad neutral test results!


AS Promised, here are the test results from our little real-world test of a lost neutral on a multi-wire branch circuit:

The first picture is 2 keyless lampholders both with a 100 watt bulb in it, to simulate a balanced load, power off. You can see the 40 watt bulb behind them waiting "on deck"

2nd picture is with them both on, with the neutral intact. I measured input voltage at 249.7, with an amp reading of .77 on the right-hand bulb. Neutral amps read at .001. Input voltage varied between 249.6 and 250.2 for the duration of this test.

On the 3rd photo, I detached the neutral. The voltage now read 122.1 for the left bulb, and 128.0 on the right. Measuring the total amps of the circuit showed .73.

The brightness to the naked eye showed no difference. I'm sure there was a minute difference, but staring at lit 100 watt bulbs was not discernible.

4th photo shows the left bulb with a 40 watt globe type (easier to tell the difference this way). Amps showed .33 on the 40 watt bulb, .73 on the 100 watt bulb, and .4 on the neutral.

5th photo shows what happens when you lose the neutral with unbalanced loads on the legs. The 100 watt bulb got real dim, and just glowed, while the 40 watt bulb got real bright. I was able to measure the Voltage on the 40 watt bulb at 203.4. Total Amps with this series load was .42

6th photo shows the same scenario, but with the voltmeter measuring across the 100 watt bulb. You can clearly see it is reading 46.87 Volts.

The bulk of the supply voltage is hitting on the 40 watt bulb, which will fail prematurely if left connected in this manner. Usually, such tests will blow the smaller bulb in short order -- sometimes even explosive! I kept my distance when I pulled the neutral.

The readings are essentially double what I measured doing the same thing with a 125 Volt feed.
Attached Thumbnails
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MWBC bad neutral test results!-lightbulbs-006.jpg  

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Old 04-02-2010, 03:50 PM   #2
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MWBC bad neutral test results!


With the exception of the bulb not blowing up, that's about what would be expected.

By the numbers (based on stated wattage and assuming 120v):
40watt bulb has 360 ohms of resistance
100 watt bulb has 144 ohms of resistance

When in series and a 240 volt source
40 watt bulb should have 171 volts across it
100 watt bulb should have 69 volts across it

The 40 watt bulb (with the higher then normal voltage) burns brighter than it should and gets hotter, while the 100 watt bulb (with lower than normal voltage) burns dimmer than it should and gets cooler.

Generally, when things get hotter, the electrical resistance goes up. So compared to the norm, the resistance of the 40 watt bulb should go higher than calculated, the resistance of the 100 watt bulb should go lower than calculated, and therefore the 40 watt bulb gets even MORE of the voltage drop.

The question now becomes how long can the 40 watt bulb last with 200 volts pumping through it.

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Old 04-02-2010, 04:14 PM   #3
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MWBC bad neutral test results!


Great test !!
Thanks for taking the time to do this & posting the results
Its always nice to see a real world test & results
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Old 04-02-2010, 07:29 PM   #4
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MWBC bad neutral test results!


Thanks for doing the test...nice pics.

Also, thank you for starting a new thread. The old one was getting difficult to navigate!

Question about HooKooDooKu's reply: Why does resistance increase as temperature increases?

Thanks again kbsparky.
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Old 04-02-2010, 07:47 PM   #5
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MWBC bad neutral test results!


"The brightness to the naked eye showed no difference. I'm sure there was a minute difference, but staring at lit 100 watt bulbs was not discernible."
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Old 04-02-2010, 09:53 PM   #6
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MWBC bad neutral test results!


Nice work!!!!
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Old 04-02-2010, 11:43 PM   #7
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MWBC bad neutral test results!


The way KB did set up the photo on the single phase system and it will work the same way with Three phase MWBC there is no differnce of reaction so you can understand from electrician's point of view we have rules.,,

JAMAIS { NEVER } lift MWBC netural unless all that affected circuit is off and tested with voltmeter or wiggy.

And with MWBC you have to keep your guard up unless I will say test it twice the reason why I mention that due somecase you will find one breaker or fuse at the one spot and find the second part of MWBC on other breaker or fuses.

That one reason why the code did strictly enforeced on 2008 NEC code cycle now all MWBC it will required common trip breaker { some area may allowed handle tier on it }

I know some of the readers may ask me this question do we have MWBC in France ? the answer is Oui { Yes } we do the only differnce is the voltage that it.

Merci,Marc
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Old 04-03-2010, 12:53 AM   #8
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MWBC bad neutral test results!


common practice for us at work is even though a MWBC is off to check the voltage on the neutral to ground to make sure nothing else is tied into the neutral as well.
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Old 04-04-2010, 08:29 AM   #9
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MWBC bad neutral test results!


Awesome test kbsparky. Good thing your test involved a couple inexpensive light bulbs and not a nice flat screen TV...
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Old 04-05-2010, 08:05 AM   #10
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MWBC bad neutral test results!


Quote:
Originally Posted by jlmran View Post
Why does resistance increase as temperature increases?
Electricity is all about moving electrons.
Temperature is all about vibrations (at the atomic level).
The higher the temperature, the greater the vibrations.
The greater the vibrations, the more difficult it is for electrons to move.

This is why "super conductors" are all VERY VERY COLD!!! At the atomic level, when things get cold enough, everything lines up and stops vibrating allowing electrons to flow with no resistance.



Batteries, on the other hand, are sort of the opposite. Because batteries produce electricity from a chemical reaction, cold temperatures make that chemical reaction more difficult.
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Old 04-05-2010, 06:29 PM   #11
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MWBC bad neutral test results!


Does this mean ice has less resistance than water?
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Old 04-05-2010, 08:56 PM   #12
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MWBC bad neutral test results!


Quote:
Originally Posted by jlmran View Post
Does this mean ice has less resistance than water?
that just poked a massiver hole in the "colder is better" theory being as ice hardly conducts at all. Infact if the ice itself is cold enough at best all you'll see is a static discharge. Also food for thought...de-ionized water if it's pure enough may not even be able to carry enough current for you to even feel it at 120V.
Just because something gets cold doesn't make it a superconductor though...there are other factors involved. This isn't an attempt to disprove super conductors are cold....just clarification that not everything has a lower resistance when cold, But then water is in a totally different state as well when frozen.

Last edited by andrew79; 04-05-2010 at 09:01 PM.
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Old 04-06-2010, 01:20 AM   #13
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MWBC bad neutral test results!


Quote:
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Does this mean ice has less resistance than water?
Ice is a crystiline form of mater, while water is liquid. Obviously two different forms of mater composed of the same chemical elements.

But when you are talking about copper and similar electrical conductive metals, the colder they are, the less resistance they have. Most metals will become super-conductive if they are made cold enough (i.e. pretty much absolute zero, which is −273.15C or -459.4F).

About 15 years ago, super conductors were in the news because materials were being discovered that becames super conductors at the relatively warm temperature of liquid nitrogen (−196 C; −321 F). That was significant when you consider at the time (and maybe even today) the cost of liquid nitrogen per gallon was cheaper than milk. BUT the main problem with these high temperature super conductors was that they were generally ceramics... which ment they were very difficult to turn into a wire.
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Old 04-06-2010, 06:56 AM   #14
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MWBC bad neutral test results!


Quote:
Originally Posted by HooKooDooKu View Post
Ice is a crystiline form of mater, while water is liquid. Obviously two different forms of mater composed of the same chemical elements..
When the state changes (say from liquid to solid) the atoms may re-organize, say to a regimented arrangement which most crystalline structures have. Then the conductivity versus temperature characteristics may be totally different.

I would guess that ice at 31 degrees (F) has a greater resistance than ice at -200 degrees.

But many gases have a lower resistance at higher temperature. This is one of the reasons why fluorescent lamps (also mercury and sodium vapor lamps) need ballasts. The ballast has a fixed resistance or a controlled resistance so the current can never exceed a certain amount. (current always equals voltage times resistance) Were it not for the ballast, the current would increase as the lamp warms up and something would have to give for example the breaker would trip, For gases it gets more complicated because the pressure also affects the resistance and for a given amount of gas confined to a certain container (such as a bulb) as the temperature increases the pressure increases also.
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Old 04-06-2010, 08:23 AM   #15
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Quote:
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But many gases have a lower resistance at higher temperature.
When you are dealing with a gas, you are dealing with molecules that are bouncing around in mostly empty space. There is so much space between molecules, that the temperature/pressure formula (PV=nRT) holds regardless of the size of the molecules.

Similar to a solid, the hotter the gas, the faster the molecules move. But in the case of a gas, this means that the molecules bounce into each other more frequently providing more opportunities for the electrons to jump from one molecule to the other; hence the lower resistance at higher temperature.



Quote:
Originally Posted by andrew79 View Post
de-ionized water if it's pure enough may not even be able to carry enough current for you to even feel it at 120V.
I didn't think about it at the time, but this gives a bit of a clue why ice is not as conductive as water. In the case of water, pure water doesn't conduct electricity very well because the H2O molecules are very stable. But what water does is provide a medium for ions to free move. So when water is conducting electricity, the H2O is just the medium, but the ions (disolved minerals, etc) move the electricity. When you freeze the water into a crystal structure, the ions are no longer free to move around like they did in the water.

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