open neutral...big problem

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westelectric

Senior Member
Location
westchester county, NY
I have an issue where a mwbc left a panel. 1 cir was a fridge, 1 was a micro. By accident the neutral of the mwbc was opened. I am being told that both appliances are damaged. I understand this is possible but can someone explain how this happens. I understand that a load must have been applied, for example, someone turned the micro on or the compressor in fridge turned on creating high or low voltage. I just would like some clarification as to what actually happened so i can understand. Thanks
 
LEO2854

LEO2854

Esteemed Member
Location
Ma
westelectric said:
I have an issue where a mwbc left a panel. 1 cir was a fridge, 1 was a micro. By accident the neutral of the mwbc was opened. I am being told that both appliances are damaged. I understand this is possible but can someone explain how this happens. I understand that a load must have been applied, for example, someone turned the micro on or the compressor in fridge turned on creating high or low voltage. I just would like some clarification as to what actually happened so i can understand. Thanks
Click to expand...
:-?
What is mwbc?
 
LEO2854

LEO2854

Esteemed Member
Location
Ma
westelectric said:
I have an issue where a mwbc left a panel. 1 cir was a fridge, 1 was a micro. By accident the neutral of the mwbc was opened. I am being told that both appliances are damaged. I understand this is possible but can someone explain how this happens. I understand that a load must have been applied, for example, someone turned the micro on or the compressor in fridge turned on creating high or low voltage. I just would like some clarification as to what actually happened so i can understand. Thanks
Click to expand...

Check out the link in sparky's post it has good info.
 
D

dbuckley

Senior Member
Location
Canterbury, New Zealand
The example linked to is OK for two simple loads (such as a couple of lamps), but it understates the dynamics of the real world.

In a neutral failure scenario, rarely are the loads as simple as two lamps in series; there is usually a network of loads on each half. The microwave when off has just the electronics that operates the display and keypad. Open the door and that adds the interior light. Press the start button and the transformer and the motor for fan and rotation and the light comes on. So there are four separate independent loads in a typical operating microwave.

Depending on the fridge, there may be one or more lights, one of more compressors, and maybe heater(s), and maybe electronics for the display.

But, as noted in the presentation, Ohm's Law applies, so you can (theoretically!) work out what the actual impedances of the two appliances are, and thus work out what voltage is present across each one when the neutral opens.

Rarely will the impedances of the appliances be such that each still gets 120V across it. One will have more than 120V, the other less. So what happens?

That's when it gets interesting.

The weakest link on the appliance with the higher voltage across it will eventually fail. When it does, the impedances have changed, and the voltages will re-balance. Thus the next weakest link comes under stress.

Note that the weakest link is the weakest link at the actual applied voltage.

So what happens is a sequential failure of each element in the complete circuit until you get to the point where something can take the over-voltage continuously in a stable state, or on one side all loads have burned out so current no longer flows.

Thus in most real world neutral failures, and especially when its at the service not at the MWBC, pretty much all parts of all connected devices will get trashed.

Neutral failures are ugly.
 
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H

hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
I had a worker do this exact same thing with the exact same loads (strange), and I too wanted to find out why both appliances on opposite sides of the MWBC was damage, as in most cases the lighter load appliances opposite of the heavier load appliances will most times always fail but the heavier load appliance is ok , but the microwave failed and the refrigerator also failed, like in the OP the micro wasn't in use, so measuring the resistance of the failed microwave I saw why it took out the refrigerator, the resistance of the micro was .09 ohms, which meant almost a dead short, which would have sent almost the full voltage to the refrigerator.
What I found was when the wrong neutral was lifted the refrigerator being the heavier load first sent over voltage to the microwave damaging the control power supply causing the full wave bridge rectifier to dead short out, this then flipped the over voltage back to the refrigerator which took out its power supply and electronic controls, both of these controls are always on so even when not in use they will be damaged, but I didn't expect the flip-flop, and the only fuse in the microwave was a 15 amp slow blow, so it was enough to damage the controls in the refrigerator which I suspect wasn't running at the time, I think if the compressor was on line at the time it might have caused the 15 amp fuse in the micro to open, and might have protected the refrigerator, but that is just a guess.
but this was an eye opener, and the first time I saw damage on both sides of a MWBC.

I have since seen an even worst case at our city hall where a city worker was remodeling a bathroom, and went to tie in a new light in a junction box that seem to be in the right location right above the light, but when he opened the neutrals on a 120/208 3-phase MWBC, he floated neutrals going to all the EMS 911 computers and radios system, luckily for the most part almost all of them had dual conversion UPS and it took out the battery charges, again these shorted out which shifted the high voltage to the next phase which again took out these UPS and then the last phase was hit, all the UPS systems was dual conversion so the radios and computers are always running on battery power, and the charges just act as a power supply and keep the battery's charged, so most of the equipment wasn't damaged,
but three systems was, that had single conversion UPS which did supply utility power to the equipment, and the total damage was over $19,000, that was the end of the city ever allowing a city worker to touch any electrical.

Yea I know I ramble too much,:roll:

I just love to share things I have run into hopfully to help others when they run into the same problems.
 
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G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100901-0702 EST

The light bulb example is wrong. The resistances change when the voltages change.

This example should be done with Ohmite power resistors as the loads. Within the power rating of an Ohmite power resistor (other brands can be used) the resistance is relatively constant. Not so with a tungsten filament incandescent lamp.

.
 
480sparky

480sparky

Senior Member
Location
Iowegia
al hildenbrand said:
Really nice slide show, Ken. :)

Is there a way to indicate " Ω " for the resistance calcs? Does ALT 234 work in your slide program the way it works here?
Click to expand...


I thought they already had "Ω" in it. Every slide was built from a bitmap..... there's no text other than what's along the top.
 
reddy kw

reddy kw

Member
Location
Mudville, upstate New York.
dbuckley said:
The example linked to is OK for two simple loads (such as a couple of lamps), but it understates the dynamics of the real world.
In a neutral failure scenario, rarely are the loads as simple as two lamps in series; there is usually a network of loads on each half. The microwave when off has just the electronics that operates the display and keypad. Open the door and that adds the interior light. Press the start button and the transformer and the motor for fan and rotation and the light comes on. So there are four separate independent loads in a typical operating microwave.
Depending on the fridge, there may be one or more lights, one of more compressors, and maybe heater(s), and maybe electronics for the display.
But, as noted in the presentation, Ohm's Law applies, so you can (theoretically!) work out what the actual impedances of the two appliances are, and thus work out what voltage is present across each one when the neutral opens.
Rarely will the impedances of the appliances be such that each still gets 120V across it. One will have more than 120V, the other less. So what happens?
That's when it gets interesting.
The weakest link on the appliance with the higher voltage across it will eventually fail. When it does, the impedances have changed, and the voltages will re-balance. Thus the next weakest link comes under stress.
Note that the weakest link is the weakest link at the actual applied voltage.
So what happens is a sequential failure of each element in the complete circuit until you get to the point where something can take the over-voltage continuously in a stable state, or on one side all loads have burned out so current no longer flows.
Thus in most real world neutral failures, and especially when its at the service not at the MWBC, pretty much all parts of all connected devices will get trashed.
Neutral failures are ugly.
Click to expand...

It's cheap and easy to run one 3-wire to get 2 circuits, but it's not sound engineering. Is it possible MWBC's are obsolete in 21st cen. residential wiring? Along with danger of open neutrals, they don't work with AFCI or GFCI breakers. When MWBC's were prevalent 100 years ago, there were no electronics. Houses had resistive loads, almost no leading/lagging loads. 3-wire MWBC's were NOT meant to be used for electronics or Sub-Zeros.
14/2/2 and 12/2/2 give us '2 for 1' cable pulls without any issues associated with "Edison Circuits" (as they used to be called). More expensive then 14/2 or 12/3? Yes, but far less expensive then having to buy your client a new frige and microwave.
 
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al hildenbrand

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
reddy kw said:
It's cheap and easy to run one 3-wire to get 2 circuits, but it's not sound engineering. Is it possible MWBC's are obsolete in 21st cen. residential wiring?
Click to expand...
I dunno 'bout that. A MWBC with reasonably balanced load running will have better voltage "control" as the voltage drop of the neutral will be canceled, thus halving the voltage drop each side's load experiences.

Regardless of the load type (early 1900s or early 2000s), the voltage control on the end of a MWBC is still good engineering to me.
reddy kw said:
Along with danger of open neutrals, they don't work with AFCI or GFCI breakers.
Click to expand...
With respect to AFCIs, that may be changing. The new GE MOD 3 AFCI in the single pole 15 A and 20 A sizes can be used to protect a MWBC, as shown in a GE diagram that Dennis Alwon posted here. Note that the MWBC neutral is only connected to one of the single pole AFCIs.

GE is, evidently, accomplishing this, in part or whole, by dropping the GFCI-like component of their breaker circuitry.

GE has released very little information about this development, even though the breakers are already out in distribution. I've even seen them on the shelves of the local Big Orange store just last week.
 
480sparky

480sparky

Senior Member
Location
Iowegia
hurk27 said:
Ω+<=>???∂∆∏∑−∕∙√∞∟∩∫≈≠≡≤≥ well I can get them but only from copying and pasteing from the Character Map:confused:
Click to expand...


My point is, when I created the PP slides, I didn't load it up with text.... they were made from .jpg images. The 'text' in the ladder diagram portion is part of that image.
 
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