why i HATE shared NEUTRALS!!

[dnem]

The 'hot' you got working on is dead but that neutral is still carrying a load back home

This is completely dependent on where in the circuit you're working. If you're changing a ballast with the hot leg de-energized to the fixture the neutral at the fixture will not be carrying anything regardless of the number of circuits connected upstream of that fixture.

It is not dependent on where in the circuit you're working.

The neutral present with the hot leg de-energized to the fixture is the exact same neutral that is present at every other point in the circuit. The different sections and branches of the neutral will carry different loads and will register different current flow but the entire neutral multi branch is electrically identical. If you lose the neutral HR, every single point that the neutral is present, at every single box regardless of which phase conductors are also present at any individual box, every single point will be "hot", as long as any one of the multi phase breakers are still on.

David

 

[iwire]

David I believe Trevor is talking about opening the neutral after the point the MWBC splits out into two wire circuits.

This.....

....is all you need to work on MWBCs.

Before you open any conductors that you do not the routing of simply check for current flow.

 

[dnem]

If the fear is about the grounded (neutral) conductor that is present in the fixture and this conductor is part of a multiwire circuit would the equipment grounding conductor be as dangerous? Remember that the equipment grounded conductor is tied to the grounded (neutral) somewhere in the system. One conductor will be carring as much current as the other.

Did you say you were a teacher ?
Do you know that you posted that the equipment ground and the neutral "One conductor will be carrying as much current as the other" ?
Did you really mean to say that ?

Do you think that 2 wires that are connected together automatically carry the same current ?

I case you really meant what you said, let's run thru one simple scenerio and test that statement.

One circuit and two plugs.
A black and a white from the power source (forget about the ground for now).
They run to the first receptacle box and connect to a pigtail.
They then continue on to the second receptacle box.

The load plugged into the first plug is 10amps.
The load plugged into the second plug is 10amps.
Your clip on amp meter is around the neutral feeding the second box, the current is ..... ?
10amps, right ?
Your clip on amp meter is around the neutral feeding the first box, the current is ..... ?
20amps, right ?

But isn't it the exact same neutral going into the first box as the neutral coming out ? The only difference is a pigtail to the first load.

David

 

[infinity]

Impedance has nothing to do with this concept. The neutral and the ground are only connected together at the main. If the neutral home run is disconnected, it has no path to the main. The only connection that the neutral still has is to the load. And thru the load it is connected to the phase wire feeding the load. So it is the same potential as the load.



David

I don't recall ever posting about the neutral being disconnected. I simply was speaking of a fully operational system. The point is if you go to a de-energized fixture ballast and cut the white and black wire feeding the ballast your potential for a neutral shock should be zero. Take the white conductor and insert in mouth. Again nothing. If you want to go through all different scenarios about losing the neutral somewhere else in the circuit you can. But none of my posts had anything to do with losing the neutral.

 

[infinity]

David I believe Trevor is talking about opening the neutral after the point the MWBC splits out into two wire circuits.

This.....

....is all you need to work on MWBCs.

Before you open any conductors that you do not the routing of simply check for current flow.

Thanks Bob, that's exactly what I meant. Once the circuit splits off the MWBC at the last point that the neutral is shared, it's now a simple two-wire circuit.

 

[roger]

David, you need to understand that an open neutral/grounded conductor is dangerous period, it has nothing to do with how many ungrounded conductors it may be common to.

I'm sorry you got you hurt and I'm glad it wasn't worse. If this would have happened while you were working for our company you would have been questioned by the safety committee as to why the proper safety procedures were not followed (you have already stated why you were hammered which was taking things for granted) and you would have received a reprimand.

Your safety is your business, you can't blame getting hurt on a wiring method you are supposed to be fixing, otherwise you are admitting it was over your ability/expertise, which probably would have kept you from getting hurt if you had. Sorry to be so blunt.

The bottom line is, MWBC's are as safe as the service that feeds them, and require the same respect and "know how" when working on them.

The following sentence is not meant to be demeaning, but if that is the way it is perceived then so be it.

For those who are scarred of, leary of, don't understand, or simply believe every wives tale/myth about MWBC's that they have heard, then I say, by all means, avoid them and don't use them at any cost.

For those like myself and the others in this thread that are comfortable with them, we will continue to use them and will not have a problem when trouble shooting or servicing facilities that are full of them.

We will continue to be competitive and win bids on large projects where the drawings and prints show MWBC's and we sleep well.

Roger

 

[jwelectric]

Working on buildings that are partially occupied, where some circuits must remain on for the business that is still operating, while another section is under demo and/or construction is just plain reality.

Yes I have worked this type of jobs several times and have always been sure that anyone that worked on the job understood the correct procedures in making sure that all circuits in that area were off and disconnected before allowing anyone to enter.

When you work in those environments day after day and year after year and decade after decade, you can one day have one person on your crew that makes a mistake.

You call it a mistake I call it an act of stupidity. There is no room for mistakes or accidents in the field of electricity. Always check and double check.

The existance of accidents doesn't highlight stupidity, it highlights that your chosen profession can be dangerous. Sometimes stupidity is involved, sometimes it's not.

You, me or anyone else can excuse carelessness by calling it a mistake or an accident as much as we want and it will not change the fact that this mistake or accident could have been prevented. If this mistake or accident could have been prevented then the only thing left is a plain and simple act of stupidity.

And sometimes it's someone working with a circuit that they think is off because they have confirmed that the breaker feeding the whole area if off.
There would have been 277v registered between neutral and ground, if I had checked.
If you think that means that I was stupid not to check, then so be it.

The way I read these statements is that you only half checked the circuit and assumed the circuit was off.
Please describe these actions. (a) a mistake, (b) an accident, (c) an act of stupidity

Do you honestly think someone would ground themselves purposefully ?
I can't believe you would think that.
Do you also think that people shock themselves purposefully ?

I think that a person that is working on a circuit that could be hot and is not wearing the proper PPE is asking for what ever happens.

On one job, one of my employees told me that he was done with a house 3wire multi. I inspected his panel work and looked at the HR jbox in the basement ceiling.
We flipped on the two breakers and a fuse in the TV blew up like a balloon popping.

What do you see here? I see two different people that made a _______? I am happy for you that only a fuse blew but how would you have explained it if the same ?mistake? had caused a death. Now that would have been a great price to pay for a ?mistake.?
Why is the TV plugged into a receptacle that has never been checked? This is why I call it a act of stupidity and not a mistake.

You do make some good points about safety proceedures. But your harsh accusations show extreme ignorance of reality.

I am sorry if you think that I am being harsh but I intend to be harsh.
Safety is not a game to be played nor is it something to be ignored.
Every ?mistake? that you have posted above is something that should not have happened and would have been avoided with the proper safety training and procedures.

One amp of current flowing through the human body is almost certain death. There is no room nor excuse for mistakes or accidents. I have one rule that is applied to safety in the work place and there is NO forgiveness. NO MISTAKES = NO ACCIDENTS

Should I have an employee who missed the wire in the box you mentioned above that person just won a chance to find out how many utility poles there are between the job and his house and I will not even offer to give him a tin can to kick as he walks along.
Safety first and always check then double check and go back and check again.
It is not only your life that you are dealing with but the life of others.

 

[jwelectric]

The following sentence is not meant to be demeaning, but if that is the way it is perceived then so be it.
Roger

Roger you posted while I was doing my post. I think that you made a fine statement and let me say you made me proud.

 

[roger]

Thanks Mike.

Roger

 

[infinity]

Roger you posted while I was doing my post. I think that you made a fine statement and let me say you made me proud.

Ditto exactly what Mike said.

 

[George Stolz]

David, one note I'd like to make: A tick tracer would have glowed had you put it to it before touching the neutral.

If the neutral were removed from the service, it's no longer grounded, hence the shock hazard. If it's not grounded, and energized, then it would light up a tick tracer.

All things being equal, I stick a tick tracer in anything I'm working in if the building is hot. (Exception: one temp breaker to a specific outlet installed in the (only) panel. Then I take things for granted. Otherwise, the tick tracer goes in.

 

[brother]

Wow, i didnt think that my Posts on 'why i hate shared neutrals' would cause such a discussion.

jwelectric, I hear what you saying, and thankfully I personally have NOT been hit by the neutral. But i do believe some of your comments are little off. just my honest opinion.

Its just that i see more people make mistakes on these neutrals than on other things when trouble shooting. Part of it is 'not being properly trained' and the other is inexperienced. Only on certain situations do i see a reason to 'send someone home' for making errors.

Even though most of the projects im on we do use shared neutrals alot and they are always there. Just that in some of the old remodles its a pain when you can see that someone before you had added these circuits and did NOT check to be sure the neutral wasnt already overloaded.

With AFCI's , you wont be able to share neutrals, it causes it to trip. So says GE. Even in my own house, (which i plan to change soon) They shared neutrals on the GFCI (which angers me greatly) and sometimes when my frig kicks on the gfci pops and kills power to the other outlets, but the frig stays own. Im sure the contractor who did this wasnt thinking and or didnt care!! Cheap labor.

Im just thankful that i was properly trained and learned my trade and from OTHERS mistakes /and some of it i learned in the field. But i would NOT use a BLANKET statement and say that all of them were stupid.

 

[roger]

Once again, Brothers last post points out that it is the unqualified who cause the problems not the wiring method.

Part of it is 'not being properly trained' and the other is inexperienced.

and

that someone before you had added these circuits and did NOT check to be sure the neutral wasnt already overloaded.

nuff said.


Trevor, thanks for the kudos also.

Roger

 

[dnem]

You, me or anyone else can excuse carelessness by calling it a mistake or an accident as much as we want and it will not change the fact that this mistake or accident could have been prevented. If this mistake or accident could have been prevented then the only thing left is a plain and simple act of stupidity.



The way I read these statements is that you only half checked the circuit and assumed the circuit was off.
Please describe these actions. (a) a mistake, (b) an accident, (c) an act of stupidity

The difference in the three:

A mistake can lead to an accident but the mistake is not necessarily an act of stupidity. Stupidity is the mental inability to grasp a simple concept or the conscious moral choice to disregard something you know and realize to be right. I don't think you realize how offensive your words are. And how you're misusing the word.

By reading your posts so far, I also have not seen reason to believe that you really understand the entire concept of how the MWBC works. I can see that you understand how to turn everything off and seem to think that your unwavering determination to kill all power is all you need to know. But there is no substitute for understanding how the circuit(s) work.

I've been shocked by more than 120v before. You apparently haven't. Do you think that means that whatever you say is right while I am wrong ?

David

 

[amptech]

Brother, I can't believe you are suprised at the resopnse to your "I hate MWBC" thread. Whe I first saw this topic a few days ago and read your initial post I figured either you were bored and wanted to start a lively debate or you were descended from one of those people who campaigned heavily against the automobile a hundred years ago because it was suicide to ride in a buggy sitting on a tank of explosive liquid. The MWBC debate rages on this forum and others from time to time and I don't think anyone ever changes their mind or has their opinion swayed one way or the other. As for me, I'm a proud user of the MWBC. I've been in the trade long enough to know what's fact and what's not about MWBCs and have taken the time to learn and understand electrical theory so I can sort the facts from the chicken little stories. My position is if you don't like MWBCs don't install them.

 

[jwelectric]

jwelectric, I hear what you saying, and thankfully I personally have NOT been hit by the neutral. But i do believe some of your comments are little off. just my honest opinion.

But i would not use a blanket statement and say that all of them were stupid.

I have taught safety classes for five years and I am of the firm belief that any ?mistake? or ?accident? can be avoided. If this is a true statement then it only stands to reason that any ?mistake? or ?accident? was due to something someone did that was ?stupid? thus it is not a mistake or an accident it is an act of stupidity.

I have said in the past, I am saying now and I will say tomorrow that there is no such thing as mistakes and accidents in the electrical field only stupidity.

If you get hurt while at work there is something that you could have done that would have prevented this from happening so it is your own stupidity that got you hurt.

Only on certain situations do i see a reason to 'send someone home' for making errors.

In the comment I made

Should I have an employee who missed the wire in the box you mentioned above that person just won a chance to find out how many utility poles there are between the job and his house and I will not even offer to give him a tin can to kick as he walks along.

I do not call this a mistake nor an oversight. This is incompetence plain and simple.
I call a mistake when someone forgot to add cheese to my buger or got me a coke instead of tea.

Again, There is no room for mistakes or errors in the electrical field. Remember the life you save could be your own.

Here is a sernio that I sometimes use in the classroom.
I am sitting at a red stoplight and the light turns green. As I start to enter the intersection a car coming from the left hits me in the side.
Who is at fault?
This question has nothing to do with who will get the ticket no who is liable for the cost. The question is, who is at fault?
Before you answer that question let me ask this. Is there anything that I could have done to prevent this accident?

 

[don_resqcapt19]

David,

I've been shocked by more than 120v before.

Do you know that working on energized circuits is almost always a violation of the OSHA rules?
Don

 

[amptech]

"If you get hurt while at work there is something that you could have done that would have prevented this from happening so it is your own stupidity that got you hurt."
Mike, this statement it a little too broad. My father-in-law, while working as a lineman, had his boss call in to control while he was working a 14kv primary and accidently order the wrong circuit closed. He received a 14kv jolt that should have killed him. The only thing he could have done to prevent this was to have stayed home that day, but unfortunately he didn't check his crystal ball before he left for work that day. As long as you work with other humans there remains the possibility of getting hurt through no fault of your own. I lost my left eye 25 years ago on the job. I was wearing safety glasses and had nothing to do with who/what launched the projectile that took my eye. I guess I'm to be counted among the countless "Stupid" who have the misfortune to be in the wrong place at the wrong time. What, in your learned opinion, can someone do at work to prevent the careless actions on another party from causing them injury? I'd like to know because I've thought about it a lot the last 25 years and I still got nothing.

 

[dnem]

I've been shocked by more than 120v before.

David,

Do you know that working on energized circuits is almost always a violation of the OSHA rules?
Don

Noone was intentionally working on the circuits while they were live. I'll post the example one more time so you understand the situation.

We were reworking a large drop ceiling area in an office. The multi was a 4wire 277/480v. We shut off all three circuits and demoed certain connections and moved lights around. Obviously the neutral HR was lost in one of those disconnections, which I would find out the next day. When we left, we left the yellow and orange circuits off because they were feeding lights that were under construction but we turned the brown circuit back on because it feed lights that weren't being disturbed and they provided perimeter lighting for office areas that were still occupied and not part of the construction area.

The next morning we went back to work, noone thought about any danger from the brown circuit because we were not working on any brown circuit lights or even working in any 4 squares that had the brown circuit present. But the shared neutral is electrically the same at every point in the multi. Just because it's only present with the deenergized yellow phase in a jbox doesn't mean anything. The neutral that I touched wasn't anywhere near any box that contained the brown. The brown phase didn't even enter the same room that I was working in.

That second morning, when we checked the circuits that we were working on to make sure they were still off, none of us thought about the neutral. I could have ended up paying with my life. It was a lesson that I won't forget and I learned it without anyone calling me stupid.

Mike seems to think that using put-downs shocks or shames people into being more aware and using their heads. I think it just shows a teacher that likes to call his students stupid and thinks he'll use the same language here.

David

 

[jwelectric]

Stupidity is the mental inability to grasp a simple concept or the conscious moral choice to disregard something you know and realize to be right.

I like your definition of stupidity. Here is what Merriam-Webster?s Collegiate Dictionary says
Stupidity. The quality or state of being stupid
Stupid. Acting in an unintelligent or careless manner, marked by or resulting from unreasoned thinking or acting

I think that both you and my dictionary say about the same thing.

By reading your posts so far, I also have not seen reason to believe that you really understand the entire concept of how the MWBC works.

You will just have to take my word that I fully understand the MWBC.

your unwavering determination to kill all power is all you need to know.

This is not what I am saying. What I am saying is that SAFETY comes first and if this means turning the circuit off then turn it off.

I've been shocked by more than 120v before. You apparently haven't.

I was putting the last little tape on a stress cone when the switchgear I was working in was energized. This happened when I was twenty and I am now 54. I still cannot grow hair on my legs due to the burns I received.
Till this day I ask and answer the question, who was at fault? I was because I did not tell anyone that I was returning to the switchgear after it was finaled. I did something that was very STUPID and got burned because of my own STUPIDITY.

Do you think that means that whatever you say is right while I am wrong ?

Not at all and as most on this forum will be glad to tell you I say some pretty stupid things sometimes. (Anyone that agrees feel free to quote me on this statement and agree.)

I will agree that my words are harsh and I mean for them to be harsh. Through stupid actions people get hurt and killed. It is not the MWBC that is a danger it is the people that are a danger.
My old 12 gauge Fox has a three inch magnum shell in each chamber and sitting there in the corner with its ears at rest there is no danger at all. The danger comes when someone with out knowledge of how she works picks it up and tries to lay one of her ears back. (The left ear does not catch unless it is released real slowly and she will bark real loud)

There is nothing I am saying directed to you nor am I implying that you are stupid. I am only saying that the only time that a MWBC is dangerous is when the unskilled tries to work on them. To try and work on any electrical circuit with out the proper training would be nothing short of stupid.