divorced neutral bar ?

[plt]

I must be missing something. How can the neutral bar really be divorced from the ground when you complete a circuit in the meter can that creates a closed circuit between the grounding conductor, the panel and the grounded conductor? I think it is silly to bond the neutral bar to the box but at this point what is the difference if you are going to bond the neutral to the ground in the meter can and then bond the can to the ground and the pipe and then bond the cabinet to the pipe too? Why all the fuss about isolating the neutral in the cabinet if it is bonded to the same grouning conductor as the cabinet anyway?
thanks,
Richard

 

[hurk27]

Re: divorced neutral bar ?

First if I understand your dilima.
The NEC requires the neutral to be bonded to the groinding at the First point of disconect. After this connection it must be kept seperate. This is to insure that a gound fault will trip any breaker or fuses in the system. the reasone for keeping the grounding conductors and neutrals seperate after the main bonding jumper is to prevent neutral current from shareing the grounding pathway back to the panel. also if there is any voltage drop in the neutral as there will be inthe event of a fault. this voltage drop will be on everything that has a ground connected to it and would present a shock hazard to anyone in contact between the faulted circuit grounding and the grounding of another circuit or anything that has a seperate path to the panel I.E. the water pipes. Here is an exampale:
Lets say a home owner is washing some clothes and just happens to be leaning against the electric dryer and has his hand on the washer, when the motor in the washer shorts out between the hot and neutral now this short current can have as high as 3ka to 5ka amps on it and will create a very large voltage drop between the dryer and washer since they are fed by differant circuits. now since the grounding is the same as the neutral (not a seperate conductor) he will get the full effect of the voltage drop across his body. Also this allows stray currents to form on the grounding that should not be allowed to happen.

Also the reason the meter can also has the neutral bonded is if a fault were to occure in the service entrance conductors you need a good bond to not allow the fault to open a conductor inside of the house and hope it will blow somthing open at the pole or at least outside of the building. The primary fuses at the transformer are not there to protect the secondary conductors and in most cases are over sized to maintain continuty of electricity they are only there to protect the primary conductors that feed the transformer. this can and will most of the time allow for a very large current in the event of a fault to be placed on the service entrance conductors and have caused many fire's when the service equipment was eather not installed or maintained. So you want that connection as close to the service drop as possable.
Maybe Charlie from IPL can give som insite on this.

Hope this helps and maybe someone else can explain it better.

[ May 23, 2004, 03:41 AM: Message edited by: hurk27 ]

 

[roger]

Re: divorced neutral bar ?

Wayne, I thought that was an excellent explanation, and here is a graphic from Mike Holt that is in line with your explanation.

The same graphic with different wording explaining the metal bonding.

Roger

[ May 23, 2004, 10:06 AM: Message edited by: roger ]

 

[Guest]

Re: divorced neutral bar ?

I've re-read this question several times. Is the question solely about the Main panel, or is it also about remote Distribution panels ("subpanels")?

In the Main panel there should only be one type of bar (buss) for the grounded-conductors ("neutrals") and the grounds to land on. This bar is bonded to the pan of the Main panel. This bar bonds the neutrals, grounds, and pan. There can be more than one physical bar, but they will be identical in function and bonded to each other and to the pan.

In the Main panel a "ground" wire is also run from this bar to earth. There are many options for earthing: ground rods; ground plates; ground rings; Ufer's (capitalized in honor of Mr. Ufer); etc.

Now in remote distribution panels (commonly called subpanels) the NEC requires that the branch circuit grounds land on a grounding bar, while the grounded conductors (neutrals) land on a neutral bar. The ground bar is bonded to the subpanel pan, while the neutral bar must be isolated from the subpanel pan. [Note: There are exceptions where certain circuits like Isolated Ground are allowed to have their ground wire pass through the subpanel and not land until they hit the Main panel. But, that's another topic].

The grounding system would be more accurately called a bonding system because as you point out in the end they're both (ground and neutral) earthed at the Main panel.

The neutral is a grounded conductor. It is intended that this conductor will carry current.

The ground is a bond and per the NEC it is eventually earthed. The bonding and earthing provide different (almost mutually exclusive) functions.

The previous two posts cover the details.

 

[gpedens]

Re: divorced neutral bar ?

Also the neutral normally carries current. If you ground it at more than one place you now have more than one "neutral" to share the current. As you say anything grounded after the first point will now have current flowing in it. Including the pipe or appliance housing. Current times resistance is voltage but in this case it is very low until the fault occurrs. Another reason for only one ground point is even faults outside will now have a path for current flow. More than one ground can also cause noise signal problems in your phone or tv or computer. As little as one volt may cause your computer to see a zero instead of a one. The main thing is the one point ground is for safety, but it will cause problems with tv, computer, phone and other low signal stuff.