I realy enjoy this site , it is very imfomative. I have two questions regarding neutral being tied to ground. I am pretty sure when I first stared in the trade in Canada , that different cities or communities would have different codes in dealing with when your neutral would be tied to ground at the main panel comining into the house. There would be a bonding screw that Would tie the neutral to ground. Some inspectors would have us remove that screw and others would make us leave them in , (according to my memory). Is it to do with the utilities upstream or am I out to lunch on this one. My second question is along the same lines. Please correct me if I am wrong.
If I was to put a large 3 phase sub panel donwstream from a main panel , could I not just run four conductors from the main panel and then put in adequate grounding at the sub panel end with ground rods .The cost saving for not running that extra conductor is the reason. I would only need a 15 foot piece of ground rather then a 400 foot piece. (assuming the megger resistance reading at both locations is the same) I have had an electrical inspector tell me that by having a seperate ground for that sub panel that I would create a floating ground that would affect some communication equipment. He said that 5 conductors would always need to be run back to the main panel or main point of distribution . Help me out!
#1- you are desribing an MBJ ( main bonding jumper). on a typical service here it would be necessary, i don't know if Canada has separate N&G from the serving X-former, but this is the only reason i can see for NOT having the MBJ at either meter or panel...
#2- 250.32 would cover this, would this be an outbuilding? Or is the main & sub under one roof??
Re: main service neutral tied to ground#10242 06/03/0203:39 PM06/03/0203:39 PM
Greg, As far as your first question, the NEC requires a main bonding jumper at the service disconnect. In a small panel the green screw is the main bonding jumper. As far as the second question, an EGC (equipment grounding conductor) is required to be installed to the subpanel. This is the only way a ground fault can be cleared. Grounding electrode systems will rarely carry enough current to cause the OCPD to open. The NEC permits the metallic conduit to be the required EGC. Don(resqcapt19)
Re: main service neutral tied to ground#10243 06/03/0205:26 PM06/03/0205:26 PM
My understanding is that per NEC requirements there is always a neutral-ground bond on American services.
In Europe, however, this is not always the case, although I'm afraid I have no details on what is normal/expected in Bosnia.
In some systems on our side of the Atlantic, grounding is just to a local rod and a main GFI is used to enable ground faults to be cleared. Take a look at diagram #3 here for an outline.
That said, however, I would not want a separate ground rod on a sub-panel, because as your inspector has pointed out it can result in slightly different potentials on the ground system of the two sections. If you have interconnected equipment running from both sections, it could easily cause interference, audible hum, data problems, etc. depending upon the equipment in use.
Re: main service neutral tied to ground#10244 06/03/0205:59 PM06/03/0205:59 PM
The separate ground system (diagram #1) has a lot to recommend it, e.g. no neutral current flowing through the parallel path of water pipes as has been a common point for discussion here. The downside is the amount of copper used to run a 5-wire service (or 4-wire plus cable armor) around the neighborhood. I'm not sure if there are sny services of this type still used in Europe. They use the other two systems (diagrams #2 and #3) extensively).
Re: main service neutral tied to ground#10246 06/04/0212:40 PM06/04/0212:40 PM
Thanks for the info.I believe I understand the housing question I asked but i am still a little confused on the second question. Let me give more detail. On this camp we are running on 4-500kw catepillar generators. These generators are all syncronized on the same sine wave and have one common power distribution unit, from which this entire camp is fed. It is at this power distribution unit that neutral and ground are tied together . It is very hard to get five conductor cable here because it is never used on three phase systems, and running pipe is out of the question (doesnt exsist) dont ask me why. I want to put a new distribution panel about 600 feet or so from the main power distribution unit, and my local emloyees have run a four conductor cable the full 600 feet , installed the new distribution and hooked up the 3 phases and the neutral. Now in that new distribution panel they have run a 20 -30 foot piece of ground wire to a suitable location and hokeed it up to 3 -10 foot ground rods. The size of the cable and grounding is good!! They did not run the ground all the way back to the power distribution unit. In this new distribution panel they have seperated the neutral and ground. Now if there is a ground fault it does not go all the way back 200 meters to the main P.D.U. which in my eyes seems acceptable, but if there is a problem between the phase and neutral at the new distribution, the fault would have to go to earth at the P.D.U. If I tied neutral to ground at the new distribution panel is this acceptable, . Are there any safety concerns?Does this make sense to anyone? I believe for them it is a cost saving of copper. Would I be better off running a seperate ground 600 feet back to the main P.D.U.? Thanks for your help Always learning or not!
Greg Smith Camp Black Bear Bosnia
Re: main service neutral tied to ground#10247 06/05/0207:19 AM06/05/0207:19 AM
O.K., as I understand it then, you have the neutral grounded only at the generators/power distribution unit, and the ground bar on the new panel is grounded just to the local rods.
I know this wouldn't be to code by the NEC, but this is generally accepted practice in Europe, though you must use a main GFI because the loop impedance will be too high to clear a ground fault with a normal OCPD.
If the new panel is metal clad, then the GFI would have to be either at the genny end of the feeder, or fitted in a fully-insulated enclosure in front of the new panel.
Re: main service neutral tied to ground#10248 06/05/0210:30 AM06/05/0210:30 AM
Greg, I think that under the NEC you would treat the remote distribution panel as a second building or structure and rebond the neutral at that panel. If the neutral is not rebonded to the equipment grounding system at that point there will be no way to clear any ground fault. That would be a very unsafe installation. Don