Ground/Neutral

[TheOz]

Hi

I am sure this has been answered in a previous post, but I searched and couldn't find it. So I apologize in advance for going over old ground. And for sounding stupid, but I'm relatively new to the field as a journeyman.

In my current position I have seen numerous panels where the Neutral and the Ground blocks are connected together or are actually the same block and I can't seem to get a straight answer out of any one I talk to.

So what's the rationale behind having a seperate ground and neutral wire?

And why can't I connect the neutral in say a light circuit to the ground if the neutral was open circuit or something like that? My question stems from seeing an installation where that had been done. The feed to a light fixture had an open neutral, so the previous electrcian had connected the neutral from the fixture to the ground. I questioned it - but no one seemed to know why it was right or wrong.

Thanks for setting me straight.
TheOz.

 

[charlie b]

Doesn?t seem sensible on the surface, does it? It is a common question, and the answer is indeed not obvious. It might seem that if the neutral and ground are connected at one point, let us say at the main panel, then they are essentially the ?same electrical point.? So you should be able to connect them anywhere, and not change the fact that they are ?same electrical point.?

But the answer is that they are not quite exactly the ?same electrical point.? Suppose you connect neutral to ground both at the main panel and at a sub-panel (or as you suggest, at a specific load). There will be some distance of neutral wire between the two points. This wire should be carrying all, and I mean all, of the current from the load back to the source. If however you have connected neutral to ground at the sub-panel (or load), then you have given the current a parallel path to take, on its way back to the source. Some (perhaps about half) current will go back to the source along the neutral, and the rest will go back to the source along the ground wire. The real problem is that there are other parallel paths as well, because the ground wire, the ?equipment grounding conductor,? is also connected to all equipment enclosures and to all metal conduits. As a result, if you touch a conduit you could be touching something that is carrying current at that time.

By forbidding the connection of neutral to ground anywhere other than at the main panel (or at the first disconnection means of a separately derived system), you prevent current from flowing in the ground path.

Does that make sense?

Welcome to the forum.

 

[TheOz]

Hi Charlie

Your answer does make sense - and a whole lot more than the ones I've been getting from those around me.

As a follow up - to solve the problem in the light fixture, could I just connect the open neutral wire to another neutral in the attic? Even if it is not the same circuit? (That's what was done to fix the problem I mentioned in my first post of having the open neutral connected to the ground).

Thanks
theOz.

 

[crossman]

And the parallel path for current creates a shock possibility for persons. If there are grounding connections between the neutral conductor and the equipment grounding conductors or non-current carrying metal parts downstream of the service, and the neutral conductor becomes disconnected or has a faulty connection, the non-current carrying items could carry a significant current. Now, if these other paths become disconnected, a person could be shocked, say from a disconnected green wire to the other side of the green wire. Or from one piece of a conduit with a broken fitting to the other side of the conduit run.

Using the neutral for equipment grounding downstream ofthe service poses the same hazards. If the neutral is lifted from the terminal, the equipment enclosures could become energized.

 

[K8MHZ]

Hi Charlie

Your answer does make sense - and a whole lot more than the ones I've been getting from those around me.

As a follow up - to solve the problem in the light fixture, could I just connect the open neutral wire to another neutral in the attic? Even if it is not the same circuit? (That's what was done to fix the problem I mentioned in my first post of having the open neutral connected to the ground).

Thanks
theOz.

It would be functional but pose some Code and safety issues. Without going into detail, I would recommend doing it correctly.

Switching neutrals to operate equipment would also be functional, but incorrect as well. As I am trying to illustrate, functional and correct are two different issues when working with potentially dangerous systems.

 

[TheOz]

Thank you all.

I have a whole lot more understanding of the problem now and know how the problem should have bee fixed (and will suggest to the boss we fix it properly).

Thanks again
theOz.

 

[vince99]

Another issue is that the splitting of currents that Charlie referred to above will also result in undesirable elevated electromagnetic fields (EMF's) around those conductors.