Code related Theory question

[crossman]

Re: Code related Theory question

I agree. As long as the grounded conductors are low impedance, the parallel paths are really not a safety issus on the matters of arcing or someone getting shocked by breaking the parallel path.

But with parallel paths, if a problem arises in the grounded conductor, conditions that could be fatal to the public are created.

Does everyone see it that way or am I missing something?

 

[hurk27]

Re: Code related Theory question

But you have to remember that not all installations will have a low impedance path.
and the NEC has to cover a wide range of installations.

If you had a long run to an out building that presented a high voltage drop on the neutral there could be a shock hazard if you got in between anything bonded to this neutral and anything bonded to the service neutral at the house. This is just one example where there is a metal path between the two buildings. When this is done in the same structure there will always be multiple paths But low impedance can not be guaranteed. There for it's not allowed.

 

[crossman]

Re: Code related Theory question

What comes to mind is the maximum voltage drop allowances on feeders = 3% maximum. The impedance of the neutral should be within these tolerances, which keeps a safe voltage level on other paths.

Again, I agree that the parallel paths are undesirable. I just think the reason is because of what can happen when problems with the neutral arise, not so much what is going on under normal circumstances. I cannot think of any "normal" situations where the neutral impedance would be high enough to cause significant hazards with the parallel paths.

The best thing about this forum is that it has gotten me to thinking about a lot of things and re-examining my knowledge base. And this is a very good thing.

 

[crossman]

Re: Code related Theory question

Related note: High impedance neutral a.k.a. "wait, where is the neutral??"

Back when I was working with my tools, my tool buddy and I were sent to do a job at a surplus food warehouse (Don't remember what we were even doing). Anyway, this warehouse at one time had been a machine shop. They had previously removed all the machine tools and had added some walk-in freezers (years before).

The service was a 120/240 high leg delta. The structure was wood. We were up in the attic doing whatever we doing. Someone had previously run 1/2 EMT hanging from the roof structure (wood) with ceiling wires. This run supplied the lights in the area. I inadvertantly bumped the pipe, causing it to wiggle, and sparks started flying from the couplings and j-boxes in the run!! "Holy cow what is going on??"

Anyway, we got ourselves out of the attic so we could think about this before we ended up hurt. We went over to the sub-panel where the branch circuit originated. There was a gutter with three 3-phase 60 amp disconnects feeding the freezers and a small single-phase panel. This stuff was very old, very corroded, and loose fittings everywhere. We investigated and found the problem. The panel did not have a neutral, just two hots... the feed to this area was 3 phase 3 wire 240v. And they were using all the conduits as the neutrals!! And this place was literally on the verge of burning down. When we were taking the gutter cover off, any bumping or jarring of the enclosures would send sparks coming off the loose nipples and locknuts to the disconnect switches.

After a call to my boss to get out there pronto to discuss the situation with the owner, the owner wisely agreed to redo the entire mess. We shut the junk down and worked a bunch of overtime to put in a new feeder, guttter, panels, etc.

Then on to the repair of the lighting circuit that had been arcing and sparking up in the attic. I removed a cover from a 4s box and found half of a cast locknut in a solidified pool of metal . I am serious. The top of the locknut had melted off of the threads of the connector and was laying in the bottom of the box. Come to find out, this conduit had previously fed a single phase 240v vent fan and someone had added 120v lights to this and used the pipe as the return path!!

How this place hadn't burned down is a mystery to me.

So that is an example of a parallel path around an extremely high impedance neutral...

 

[rattus]

Re: Code related Theory question

Crossman, obviously the place you describe did not have a rigorous maintenance program! Perhaps it is not feasible, but it would be good if all structures had a periodic inspection. After all we inspect our vehicles once per year. I have seen similar travesties, zip cord wired into ceiling boxes, outside lights waving in the breeze, EGC used for neutral, and I am not even a practicing electrician. The best one though was the house wired with alarm wire!