PE conductor as N conductor, again

[Cheetah]

Situation:
Machines at various sites are fed with 208Y/120 three phase, with L1, L2, L3, and PE (protective earth) supplied to the machines. The neutral (N) conductor is not supplied to the machines. Inside each machine control panel is a jumper from PE to N. Some machines have L-N loads, some don't.

I advocate removing the N-PE connection on every machine and providing the N conductor to those machines with L-N loads. I have advocated this from a position of safety, stating that a "potentially lethal shock hazard will exist" should the PE connection to the machine be lost. Another engineer remains unconvinced that this is a legitimate safety issue.

I am looking for precise documentation and examples of how this is really a legitimate safety issue. Can you help?

 

[bphgravity]

This is a violation of the NEC. The equipment grounding conductor (what you are calling "protective earth"?) is only permitted to be connected to non-current carrying metal parts of the equipment served. A connection to neutral conductor is in direct violation of 250.24(A)(5).

With this jumper installed, normally operating currents will flow on all metal parts. A person in contact between this equipment and other grounded or bonded equipment will become a part of the return path for this normal operating current.

If line to ground loads exists, a grounded (neutral) conductor is required to be run with the ungrounded supply conductors. This issue should be corrected before the equipment is operated any further.

 

[charlie b]

If you have a line-to-neutral load as part of one of your three-phase machines, and if you don't bring a neutral wire to the machine from the source, and if you connect the load's neutral wire to the equipment ground (what you call PE), then current will flow back to the source from the load's neutral point via the PE conductor. That is the circumstance that the "other engineer" is saying is acceptable. If that were the entire story, it would be acceptable. But there is more to the story.

It boils down to the fact that current will take every available path back to its source, and it is not going to care if a human is part of one of those paths. The important rule is the "Current Divider Rule": Current will flow in all available paths in inverse proportion to the resistance of each path.

There is one path from the load to the source, via the PE, as described above. However, the point on the PE to which the load's neutral is connected is also connected to the case (any external metal part) of the machine. The machine's external metal parts are connected to all other external metal parts of every other machine in the building, because of the pipes or conduits or metal bolts that fasten each item to the floor. Therefore, as Bryan has said, every one of those metal items will have current flowing.

That will create a potential difference between any two metal items. The amount of the potential difference (or voltage drop) will be equal to the resistance of the path times the current in the path. The closer two items are to each other, the lower will be the potential difference between them. But it will not be zero. Never.

Therefore, as a human touches any two metal times (like the case of one machine and the metal floor beneath their feet), some amount of current will flow through that person's heart. It might not be much. It might even be so low as not to be noticeable. But it will happen. Every time. No exceptions.

Would you be happy about that?

 

[realolman]

I like your explanation Mr Beck.

Why would anyone think it was OK to not run a conductor, but use the conductive parts of everything to carry the load?

If the load is that small, put in a little transformer.

I can see someone doing it out of ignorance, but I can't see defending it once it's pointed out.

Must be a sanitation engineer.

 

[bphgravity]

Why would anyone think it was OK to not run a conductor, but use the conductive parts of everything to carry the load?

The same reason engineers don't specify EGC's to be installed to parking lot and street lighting and at the same time specify 10' ground rods connected to the base. Many engineers don't know code, many never received a single lesson on code while in school, and many try to apply the rules that utilities use for there systems on premise wiring.

 

[Cheetah]

Why would anyone think it was OK to not run a conductor

The N conductor was not run because "it was not needed". The correct answer is "it was not needed then". Future machines will have the N conductor regardless.

 

[Cheetah]

Gentlemen, I respect and appreciate your statements. They provide me with confirmation that the safety issue is legitimate, and we will get it fixed. Thank you.

 

[ardy]

Just for my curiosity,what is the "N" used for? If the mach. uses it for control
pwr. then they would have a transformer (control) on the mach. and the conductor would not need to be pulled.

 

[bphgravity]

Just for my curiosity,what is the "N" used for? If the mach. uses it for control
pwr. then they would have a transformer (control) on the mach. and the conductor would not need to be pulled.

You still can't make a grounding connection to it.

 

[Smart $]

Just for my curiosity,what is the "N" used for? If the mach. uses it for control
pwr. then they would have a transformer (control) on the mach. and the conductor would not need to be pulled.

Some manufacturers of machinery use a standard terminal block for several variations on a machine design, where one design may have one or more L-N loads and another has none. In the latter case, they simply don't use the N terminal for internal wiring.

I don't know if that is the case here...

 

[Cheetah]

what is the "N" used for?

On some machines the N conductor is not used at all. On others it is used for typical L-N loads (120 Vac).

This whole situation exists because the N conductor was not included in a 208Y/120 power feed to a machine. This to me is analogous to buying a new car and not including the spare tire because "I don't need it." Eventually you might.

 

[Cheetah]

The machines in discussion use a standard terminal block for a 5 wire 208Y/120 feed. On the "line" side there is no N conductor. On the "load" side there is an N conductor that runs throughout the machine, and it is bonded to the grounding conductor.