Objectionable Current?

[bennie]

This topic is associated with the 3 wire feeder to panels, and parallel paths for neutral load current.

Electro-magnetic fields are not what objectionable current is addressing. Noise in sensitive electronic equipment is also not a consideration.

What is left to be considered objectionable?

 

[pierre]

Re: Objectionable Current?

Neutral load current flowing on the EGC, back to the source.

Pierre

[ October 05, 2003, 11:05 PM: Message edited by: pierre ]

 

[bennie]

Re: Objectionable Current?

Why is it objectionable? does anything get damaged, ignited, or electrocuted?

 

[dereckbc]

Re: Objectionable Current?

Originally posted by bennie:
electrocuted?

Yes. Current flowing on the raceways can build up a high enough voltage drop to electrocute.

 

[pierre]

Re: Objectionable Current?

Bennie

Current can flow on the equipment grounding conductor, which might be a raceway, for an extended period of time. Two scenarios can occur. Touch potential leading to shock or electrocution, and if the conditions are right, loose connections in the ground fault current path will eventually lead to overheating/sparking and possibly lead to a fire.

Pierre

 

[brian john]

Re: Objectionable Current?

And should the neutral open the ground connection that set up the parallel path will carry the neutral current. If the ground connection is not up to the task (6/32 screw pinching a no 12 AWG.) possible fire.................

 

[karl riley]

Re: Objectionable Current?

Although the NEC does not address EMI or possible health effects, these are a part of the real world electrical scene, so we ignore them at some risk, professionally. Here is an example from my experience:

I was called in by the Chief Electrician of the PUC of a northeastern state to deal with a problem in the 6-story PUC building in the State capital. The ground floor computers were jittering in the offices of the State Environmental health dept. Not only was this unacceptable visually, but some of the personnel had read enough about the health research to be uneasy about sitting in 10 mG magnetic fields every day.

My investigation showed that they were being affected by the net current magnetic field from two bus ducts which ran under their floor, supplying 1/4 of the building. I checked a couple of the subpanels on upper floors supplied by these busses. Net current on the feeds (missing neutral). Measured current on various metallic pipes, etc. Checked some of the circuits with a clamp-on ammeter. Net current due to some missing neutral current.

I found that these circuits fed small subpanels located in offices around the floor. In each subpanel that I opened up, neutral and equipment grounding conductors shared the same bus. The result added up to a large net current on the buss ducts as well as on various pipes and conduits.

They had to hire an electrician to spend many days going around to each subpanel and installing a ground bus and separating neutrals from grounds.

Though the original complaint was not NEC related, the cause was NEC violations.

Just as Bennie doesn't see anything wrong with this situation, the electrician who ran his grounds to the neutral bus also didn't see anything wrong with it. Many, many electricians don't see anything wrong with connecting neutrals and EGCs anywhere. That's why we have Forums like this now. They were not available when this building was wired.

Karl

 

[Guest]

Re: Objectionable Current?

Originally posted by dereckbc:

Originally posted by bennie:
electrocuted?

Yes. Current flowing on the raceways can build up a high enough voltage drop to electrocute.

Are you speaking to electrical "potential" between a raceway and the earth?

 

[bennie]

Re: Objectionable Current?

Karl: I am trying to improve my literary skills.

I did not intend to indicate approval of interconnecting the ground/neutral with the equipment ground on the load side of the service switch.

I was attempting to illustrate the NEC does approve this procedure, when small amounts of current are involved. The only interest in the NEC is to prevent heating effects from objectionable current. The NEC does not deal with EMI. The focus is only on fire prevention.

Objectionable current is when heat or arcing is generated that can ignite other material.

The only method to prevent EMI on a premises wiring system is to have an ungrounded separately derived DC system.

All opinions are my own, and do not reflect on other persons opinions.

 

[bennie]

Re: Objectionable Current?

My feeble attempt to point out that the NEC does not use the term "sub-panel" is due to the fact it is technically incorrect. A sub-panel does not exist, except in some individuals imagination.

All panels are service panels. The service panel distributes power from the utility to the premises wiring system.

See definition of "premises wiring system". Notice the absence of panels, switches, load centers, fuse blocks, overcurrent devices, feeders, or service equipment on the laundry list.

The distribution equipment is not part of the premises wiring system by definition. The service equipment is where the utility "supply" changes to premises "load" wiring. Panel location is not specified for services, only the service disconnect location is specified.

After a service disconnect, is the panel still a service? I hope so.

Again my standard disclosure and disclaimer apply. This is my opinion based on existing installations, that changed, not from code changes but from official interpretations.

 

[dereckbc]

Re: Objectionable Current?

Originally posted by awwt:
Are you speaking to electrical "potential" between a raceway and the earth?

Yes

 

[karl riley]

Re: Objectionable Current?

Bennie, I realize you do not recognize the existence of subpanels, and do not think that a subpanel is on the load side of the service disconnect. You say that the NEC agrees with you. Would you care to ask Mike Holt or Joe Tedesco or any person whom you think can speak for the NEC if they agree with your interpretation? Or have you decided that no-one will agree with you because they just can't read the NEC?

As to safety, please re-read a couple of the posts above. "Arcing in concealed spaces" is the main safety concern usually stated in relation to neutral current coursing around the building on metallic paths, many of which were never designed to carry electric current. Electrocution is possible if the neutral opens.

Imagine being told by an inspector that you can send current all over the building on anything which will carry it. "Dont't worry about bonds - a nail touching some sheet lathe will do. Don't worry about connections, just as long as they touch, the current will flow. Have fun and get back to me when you want me to sign off on the job".

Bennie, I know you will not change your opinions. But thanks for the opportunity you give us for clarifying these questions.

Karl

 

[al hildenbrand]

Re: Objectionable Current?

This thread puts me in mind of the thread last year that was called Touch Voltage where we considered the voltage divider created when a line shorted to the EGC. It was easy to see half of the line to ground voltage would be on the EGC close to the location of the short.

A person, who is grounded and who is touching the EGC provides yet another current path during the fault. This is the shock hazard.

When the neutral acts as the EGC, in the case of a "subpanel" fed as Bennie suggests, the touch voltage on the panelboard enclosure will also be half of the line to ground voltage if there is a line to neutral or a line to ground fault at that location.

 

[bennie]

Re: Objectionable Current?

Karl: There is no section in the NEC that stipulates the only method for preventing objectionable current is to separate the ground/neutral from the equipment ground. This procedure has been created by authors of the manuals explaining the intent of the NEC.

There is other means for preventing objectionable current, 250.6 lists the procedures.

250.30(1) Exception no.1 addresses objectionable current on premises wiring systems, when separately derived, and the bonding is at both the system and the source.

 

[bennie]

Re: Objectionable Current?

Al: Are you saying a neutral to equipment ground conductor will have a maximum of 60 volts to ground, on a 120 volt circuit, at the point of the fault?

I agree with this, each half of the circuit will drop 60 volts.

Along with surface resistance, the touch potential should not be a problem. All light poles, sign and traffic signal poles should have an asphalt surface. Asphalt has a high resistance.

 

[al hildenbrand]

Re: Objectionable Current?

Bennie,

the touch potential should not be a problem

I believe an individual's problem is inversely proportional to how well the individual is grounded. The lower the resistance of the individual to ground, the greater the problem. Our examples in the bygone thread were coming up with through body currents in excess of 35 milliamps for the time that it took for the overcurrent protection to clear the fault.

 

[short circuit faulted]

Re: Objectionable Current?

Bennie,
My consideration involving "objectionable currents" down stream from "main" service panel to "other" service panels would be ground fault protection....IE Main lug "other" service panels fed from 277/480Y "main" panel via GFI breakers. with EGC & Grounded Conductors tied together at "other" service panels...return current on grounded conductor drops below threshold by bleeding over to EGC causing nusansance (sp?) tripping...

Steve