Neutral Current Where It Doesn?t Belong

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tryinghard

tryinghard

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Returning current belongs on conductors, what are some of the hazards/problems when it exists on non-current-carrying items (parallel) to the intended grounded [neutral] conductor?
  • Is there dangerous touch voltage potential?
  • ??
 
tryinghard said:
Returning current belongs on conductors, what are some of the hazards/problems when it exists on non-current-carrying items (parallel) to the intended grounded [neutral] conductor?
  • Is there dangerous touch voltage potential?
  • ??
Click to expand...

VD on the neutral conductor can cause stray voltage when grounded at the building. Always best to balance loads to reduce the amount of current flowing on the neutral.
Anytime you have voltages present between grounded and ungrounded metallic objects/systems like electrical equipment and piping (3-wire washer-dryer with grounded neutral comes to mind) there is an increased likelihood of tingle voltage being felt.
Neutral-to-earth voltages create huge problems, especially around swimming pools and animal barns. Also ground-faults occuring in various types of pumps and in underground conductors can cause stray voltage.
"Objectionable current" does not have a definition in the NEC, yet. But 250.6(B) outlines ways to deal with it. :)
 
mdshunk said:
The resultant EMF will make you grow a third arm and make it hard for you to communicate with the mother ship.
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Yeah, but before that happens, the issue is that current causes voltage drop, which translates to other than 0 volts to earth/ground/etc. on what should be firmly grounded accessible surfaces.
 
If normally operating current is permitted to flow on normally non-current carrying metal parts, anything or anyone that comes in contact with the the metal part and another grounded object is subject to current flow.

Keep in mind the current will flow on any all all paths back to the source. That path back to the source is suppose to be isolated and insulated from unintentional contact.

Creating a potential between metal parts of the system and/or condcutive parts and the earth is a serios hazard and needs to be avoided.
 
080604-2107 EST

I suggest that you should treat the neutral wires within a building the same as an ungrounded hot wire.

The theory behind the EGC (equipment grounding conductor) is that it is connected to earth ground at the service entrance main panel and to all other exposed metal conducting surfaces and under non-fault conditions has negligible current flowing in the EGCs. Thus, every exposed conductive surface you could contact would be at essential the same potential. Theoretically this includes the earth as a conducting surface. Therefore no shock hazzard.

There are problems with the earth, but if there were no outside current sources, then it would also be at the same potential.

Since a hot wire could short to one of these conductive surfaces and one wants to trip a protective device it means we must bond the neutral to the EGC at the service entrance. Also this is the only allowed bonding point in the building or else you would have neutral current flowing in the EGC under non-fault conditions. Note: In most locations dropping the hot wire on the earth will not trip the protective device.

In general the earth is not a very good conductor and that is why you can not use a ground rod at a machine with no connection to the EGC. If the independent ground rod in the earth at the machine provided a 10 ohm path back to the ground rod at the service entrance and I connect 120 V to this ground rod, then the current is 12 A and I will never trip a 20 A breaker.

Consider a neutral of 0000 copper wire 300 ft long with 200 A flowing thru it. The resistance is 0.049*0.3 = 0.015 ohms and the voltage drop over this length is 3 V. Probably not a problem for a human. Now suppose you have a bad connection in the neutral. The voltage difference can be much greater maybe up to the source voltage, at least for small currents that could be lethal.

For a different comparison consider a #12 wire 100 ft long and 20 A. This produces a voltage drop of (1.6 * 0.1 = 0.16 ohms) 0.16 * 20 = 3.2 V. Now connect a DeWalt radial arm saw to this wire and at startup the inrush current is about 80 A and the voltage drop would be about 13 V on each of the neutral and the hot line.

Any device that uses the neutral as the EGC is a very bad design in my opinion.

As a side point. Consider a CNC machine using a direct RS232 connection to a remote computer. This has real problems when a short occurs from a hot wire to a chassis. Assuming the EGC is of the same diameter as the hot wire and we short these together, then the peak voltage difference between the computer and the CNC will approach +/- Vrms of the source for the hot wire * 1.414 / 2. This is because we have a voltage divider of 1/2 of the source voltage. This magnitude of voltage from a 120 V or higher source will destroy RS232 components, possibly other components, and maybe the RS232 connecting cable. There is no fuse or circuit breaker that could open fast enough to protect these components.

That is one reason we make an optically isolated system for RS232 communication. This is good for short time peaks of +/- 2000 V. I have put a 1000 V RMS (+/-1414 V peak) 60 Hz sine wave between common (chassis, ground) and a lead in our interconnect cable. Before, during, and after this connection there was no damage, and data was transmitted error free.

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There is an excellent article in ECM magazine on this whole issue. EGC's carrying anthing besides short term fault current have the potential to be dangerous.Most of the time it probably is of such low magnitude that it realy isn't much of an issue, and perhaps the most harm that would come would be a temporary inability to communicate with the mother ship, however being extremely anal about electricity in general it bothers me if all the electrons are not marching in thier proper places.:)
 
iaov said:
There is an excellent article in ECM magazine on this whole issue. EGC's carrying anthing besides short term fault current have the potential to be dangerous.Most of the time it probably is of such low magnitude that it realy isn't much of an issue, and perhaps the most harm that would come would be a temporary inability to communicate with the mother ship, however being extremely anal about electricity in general it bothers me if all the electrons are not marching in thier proper places.:)
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Scenario:

Last January I worked in Indiana. While there, someone swiped some bare copper grounding wire from an apartment (I think). Electrician (IBEW LU 697) gets the job of making the repair. In the rain, he makes a complete path with the two ends of the cut and is killed on the spot. Only 56 volts across the two ends.
 
K8MHZ said:
Scenario:

Last January I worked in Indiana. While there, someone swiped some bare copper grounding wire from an apartment (I think). Electrician (IBEW LU 697) gets the job of making the repair. In the rain, he makes a complete path with the two ends of the cut and is killed on the spot. Only 56 volts across the two ends.
Click to expand...

I hope I'm not just dumb, but I really don't understand the work description. What was replaced, the equipment grounding conductor, the grounding electrode conductor? Between what points?
 
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