Can somebody explain objectionable current?

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Benihana

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Electrical Engineer
I'm struggling to find an explanation that actually explains it.

I also understand that it can happen when neutral-ground is bonded in sub-panels, but I do not understand why that creates it and not at the service entrance where neutral and ground are bonded.
 
From the utility transformer to the main service, there is no EGC. The neutral is the zero-volts point of the system. At the service where the electrodes and grounding system connect to the neutral, that point establishes the zero-volts point for the premises.

Beyond that point, except for OCP, we treat the neutral as an ungrounded conductor. The grounding system is designed so an accidental contact between an energized conductor and a grounded surface trips the OCPD like a line-to-neutral contact would.

A service is like a sub-panel without a separate EGC. As I said, the service neutral defines the zero-volts point for the premises, even with voltage drop along it. But, beyond the service, voltage drop could cause voltage on surfaces we expect to have none.
 
Thanks. So what exactly is objectionable current though?

Does it occur at sub-panels with a neutral-ground connection because the neutral is current carrying conductor, and if bonded to the ground at those points, it would cause current to flow on the ground wire even when there isn't a ground fault?
 
Yes, and the concern is that such voltage drop can cause surfaces that we expect to be at zero volts to become energized and create a shock hazard.

The reason we used to be able to run 3-wire feeders to detached buildings was because zero volts was re-established at the building with electrodes.

Note that the existence of another metallic pathway like water piping removed that allowance because it was a more effective zero-volts reference.
 
My answer is we don't know what objectionable current is because the NEC does not define it. The NEC is structured to prevent current flowing on things downstream of the service that are not supposed to normally carry current, like conduits. One might assume that current flowing on a raceway is "objectionable", but again the NEC does not define a specific current level. Current flowing on service raceways is generally considered unavoidable, and the "alterations" in 250.6(B) are never enforced, so I guess it is not considered objectionable. IMO 250.6 should just be deleted.
 
LarryFine said:
A service is like a sub-panel without a separate EGC. As I said, the service neutral defines the zero-volts point for the premises, even with voltage drop along it. But, beyond the service, voltage drop could cause voltage on surfaces we expect to have none.
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Can you elaborate on this?

How could voltage drop cause voltage on surfaces we don't expect?

I can't picture how this scenario would arise.

LarryFine said:
The reason we used to be able to run 3-wire feeders to detached buildings was because zero volts was re-established at the building with electrodes.

Note that the existence of another metallic pathway like water piping removed that allowance because it was a more effective zero-volts reference.
Click to expand...

Is the metal water pipe a more effective zero volt reference because it's in contact with the earth?

Better than a neutral that is connected to a GES at the second building?

If no GEC is pulled with a feeder to the second building are you saying there can be a voltage potential between the neutral and the metal pipe?

Rob G - Seattle
 
marmathsen said:
How could voltage drop cause voltage on surfaces we don't expect?

I can't picture how this scenario would arise.
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Imagine a sub-panel fed by a 3-wire feeder, so the grounds and neutrals are tied together.

If there is voltage drop (rise?) on the neutral, that voltage is also present on equipment grounds.

Everything you expect to be electrically grounded can become a shock hazard.

marmathsen said:
Is the metal water pipe a more effective zero volt reference because it's in contact with the earth?
Click to expand...
No, because it is (presumed to be) bonded to the service neutral.

marmathsen said:
Better than a neutral that is connected to a GES at the second building?
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No, better than the neutral that is part of the feeder.

That GES does nothing to reduce neutral voltage drop.

marmathsen said:
If no GEC is pulled with a feeder to the second building are you saying there can be a voltage potential between the neutral and the metal pipe?
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Yes, and there's no good solution.

If you bond the neutral to the pipe, it will be forced to carry >>>objectionable current<<<.

If you don't bond it, there can be voltage between it and the building's EGC/GEC system.
 
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One example of objectionable current is current on an EGC when there are two bonds between neutral and the EGC. The EGC between the two bonded points will carry some of the neutral current.
 
mtnelect said:
Stray current is generated by MGN system the utilities uses and is pumped into the ground by them.
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Is an observation worth discussion, but is off topic for this thread. OP is not going to get his utility to abandon their MGN distribution system, and if it is part of his problems the conversation should shift to how to deal with it because the MGN itself is not going away.
 
Related to this topic, perhaps of interest:

NEC 2023 - 404.22 contains a very specific exception (for e.g. smart / humidity sensing / etc switches in switch loops) that allows (very small) currents on the EGC during "normal" (non-fault) operation. So, not all "normal" currents are objectionable. The standard UL 873 (and others) specifies the maximum for this current. Probably way below 5mA. Verified this with a US switch manufacturer and the local AHJ.
 
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