Question about bonding ground and neutral

[mollymae]

TL;DR Summary

Why does current not flow through the ground path if it is bonded to the neutral?

I am not an electrical engineering student, but a lowly apprentice electrician. I learn both on the job and also take classes for my apprenticeship.

I recently wired my first transformer and I understand that the neutral and ground are bonded together in the transformer or in the service. What I don't understand is, if the neutral is a current carrying conductor, which is then bonded to the ground conductor, why does current only flow back to its source and not on the ground path? Basically, why is the ground path not hot all of the time? I asked my journeyman this and he said it follows the path of least resistance. Okay, sure, but in class they always tell us that the "least resistance" idea isn't true, and that current in fact will take all paths.

Thanks in advance and I may be back to ask more elementary questions later. I am inquisitive at work as I want to understand as much as possible but honestly sometimes I think the electricians I work with don't always know the answers. They're not dummies (they know much more than I do) but they aren't engineers either.

 

[hutchphd]

The neutral current path offers a very low resistance path for the current, and so most of the power follows this path. The local neutral ground is important to ensure that no large potential differences can accrue over time (i.e. the V=0 point remains good). The electrical resistence of dirt (particularly if dry) is not zero.

 

[Baluncore]

Fuses protect conductors from fault overcurrents, that might melt the insulation or the wires.

The ground path is bonded to neutral, to protect insulation from overvoltage damage, that could arise from an electrical discharge such as lightning or a switching transient.

Your national electricity code will specify the convention and requirements.

 

[DaveE]

Don't assume that EEs know much about this subject. It's a broad field and most don't ever deal with power distribution. Even if they know some of the theory, it's the actual practice that matters. Grounding can be a complex subject and different jurisdictions may have different approaches. The important thing here is to know and follow the electrical codes applicable for your situation. Mixing different schemes can be troublesome or dangerous. This is an area where the codes really do matter.

That code book is really thick for a reason, much of those requirements were learned in the school of hard knocks, not at universities. You may not always know why, but invariably someone does, and they thought it was important. If for no other reason, it's nice when you see something built "the normal way" and a PITA if someone was creative and thought they knew better. We often don't know what we don't know.

 

[Borek]

TL;DR Summary: Why does current not flow through the ground path if it is bonded to the neutral?

I suppose the shortest answer is: it does.

Just in a properly made circuit and in a typical situation the current is very low.

 

[Baluncore]

What I don't understand is, if the neutral is a current carrying conductor, which is then bonded to the ground conductor, why does current only flow back to its source and not on the ground path?

There is a neutral wire only, no earth wire is provided on the transmission line. The neutral to earth bond is usually on each power pole, and once in the customer's meter box.

When you consider the wires at the top of a power pole, the average voltage should be zero, so the electric fields cancel and there is no voltage capacitively coupled onto nearby objects.

The total current in all the wires should also be zero, so the total magnetic field will also be zero, and energy will not be radiated. That makes a transmission line, where the current flowing in any one wire, will be equal and opposite to the sum of the directional currents in all the other wires.

There is no place in that system for an unbalanced earth current, except during a momentary fault condition.