Question about bonding ground and neutral

[mollymae]

TL;DR

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 resistance 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.

 

[Dennis]

On a transformer where you bond the X2 terminal to ground with a green wire, the second wire connected to the X2 terminal becomes a neutral conductor because the now grounded frame and the wire you connected to X2 are at the same potential. So that effectively makes that conductor neutral that will not do anything if it is grounded to the frame. The other wire that will be connected through a protective device like a fuse before going to the X1 terminal becomes your hot wire. If the hot wire touches the metal frame it has a return path to the bonded X2 terminal that you tied to the frame of the enclosure and so it will blow the fuse.

The ground will not have current flowing into it as long as the hot wire is not touching the frame

If you don't tie one of the output legs of the transformer to the frame, then you have a floating transformer, and so if any single conductor touches the metal frame, then nothing happens since there is no return path back to the transformer.

 

[sophiecentaur]

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

Basically, why is the ground path not hot all of the time?

It's because the load at the other end is connected between the L and N and is not connected to the Earth / frame / ground. There is (should be) no path for the load current via the far end of the earth conductor . By the time you get to the far end, the resistance of the neutral conductor will produce a finite voltage relative to the earth due to the return current flow, despite them both being connected at the transformer.

If you connect the neutral to the earth downstream of the consumer unit at the load end the RCD will probably trip because there is now a path to carry the unbalanced current away from the Residual Current Detector. Many years ago I was taking some old unused cabling out of a house. I removed the fuses and started chopping the unwanted('dead') cables and the trip tripped. I then disconnected the L and N conductors from the bus bars in the fuse box and there was no more trouble. An induced current in the old N wire was detected by the RCD.