Why use separate buses for grounds and commons in subpanel boxes?

[Stephen Tashi]

TL;DR Summary

Why are ground wires and common wires put on separate isolated buses in subpanel boxes, but not in the main panel box?

My understanding of (USA) household electrical wiring is that in a "main panel" breaker box, the ground wires and the common wires may go to the same bus. In a "subpanel" box, the wires and commons must go to separate, electrically isolated buses. Why is this convention used?

If there is a mechanical switch that disconnects electricity to the house at the electric meter, the panel boxes that I have seen are wired as "subpanel" boxes even if they are the only breaker box.

 

[Asymptotic]

If neutral and ground are connected to one another in a subpanel, a portion of return current to the main panel flows through the neutral wire and the remainder flows through the ground path.

This is unsafe under the best of circumstances, but if the subpanel ground-to-neutral bond develops high resistance through corrosion, loosening connections, etc. and neutral connection fails completely, all return current to the main panel flows through the ground wire. Voltage on the subpanel ground wire in reference to ground at the service entrance (hence, to anything connected to the grounding pins fed from that subpanel - appliance frames, for example) will rise to lethal levels.

If there is a mechanical switch that disconnects electricity to the house at the electric meter, the panel boxes that I have seen are wired as "subpanel" boxes even if they are the only breaker box.

Sometimes ground and neutral are bonded together in the meter base rather than at the main panel box.

 

[dlgoff]

Summary: Why are ground wires and common wires put on separate isolated buses in subpanel boxes, but not in the main panel box?

In a "subpanel" box, the wires and commons must go to separate, electrically isolated buses. Why is this convention used?

This is unsafe under the best of circumstances, ...

Sometimes a picture is worth a 10³ words.

image compliments of https://www.ecmweb.com/

 

[essenmein]

To add to the safety issue mentioned above, tying ground and neutral together at two locations creates a current loop, and in the presence of other high current loads (more specifically the alternative magnetic fields from that current) that loop can flow current, at best this can make audio equipment noisy, at worst, significant ground voltages can be induced.

 

[Stephen Tashi]

Sometimes a picture is worth a 10³ words.

I find the picture confusing. What does the "main bonding jumper" connect?

 

[Averagesupernova]

I find the picture confusing. What does the "main bonding jumper" connect?

I agree. I suspect it means that the neutral bus is tied to the chassis with a jumper of some sort supplied by the manufacturer that is capable of carrying fault current until the main breaker trips.

 

[Svein]

Think of the "neutral" line as just one of the two phases necessary to get the current to flow. The "ground" is not part of the AC circuit, it is just a safety measure (therefore it is usually called "protective ground"). Different countries have different requirements for connecting the two. In my country, we have strict requirements for the protective ground, but connecting one of the phases to protective ground counts as a "ground fault" and will trip the ground fault protection part in the corresponding fuse.

 

[dlgoff]

I find the picture confusing. What does the "main bonding jumper" connect?

What's confusing? In the image it clearly says, "Main Bonding Jumper, Not Installed". Isn't this thread about "grounds and commons (neutral)"? The ground jumper is shown not connected to the neutral. Hence it should be connected like this: