Neutral and ground seperation at a sub-panel

Question

Could someone explain to me why the neutral and ground bars are never bonded on a sub panel? It's my understanding that the reason they're separated is so that the current returning on the neutral doesn't cross over to the ground and return to the main panel. But if ground and neutral under normal circumstances don't shock you then why is it necessary to separate them in the sub-panel in the first place? Is it because there is the possibility of the current crossing over to the ground that a breaker might not register a spike in current and fail to trip?

Thanks for taking time to answer my questions.

Answer 1

We don't build the system for normal circumstances. If we did, we wouldn't need circuit breakers, would we? :)

Ever hear of a "Lost Neutral" situation? This is where neutral becomes loose or broken, and is no longer pegged in the middle at 0V between the two opposite 120V phases (or at 0V in between the three 230V phases in Europe). Neutral is subject to a "tug-of-war" amongst the various phases - the more load on a phase, the harder it pulls. This causes the weaker phase's voltage to go higher than spec, which often burns out equipment.

Suffice it to say, neutral is no longer at 0V, and could be as high as 120V or 230V if only a single appliance is turned on (or still functioning).

Now, what would happen if the ground was attached to that lost neutral, i.e. the chassis of all equipment? Well that would be at a high voltage along with the neutral, and people would be getting nailed off switch plate cover screws.

That's... why you maintain ground separately at all points beyond the main disconnect.

Answer 2

I'll admit to not 100% understanding it, but I can speak to two points:

if ground and neutral under normal circumstances don't shock you

Ground normally has "nothing" traveling through it. Proof of that is that the ground wires are often bare - no need to protect yourself from them.

But the neutral wire normally has as much current flowing through it as the matching hot wire!

Why can you touch a neutral and not be shocked? Because when you touch a neutral, but not a hot, you don't complete a circuit. The truth is that if you touch a hot but don't touch a neutral or ground (or any grounded item like the physical earth (while wearing conductive shoes or standing in water, etc.) then you don't get shocked either.

The end result is that if you touch a neutral that is currently carrying current, then you will get shocked. If your grounds are commingled with neutrals then there is a possibility that your ground - not insulated, connected to metal appliance cases, metal boxes, etc. - could shock you too.

Keeping neutral and ground separate, except in one key location, prevents current from traveling on ground except when there is a fault (in which case you want the current to flow to ground rather than through you, and if there is a GFCI it will be tripped and if there is no GFCI but there is a lot of current (hot wire shorted to case of an appliance) then the regular breaker will trip.

The other use for ground is natural electricity - static, lightning, etc. In those cases it is often going through the metal case, etc. to ground, and there is no reason for it go on neutral.

current crossing over to the ground that a breaker might not register a spike in current and fail to trip

Actually, as I understand it, that is not a problem. In fact, quite the opposite. In typical US 120V/240V installations, the neutral does not have a circuit breaker. That is why it is critical that MWBCs are wired correctly - to prevent an overloaded neutral. But the flip side is that if some neutral current were to go over ground instead, the hot would not be affected. In fact, if you had 20A over neutral and 20A over ground (a 50/50 split) then you have 40A over the hot wire and get a breaker trip (assuming it was a 20A circuit).