0

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.

1
2

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.

6
  • All this makes sense. Until that unexpected power, flowing on the ground, makes it back to the main panel where it's connected to the neutral. What prevents it from flowing out the neutral there to every other outlet (and, therefore, appliance) in the house? How do we know that this unexpected power will flow from the N/G bond to the house's grounding system instead of just back throughout the house? (that's the part that's always confused me)
    – FreeMan
    May 4 at 12:51
  • (Not questioning your answer, just my understanding)
    – FreeMan
    May 4 at 13:02
  • @FreeMan how would the current get to the ground? Neutral and ground are not connected at the subpanel. May 4 at 16:13
  • They're connected at the main panel. Could the power not pass from one to the other there, then be redistributed back throughout the system? (Note: I am not an electrician, electrical engineer, or otherwise edumacated on the topic beyond what I've read online)
    – FreeMan
    May 4 at 16:19
  • @Freeman I'm not sure how any current would get on ground in the first place. With a correctly wired 4-wire system, you get lost neutral effects on the circuits, but ground is unaffected and sees no current. That being the idea. May 4 at 16:21
1

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).

2
  • 1
    Thanks for the quick response. i don't fully understand the neutral shocking you portion. In the past i've touched a neutral wire carry current and ground and haven't experienced any sort of shock, is it because since the neutral is bonded to the ground at the main panel that the current would rather go through the ground (due to its low impedance) rather than through the body?
    – user0013
    May 4 at 2:50
  • Yes, if you have current on neutral it will take all possible paths to get back to the neutral in the panel. If you are high resistance and the wires are low resistance (which is generally the case unless there are broken wires or you are wet) then almost all the current will flow on the wire and everything ends up OK. But you can't count on that, which is why there are multiple layers of protection (neutral/ground design, GFCI, etc.) May 4 at 3:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.