Replies

1. 
   john_c2 | Sep 22, 2019 11:52pm | #1

   The white and bare wires are only bonded together in the service panel -- and even there, the ground wires should be on their own ground bar.

   In a subpanel, the grounds and neutrals must be kept separate.

   The bare ground wire is bonded to a lot of things, such as outlet and junction boxes, copper water pipe -- all siirts of things. Please, please, dont try to use it as a current carrying conductor.

   You are sort of describing what is called a multiwire branch circuit, which uses a black, red, white and ground to run two circuits. There's more, but there must be three conductors plus a ground.

   Please don't try to do your own wiring. Seriously. It sounds like you know just enough to be really dangerous.

2. 
   Highdesertwoodworker | Sep 23, 2019 11:40am | #2

   I am curious about the physics here.

   First, I bought my house new, so the electrician attached all grounds and neutral of all circuits onto a single metal bar.

   Second, many older houses in the US still have only two prongs and have no ground connected, or the neutral is also the ground. My brother's house is such and I replaced most outlets there with GFI with three prongs. Equipment ground is not possible. Why? I believe the reason is simply that there is no connection. GFI for three prongs works for two prongs, except for equipment ground.

   Third, all the neutrals of all circuits have only the bar at the panel to serve as common, in no place else do they touch. The same is true for all grounds. And at the panel all neutrals and all grounds are connected to the same bar.

   Fourth, I believe the only reason why the bare wire should not function as current carrying neutral is thermal consideration. The outside can get hotter because it is not insulated. When 12 AWG is used with a 15 amp circuit breaker, the wire will not get too warm before the breaker trips.

3. 
   carbide_tipped | Sep 23, 2019 10:54pm | #3

   There are several reasons why grounds (bare or green wires generally) and neutrals are kept separate and are only tied together at the service entrance panel. Note that neutrals are called grounded conductors in code documents like the National Electrical code, to make it clear that although they are nominally at ground potential (zero volts above ground), they are conductors that carry current.

   Most of the reasons have safety at their core. A ground protects people from electrical shocks that could occur if a fault in an appliance, say, caused contact between an electrically hot wire and the metal case of the appliance. If the case was not connected to ground, the case would become energized and someone touching the appliance could receive a dangerous shock. When the appliance case is properly grounded, the fault causes a short circuit and the circuit breaker protecting the circuit will trip, both removing the hazard and alerting one to the fault.

   Now, you ask, since the grounded conductor is at ground potential, why is a separate ground connection needed? Why isn't the case of the appliance just connected to the neutral? Indeed, they often were, before the advent of a separate ground. Why isn't that good enough?

   Reason 1: The notion of independent, redundant systems as a way to increase reliability of critical functions is well established in engineering. In the case of an appliance, for example, the primary safety system is to insulate all the electrical connections so they don't touch the case of the appliance. Connecting the case of the appliance to ground becomes a redundant, independent safety system that provides protection should the primary system, the insulation, fail. This is why a so-called, double insulated appliance isn't required to have a three prong plug with a separate ground connection. A double insulated appliance has redundant (at least two) layers of insulation between energized parts and any part a user can touch. For example, it may have an insulating plastic case that serves as a redundant, independent way to keep the user from coming in contact with an energized wire. The insulation on the wire is the other way.

   Reason 2: Wire has impedance and when current flows through it, a potential difference (voltage difference) results according to ohm's law. This means that the grounded conductor no longer is exactly at zero volts, but at some small voltage above ground. If there happens to be a poor connection in the neutral path, and the appliance draws a lot of current, the voltage on the grounded conductor can rise high enough to be a hazard. Besides the safety aspect, a voltage, even a small voltage, on what is supposed to be the ground connection can lead to performance and noise problems with sensitive audio and computer gear.

   Reason 3: Wires that carry current can get quite warm, and connections to things like receptacle terminals and circuit breakers can get even hotter because they often have higher resistance than just wire. As these points go through hot/cold cycles as current flows and then stops flowing, the thermal cycle can cause gradual loosening of the connection, which raises the resistance of the connection, which increases the temperature even more, leading to more loosening, etc. Because no current flows through ground connections under normal conditions, they are immune to this type of thermal cycling and thus are more reliable over the long term.

   Now, why are the grounded conductor and the grounds only connected together at one point? If they were connected at multiple places in a building, say, you would now have current flowing through the grounded conductor, which is undesirable per 2 and 3 above. You could also have a situation where there was a failed neutral path (say due to bad connection) and *all* the current in that circuit flows through the ground, and you would never know there was a problem, but now you have lost the redundancy provided by the separate ground system. This can also lead to a bad shock to someone working on the wiring if they happen to disconnect that ground wire (normally not a risk). Finally, GFCI and AFCI circuit breakers would trip immediately on such a circuit because they trip when there is a difference in the current in the hot wire and the current in the neutral. If the neutral was connected to ground somewhere, some of the current would flow in the ground path so the GFCI circuit breaker would sense the difference and trip.

   Bottom line: The separate ground system is intended to provide an independent, redundant system to prevent electrical shocks. To serve that end reliably, there must be no current flowing through it and it must not be connected to the ground system except at the service entrance panel.