I was renovating the garage and noticed that I have a continuity between the ground and the neutral in the conduit box when I tested with a multimeter. I then checked the service panel and noticed that the neutral bus is hooked to common ground directly. (and it passed the inspection that way long time ago.)
Is this normal? Isn't the neutral rail should be connected to the ground separately and ground cables should be connected to the 3rd rail (or busbar)?
Sorry in advance if this is a stupid question, I'm just trying to be safe and it just didn't make sense that they actually loop. I always thought ground is a "less prefered" route for the electricity to travel hence it being isolated and to be forced to bypass neutral wire seems to be making more sense.
Best Answer
Neutral and ground systems must be completely separate, that's true. However, that's done for some purposes. First, it's desired to keep hot and neutral wires reasonably near ground potential (i.e. within 120V of it). Second, since hots have circuit breakers and neutrals do not, we want it so that if a wire shorts to ground, a neutral-ground short will be fairly harmless and a hot-ground short will trip the breaker.
Both of these require neutral be fairly near ground potential - though a couple volts difference would be fine. Well, "a couple volts difference" would require some sort of transformer there or something; a copper strap is a lot cheaper.
So, neutral is forced to be near ground potential by installing a neutral-ground equipotential bond, and in exactly one location. As near the service as possible, in the main panel and right where the grounding rods attach. If there were two, then ground would be used as an alternate current path for neutral, imbalancing current, and a neutral-ground break could actually electrify all the grounds beyond the break!
It's easy to see that and think "neutral and ground are the same thing, I'll just shortcut that where I care to". But that means if the system fails in a trivial way - broken neutral for instance - all the grounded things are now hot things. Neutral is for normal return currents only, and ground is for fault currents only.
Nothing should ever return current on ground. UL did approve a few smart switches which return less than a milliamp on ground and had some internal controls to ensure that this could never be enough current to shock someone. But NFPA told them to stop approving those.
You will see a fraction of a volt of voltage difference between neutral and ground. Since normal currents flow on neutral, it's subject to voltage drop : Vdrop = Current x Wire Resistance. However current doesn't flow on ground, so it suffers no voltage drop.