I have what might be a stupid question, but I can't wrap my head around it, and I can't seem to find an answer searching...

Over the weekend I ran a new circuit and breaker for a new dishwasher in my house. I had never really been in the panel before (Square D QO brand) until running this circuit, and I was surprised to see that there are two buss strips on the sides of the disconnect, both tied to the main neutral, and both neutrals and grounds from existing circuits terminated there (ie: not neutrals to one and grounds to other, but mixed). I know that the neutrals are tied to ground, but I was expecting there to be separate busses.

Nevertheless, I terminated the neutral and ground wires to separate screws, next to each other, on the buss side my knockout and breaker were. But I can't quite understand - so what is the point of the ground wire anyway, if I essentially have two wires going to the exact same point of termination at the panel?

If a hot wire came loose and hit the metal frame in the dishwasher (or any other metallic device), it would trip the breaker. So why not just tie the neutral to the frame?

 

It is not safe to expose current carrying conductors to humans. Exposed metal is connected to NON-current carrying conductors. This way, if the neutral wire becomes loose at the panel, you will not have high voltage present at the exposed appliance metalwork.

 

If the frame was bonded to the neutral you would create a shock hazard by having the current on places you touch that should be dead.

 

It can be hard to visualize. Several answers printed before me. Here's another.

Suppose you only ran two wires to your dishwasher..... a white and black wire. The black is hot.
Since the white wire is neutral and ground you connect it to the neutral wire of the dishwasher and to the cabinet for ground purposes.

You accidentally over tightened the white wire in the panel and it snapped off two weeks later. You turn the dishwasher on. Since there is no longer any neutral or ground..... the hot power appears directly on the cabinet. The dishwasher won't work without a neutral. You hold on to the grounded metal sink and reach over to the dishwasher......

The bare wire is strictly there as a safety grounding means.

 

The purpose of a ground wire is hard for me to visualize also.

The best reason I can think of, which perhaps very few others would say is the best reason, is a redundant or "safety" path for stray current.

Let's start here. We have a dishwasher or radio or other device with just two wires, black and white. Neither is connected to the cabinet, which is normal and proper. Now suppose that somehow a wire inside biodegraded or vibration caused the wire insulation to be worn through and the bare wire touched the cabinet. Now the cabinet could be live (have a voltage difference relative to other unrelated and grounded objects such that current sufficient to electrocute someone could flow) depending on what wire touched the cabinet.

The third wire, as a ground wire (equipment grounding conductor) connected to the cabinet dissipates the current that might otherwise flow through a person and electrocute him. Because current seeks to return to where it came from as opposed to go to the ground (earth; soil; dirt), the ground wire is also attached to the neutral bus bar or to metal that is in turn connected to (bonded to) the neutral bus bar.

Ground rods and other buried parts are intended to help dissipate power surges due to lightning. (Nothing offers total and absolute lightning protection.)

The ground path is supposed to be robust enough that if a hot wire touches the dishwasher cabinet or other exposed metal in a manner that a large current would flow, the breaker would trip.

The ground path is not meant to keep the equipment running if the neutral path should be broken. Namely, if the neutral path should be broken, the current flow should stop rather than use the ground path. Thus, in modern* equipment the neutral is not also connected to the cabinet. Flow of current on the ground path is supposed to be regarded as abnormal. The ground path is redundant only in the sense it provides an additional way to dissipate what could be hazardous voltage on exposed parts.

* Some older equipment, notably clothes dryers, had neutral connected to the cabinet and no separate ground wire.

 

Grounding and bonding are probably the most misunderstood function in the electrical industry.
IMO
Geo

 

Here's a couple of explanations that help me understand:

The bare copper ground wire normally carries no current. Its purpose is to provide an emergency path for any accidental contact between a hot wire in an appliance and the metal parts of the appliance that the user may come in contact with.

You can say the neutral conductor is "grounded", but you should not say it is "ground", because that term (in building wiring at least) refers to the special conductor reserved for safely conducting current during a fault.

 

Bonded -- (Conductive objects) having an essentially resistance free connection between them including in the sense that if A is bonded to B and B is bonded to C then A is bonded to C. Proper bonding is context sensitive; sometimes a #4 copper wire is needed, sometimes a #22 wire is sufficient.

Grounded -- Literally, bonded to a buried conductive object.

Electrical ground -- All those objects that are bonded, not intended to be part of any circuit path, and at least one of which is buried. Typically defined as being the zero volts reference for the system.

A radio chassis is often referred to as a ground (for the radio circuitry) because in the early days of radio, a wire to a ground rod was usually attached to it.