[vvillyd]

Thanks! In my 11 year experience in the electrical industry I have come to realize that there is a LOT of misinformation about how electrical systems work, and even the most experienced veterans in the field often don't really understand it.

[ryanmarsh]

So what would be the potential consequence if I didn’t connect the “ground” wire (green or bare) to the little brass ground screw on (let’s say) a ceiling fan when installing it?

[shreddish]

From my understanding, if the hot wire (black) were to come in contact with a piece of metal on the fan it would build up electrical charge and or allow for current to pass through the piece of the fan. Then if you were on a ladder fixing the fan and touched that piece of metal you would become a path for electricity to pass through to a lower potential.

[vvillyd]

That's exactly correct. You would get shocked if you touched the fan.

[chrisdhoover]

The way you bond is codified too. The source feed is wire-nutted with a pigtail connected to the box, and to the ground of the device in parallel. If the bonding was daisy chained it could be interrupted when servicing or changing.

[vvillyd]

If you do not properly bond the ceiling fan (that is, connect the "ground" wire) then the circuit breaker will not trip if there is a fault to ground on the fan.

That is, if the hot wire touches a metal part of the fan which is not supposed to be energized (say the metal housing), and the fan is not properly bonded, then there will be no ground-fault path back to the source to cause the breaker to trip. This could mean that the housing of your fan would be energized with (in a home in the US) 120V, and would shock anyone who touches it.

[ryanmarsh]

So when installing any electrical fixtures at home I should always connect “ground” (bond the fixture) first. I’ve been connecting it last (always with the power off but accidents can happen). This could lead to an injury if someone flipped the breaker on and I accidentally touched hot to myself or the case because the breaker wouldn't trip.

[vvillyd]

Absolutely! Yes!

First, NEVER work anything hot. There is no such thing as a safe voltage to work energized. Even a simple home receptacle circuit can easily kill you (and it happens with alarming regularity).

That said, ALWAYS work something AS IF it were hot (for the exact reason you said, accidents happen). This means connecting the safety measures first. The safest order to connect wires is: ground, neutral, hot. The safest order to disconnect wires is the opposite: hot, neutral, ground.

[ars]

> This could lead to an injury if someone flipped the breaker on and I accidentally touched hot to myself or the case because the breaker wouldn't trip.

Half right, half wrong. It would not short if it touched the metal box, and would not trip the breaker. That part is right.

However if it touched YOU not having the ground wire connected actually helps you. It means the electricity does not have as a good of a path back to the box, and you'll get a weaker shock.

With the ground wire you'll get a much higher shock, but it won't trip the breaker!! Humans have too high resistance to trip breakers, you'll just keep getting shocked.

So it's kind of a wash - with the ground, it's more likely to just short against the box and trip.

Best is just to always wire things as if they are live. Only touch the wire if you have to, always use insulated tools, even if the power is off.

[dreamcompiler]

In industrial and commercial settings, there are "Lockout/Tagout" rules. These prevent -- either through policy or physical means -- somebody from accidentally flipping a breaker back on when maintenance is occurring.

[shreddish]

Technically, would it only shock you if your body provided a path to "ground" or lower potential? For example if you were wearing insulted boots and touched the energized piece of metal you would not feel a shock, correct?

[nosianu]

Yes. That's why this is possible, here the insulation is the air between helicopter and ground: https://youtu.be/DPNK7bc2qvM?t=125

If that guy was connected to the earth he would get fried. Note that you still have to equalize whatever potential there is between you and the system, like they do in that video. Same reason why you should ground yourself and the hardware before working on your PC. Ever had a tiny spark hit you when you touched something (neither you or that "thing" being connected to the electrical grid, just static electricity e.g. from walking on the carpet)? But without connection to ground the only thing that happens is equalization of the potential between you and the target but no further flow. If you are connected to ground on one side and to a source that keeps creating an electrical potential, like the electrical grid with a power plant somewhere, there will be a continuing flow. If you are not connected to ground there won't be.

[dreamcompiler]

Correct. But if you touched the energized piece of metal with one hand and accidentally touched a grounded conductor with the other hand, you'd be dead. This is why in the rare circumstances when I have to work on an energized high-voltage circuit (don't do this!) I observe the "keep one hand in your pocket" rule.

[hyperman1]

Please don't insult your boots.

[shreddish]

hahah, they piss me off sometimes though