In order for a light fixture or any other type of appliance to be turned on, it must connect to a circuit powered with a flow of electricity. This electricity is initially generated by your utility’s power station before being fed to the power grid, which distributes it to your community, and ultimately into your home's electric panel.
From there, electricity is distributed through a web of circuit breakers and wired circuits throughout the home to supply every appliance and fixture connected to it. All that to make your life more comfortable at the flip of a switch, or nowadays, at the command of your voice.
How Electricity Works
Without getting into a scientific description of the detailed process, it's common knowledge that for an electrical appliance to work, you need to have electricity “flowing” through it, coming from the hot line passing through the appliance and returning to the electric panel through the Neutral (white) wire—the Neutral line being the return path of the current back to its source.
That flow of electricity through each circuit is facilitated by Electrostatic forces, which, associated with the electromagnetic force, has the ability to either repulse or attract negatively charged particles from their molecule and to latch onto the next closest particle, attracted by that next molecule’s positively charged protons, thus ejecting another negatively charged neutron from that molecule, and keeping the movement flowing.
This in fact creates a chain effect called an electric current. That current keeps going until the extra neutron returns back to the source, at the electric panel, ensuring the perfect balance of every molecule’s electrical positive and negative charges.
Note—Electrons always carry a negative charge, protons are always positively charged, and they both carry the same amount of a different charge.
What Is the Ground and Why a Second Return Path?
Electrical grids service a lot of different facilities everywhere and therefore require specific reference points in order to provide stable constant distributions without fluctuations and variable potential differences between different locations.
Ground, or Earth, does offer such a reference point from which voltages can be more accurately measured. It also serves many other purposes, such as a common return path for electric current, and a physical positive connection to the earth, serving as an adequate zero-voltage reference level.
In order for a circuit to operate properly, the current needs to flow from the panel, pass through an appliance, and flow right back to the source, activating the appliance. If the line is cut anywhere along that path, it will stop working—that is actually what the switch does (or the circuit breaker).
So let’s consider a typical scenario with everything initially in perfect working order. You have a line supplying the electric power, a fixture (or an appliance) that is switched on, and a return path to complete the circuit back to the source provided by the “white” Neutral wire
So couldn’t you say that this makes the bare copper ground wire a 2nd return path back to the electric panel where both lines are connected to the chassis GND of the metal box? It most certainly does, but for a very good reason.
You just have to think of the ground wire as a safety net, which a good trapezist performing his act can do without, just like your light fixture can work just fine without the ground wire, just as long as nothing lets go, breaks, or slips causing an unfortunate and serious accident.
The same with your light fixture—if your fixture suddenly becomes defective from a wire getting loose inside that accidentally makes contact with the terminal box or the metal casing of a light fixture that is incidentally switched on, it doesn’t even matter if the broken line is the Hot or the Neutral, the fixture’s casing becomes instantly live with the full voltage.
Using this scenario, let’s go ahead and check all possible outcomes.
Possibility 1 - With a Proper GND Wire
The whole circuit is wired with a 14/2 cable with a ground wire properly secured to the GND bus inside the panel, and each terminal box is properly wired up with the ground wire secured to the terminal box, and also to the green wire connected and supplied to the fixture.
1a - Hot Line Coming Loose
The hot line can come loose from excessive handling and touch the metal part inside. As mentioned earlier, when the hot line comes in contact with the metal casing being connected to the GND, it will create an enormous surge of current through the circuit, instantly tripping the breaker, thus shutting off the power to it.
1b - Neutral Wire Coming Loose
Whenever the Neutral wire comes loose, it can also end up touching the metal part inside the fixture. As before, the GND being properly hooked up means that the current could still be following a secondary return path to the source by way of the GND wire when the loose end touches the casing, while the light may or may not stay lit, or do so intermittently. Still though, no danger of electrocution.
Possibility 2 - Without Proper Grounding to the Electric Panel
Either the result of an old construction from before the mid-1960s or because of a broken GND wire within the circuit’s wiring that prevents a secure connection of the GND from the panel to every terminal box along the circuit, this can cause a serious electrocution hazard.
2a - The Hot Line Comes Loose
If the hot line comes loose from its terminal screw or breaks off and makes contact with any metal parts inside the electric box, the fixture will then become electrified, but since there is no path provided back to the electric panel, it will not trip the breaker as it should, but instead will maintain the full voltage on the fixture’s casing, with nowhere to go.
If, however, you or someone else providing a physical contact with the ground should touch it, just that touch would create the missing path to the ground, making it possible for the current to flow through and complete the full circuit, electrocuting someone in the process but without even tripping the breaker, since the flesh would provide some resistance with the resulting current continuing its ongoing physical damage.
Note—If dealing with a circuit miswired into a Bootleg ground, however, where the Neutral terminal is wired to the ground terminal or wire, the Bootleg would then provide the hot line with an instant return path to the electric panel without any resistance and trip the breaker. Luck might make it sound like a good thing in this situation. But it is still illegal as well as dangerous.
2b - A Neutral Wire Comes Loose
Finally, the neutral wire coming loose with the appliance or fixture switched ON and making contact with any metal from the fixture can have several consequences.
Low Resistance Danger
The casing will be fully electrified if the line powers a low resistance load, such as a light bulb, and could cause serious injuries if someone was to touch it.
High Resistance Danger
If the load was of higher resistance, the potential difference affecting the load as well as the person making contact would be divided between the two and could result in a serious injury of varying degrees.
Whenever you decide to do some maintenance or repairs around the house and come across a problem concerning an open ground or electrical wiring that doesn’t have the proper ground wire, just make sure it gets corrected.
If it concerns rewiring complete circuits in the house, it would probably be advisable to consult an electrical contractor to get all the facts beforehand. But for your safety as well as your family’s, don’t just leave it be.
If you’re in the process of buying a home, you should have an electrician or a reputable inspection agency inspect the electrical wiring, especially when dealing with an older house, while doing the overall inspection of the premises.