What ground means

The third prong is the ground

A North American three-prong plug has two flat blades and one round pin. The flat blades are hot and neutral: they carry the current that runs the appliance. The round pin is the ground. It connects to a ground wire (usually bare copper or green) inside your walls.

That ground wire runs back to your electrical panel. At the panel, the wiring is connected to a grounding electrode, often a metal rod driven into the earth. That's where the name comes from: ground is a path to the earth.

Normally, no current flows in it

On an appliance with a metal case, like a toaster, the case is connected to the ground wire through the cord and the third prong. When everything works, the current goes out on the hot wire and back on the neutral. The ground wire just sits there, carrying essentially zero amps.

It's a backup path. It only matters when something goes wrong.

When a hot wire touches the case

Say a hot wire inside the toaster comes loose and touches the metal case. That's a fault.

• Without ground, the case itself becomes live. Nothing looks wrong, and nothing trips. The danger waits until someone touches the case, and the current flows through them instead.
• With ground, the fault current has an easy, low-resistance path down the ground wire. So much current flows that the circuit breaker trips almost instantly and cuts the power.

Either way something gets cut off. With ground, it's the power, not you.

Never cut off the third prong

It's tempting to snip the round pin so a plug fits an old two-slot outlet. Don't. That removes the safety path, and the next fault leaves the case live. The same goes for cheap three-to-two adapters unless their tab is connected to a known ground.

If your home has old two-slot outlets, the fix is new wiring or protected outlets, and that's a job for a pro. Leave wall wiring to a licensed electrician.

Try it yourself

Look at the plugs around your home. Which have three prongs, and which have two? Check where the three-prong ones come from: usually things with metal cases or that sit near water, like toasters, kettles, washing machines and computers. Many two-prong devices say "double insulated" on their label. Just look; don't open anything up.

Good to know

• This is the safety (equipment) ground in North American wiring (US NEC and Canadian Electrical Code style). Wire colors and plug shapes vary by country.
• At the panel, the system is bonded to neutral once and connected to a grounding electrode: a driven rod, a metal water pipe or a similar electrode. "A metal rod" is a simplification.
• The breaker trips because the fault current through the low-resistance ground path is large. That current returns to the source mainly through the ground wire and the bonding, not mostly through the soil.
• In normal operation the ground wire carries essentially no current.
• Many appliances have only two prongs because they are double-insulated: plastic cases with no exposed metal that could become live.
• A GFCI outlet adds a second layer of protection.
• On schematics, "ground" also means the zero-volt reference point. That's related but not the same thing, and it's in the next episode.
• Don't use a three-to-two adapter unless its tab is connected to a known ground. Leave wall wiring to a licensed electrician.

Answer to the comment question

With ground, what trips when a loose wire touches the case? The circuit breaker. The ground wire gives the fault current an easy, low-resistance path back to the panel, so a large current flows and the breaker trips almost instantly, cutting the power before anyone touches the live case.