Eli5 – Why is there both neutral & ground wires in light switch?


Eli5 – Why is there both neutral & ground wires in light switch?

In: 5

Like a lot of stuff, electrons want to flow from places where there’s a high concentration (negative charge) to a place where there’s a low concentration (positive charge). The flow of electrons is electricity. Normally, we get fancy and set up a circuit – basically we make the electrons flow in a circle. So your system has a hot wire, where you pump the electrons down, and a neutral wire, where the electrons come back where they came from.

Ground wires connect to the literal ground. If something in your house gets a huge oversupply of electrons, it needs somewhere to dump them. That’s what the ground is for.

The ground is connected to the neutral wire in the circuit breaker box in most setups. A circuit breaker’s job is to detect when there’s a short circuit, letting way too much electricity through the circuit.

Say the hot wire comes loose in your light switch, and now it’s resting on the metal body. That’s a fire and shock hazard. Which is why you’ve got a ground wire – it’ll at least dump electricity into the ground, reducing the hazard. But it’s still a problem, and your circuit breaker won’t trip because your circuit isn’t shorted, it’s open – electricity isn’t coming back into the circuit. Which is why you connect the neutral to the ground. The current will go through the ground, back into the neutral, completing the short circuit and allowing the breaker to trip.

Simple version:

Yes, a lightbulb will work no matter which way the electricity flows.

But… what if the bulb is removed from the socket?

Now the switch can prevent the lead from touching your finger, or you can have the wiring reversed and the “ground” is always hot because there’s no switch on that side.

The polarized plug is purely for safety so that the switch actually does its job. And this expands for more complicated electronics as well.

Ground is not meant to be used. What I mean is that during normal operation, no current flows through the ground wire. Ground is there in case of a fault. If something happens and the current can’t make it back on the common wire, there is a second path available… Well, there’s always a second path available, it could be through you, though, and we want to avoid that. So we make the ground wire which has much less resistance than you do so electrons are incentivized to flow there instead.

In a switch, there are not a ton of things that can go wrong, but imagine if it was installed poorly and the hit wire came loose and touched the box. The outlet is screwed into the box, so there might be 120V on those screws if you touch them. I’ve also seen some with metal plate covers instead of plastic. The whole thing would be hot, then. But with a ground wire, as soon as the hot touches anything other than where it’s supposed to be, that should be grounded which will instantly trip the breaker due to a ground fault.

Btw, while we were building the house I lived in during middle and high school, they didn’t hook up the ground immediately because things were still in flux. I got a pretty good poke flipping on one of the outlets because it was floating. Probably at 120V, though it could have been less of it was just stray inductance. I lived, but that’s not always guaranteed. Best to be safe.

The ground wire is there for safety. It’s meant to carry high current, so if anything shorts directly to ground it’ll trip a circuit breaker.
With a GFCI/RCD, if anything tries to pass current to ground whatsoever it’ll cut the power.

As for why they’re both in a light switch, modern electrical code generally requires all 3 (live, neutral, ground) to be run to the box, and it’s handy to have if you want to install a shaver plug, (where available,) a smart light switch, or any other active electrical/electronic device.

(Note: IANAE, and this is not electrical advice)

Think of it like this:

You have a loop of water hose with flowing water. Water flows from live to neutral. Everything works, nothing else is needed. Or?

Well, what happens if that hose bursts? Water everywere, and this water, since it is a metaphor for electric current, is deadly. This is bad.

So, what is ground? Think of it as an outer layer on the hose, which, if the hose bursts, gathers the water and takes it back to the central. It also does it in a way which turns off the water flow (by tripping the fuse), so no more water goes into the leaky hose.

So, it provides a safe way back for the current if there is a problem, and makes sure the faulty (and possibly burning) equipment is powered off.