eli5: Whats the purpose for the ground wire?



I understand if we touch the wire and were are on the ground it creates a complete circuit. So we get electrocuted.

But how does the ground or earth wire works, so that we don’t get electrocuted?

In: Engineering

Current likes to take the path of least resistance

You’ll usually only find ground wires in machines with a conductive casing (or has a lot of conductive material present). All of these metal pieces are connected to the ground wire

Now the purpose for this is, if there was a loose wiring in the machine and it accidentally touches the conductive casing and you proceed to touch said casing, you will create a short between that wire and the ground you’re standing on and thus getting shocked.

However, if the ground was connected to that metal casing you won’t feel the shock as it’s easier for the current to flow through the ground wire than your body as your body has a much higher resistance comparatively

If there is an internal failure that exposes the housing of the unit to live voltage, it immediately shunts it to ground causing a surge that will trip the upstream protection device.

Without it, it is possible for the housing to be energized after such a failure and wait for some poor soul to walk over and grab what they think is a perfectly safe electrical device.

Electricity is really weird stuff. It likes to flow through some things better than it flows through other things.

An example is a nine-volt battery with the two little knots on the top. Electricity doesn’t like air very much – it’s an “insulator”. But it LOVES many metals and it loves salty water. So if you lick that battery, the salt water on your tongue gives the electricity in the battery a little highway that it likes much better than air, so it jumps on and you get a shock.

And some things are way way better than others. A wire? Ooh, it hops RIGHT on board. But a human body? Well, you need a lot of electricity to overcome the resistance (level of insulation) before it’ll go through your body. This is why your dry finger on that battery’s top won’t give you a shock, but putting a fork in an electrical outlet can electrocute you. (Note: please don’t try this.)

So you hook up a ground wire to give the electricity a very convenient path to go along instead of through your body and what you’re touching. So if you’re a technician that’s working on a computer circuit board on a dry day where there’s carpet, you hook up a “ground wire” to connect yourself to a metal table, which has way less resistance than the path through the motherboard and tools you’re using. Instead of a big static charge snapping through that board, like what happens when you scuff your sock feet and then touch a doorknob, the electricity follows the convenient low-resistance wire, and you don’t damage your electronics when you take a screwdriver to them.

Ground wires in three-prong electrical cords are the same. In the event something goes wrong in a household appliance that’s powered by electricity, the ground wire provides a way for the electricity to exit INSTEAD of your own body because that wire has lower resistance.

Same thing for “lightning rods” on buildings – they provide a connection to the ground that is not the building itself, and a strike flows through the conducting wire rather than setting fire to the more resistant building.

Circuit breakers work if there is a severe overcurrent in the circuit e.g. a shortcut.

If your lamp has a metal frame and a loose live wire inside touches the frame, you’d be grilled if you touch it. Now if we have a low resistance path, that’d ground the frame and short the circuit, the circuit breaker would flip and kill the circuit before anyone touches. This is what the ground wire is for.

a Ground wire is basically a safety mechanism for a circuit to safely off load any current that it couldn’t discharge thru the normal circuit.

without a ground wire this current will look for the next best conductor which could be the poor sap that touched it. the Ground wire will also ensure that in the event of a circuit failure, the discharge thru itself will cause the breaker to trip(as due to the loss, current will be way higher than its safe and if not interrupted it could burn the wires and cause a fire).

Lots of good answers, but WHY does electricity like to “go to ground?” Is the earth itself some kind of superconductor?

The ground wire gets electrocuted instead of you and when that happens it – hopefully – trips the circuit breaker so that you don’t get electrocuted too.

To add on to all the other answers (which are correct, but they say more or less the same thing, I don’t understand why everyone needs to repeat exactly what other people have already written), notice that you (generally) find a ground connection only on appliances that are enclosed in a metal housing, or have some metal parts that you can touch that could possibly get shorted to mains. You’re probably never going to find a phone charger with a ground connection, because it is all plastic, and the USB terminals are galvanically isolated from mains by the transformer. Maybe this will help make sense.

If you look at an electrical device you can imagine there are parts of the device that should have electricity in them and parts that should not have electricity in them. For example, you probably don’t want your microwave door to be electrified.

If something happens and a part of the device that you touch becomes electrified (like say maybe the insulation on a wire frays and touches the body of the device) then that poses a risk to the operator. The idea of the “ground wire” is that you can connect it to anything that shouldn’t have electricity in it, and something happens that causes those things to become electrified the electricity will go down that wire instead of going into a person.

I guess we’re not even going to get into the fact that electricity flows from, not to, ground.

Electrician here. The Grounding conductor is for fire prevention / equipment protection as a path for fault current so it’s purpose is not personnel protection. That’s what GFCI is for. Over current protection devices and the fault current pathways (ground wire)that make them work are there to stop fires and protect equipment.

Edit: ELI5! It’s to prevent fires.

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