[ELI5] What does it mean to be “grounded” against electricity, and why does it keep you safe?


[ELI5] What does it mean to be “grounded” against electricity, and why does it keep you safe?

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Electricity wants to ground out, literally go into the ground. “Grounding” a circuit gives it a path to do so that doesn’t involve ripping through you and blowing off your toenails.

“Ground” is a sometimes misleading term. It can mean “earth ground”, or the electrical potential of the literal ground under our feet. It can also mean “the negative terminal of this circuit”, which may or may not be at or near the same potential as earth ground.

One (very limited and kind of poor, but works for this) analogy might be to think of it as water in multiple tanks. If the water level in the “earth” tank is low, and you connect it to another tank, whose level is higher, then water flows from the tank with the higher level to the “earth” tank. But if the “earth” tank is at or close to the level of the other one, little or no water flows.

To be truly “safe” from electrical shock, you need to never be part of the path that current wants to take- or, to reference the above example, you don’t want to be the pipe between the tanks.

Actually being grounded- meaning, your body is at the same potential as earth and there’s a conductive path through you to earth- might actually make you *less* safe, because electricity could travel through your body.

Electricity wants to reach the ground to dissipate, and to do so it wants to find the path of least resistance. It’s a lot easier for electricity to pass through your body than air, so if your feet touch the ground and your hand touches a circuit, the path of least resistance is through your body. Grounding yourself is providing an alternate path that’s preferable for the electric current that does not go through your body.

There is two facts that we have to establish here first in terms of safety around electricity.

1. Electricity hurts you when it flows through you.
2. Electricity prefers to flow through path with least resistance. Like water, if one pipe is small and the other is big, water would flow more through the bigger pipe. In other words, electricity is lazy.

Now, according to fact 1, if there is little to no electricity flows through you, then you’re fine.

According to fact 2, electricity doesn’t flow if you have significantly higher resistance than other path.

So, usually, what we do is:

1. We provide a superbly easy way for “rogue” electricity to flow. This is usually accomplished by having the machine “grounded” (having an almost direct connection to literal ground)
2. We make ourselves as resistant as possible, via rubber-gloves and shoes, usually.

You as a person being grounded (at the same potential as the neutral wire coming into your house) is bad. If you touch one of the hot wires coming and are grounded, current will flow from the hot wire, through you, to the ground because the neutral wire is at ground potential.

A safety ground (or chassis ground) is important for devices with a conductive outer chassis (most appliances for example). Let’s say your metal toaster has a frayed hot-wire inside it. This wire touches the metal chassis of the toaster. It is now at the same potential as the hot wire. When no one is touching it it’s a open circuit, no current flow. But you come in to the kitchen and touch it while your feet are on the floor, you have created a full circuit and current will flow. Ouch

Now a number of factors come into play that determine how much current will flow. If you are on the 2nd floor of your house wearing rubber boots and your hand is dry, you might only feel a tingle because your insulated from ground potential. If you just got out of the shower and are barefoot on the basement floor, you might have a worse day.

This is where a safety ground comes into play. The safety ground is wired up to a grounding rod buried into the ground, usually where your electrical meter is. This is wired to the chassis of your metal toaster, so that when the hot wire in the toaster touches the chassis, it creates the circuit instead of you. Also this will act like a short circuit and immediately trip your circuit breaker.