Grounding: A History

The earliest instance of grounding that I can think of is when Benjamin Franklin invented the lightning rod.

A lightning rod is a metal rod attached to the top of the house, connected by a wire to another rod stuck in the ground. When lightning would strike, it would seek out the rod (since the rod has a lower resistance than the rest of the house) and electricity would flow into the ground. This prevented houses from burning down when struck by lightning, which can generally be considered a good thing.

If a lightning rod can be seen as the first real instance of grounding, then we can look at grounding as one part of the overall electrical protection system for your circuit. So in order to keep things safe, you need to figure out a way to create a pathway for the electricity to travel that has less resistance than your body, fragile circuit components, and building materials.

In the event that something goes wrong in your circuit for whatever reason, you need to direct the electric current along a very low resistance pathway. We've already talked about how to figure out what resistance is, but what kind of wire would have a low enough resistance to draw all the current?

Why not use the ground itself?

Think about it: use the planet earth as a conductor. Not only does the electricity not have very far to go to get in to the ground, but it also has a HUUUUUUUUUGE cross sectional area, meaning it has very, very little resistance. Electrons always want to go to the place of least resistance, which makes going into the ground their favourite place.

The lightning rod used this concept to great effect, and modern day power companies do the exact same thing. This also doubles as a useful way to complete your circuit: by using the earth itself as your ground wire, you can be sure that every point in your circuit (which in this case is the entire power grid) is connected to the same reference point. This means that when we now talk about voltage (for example, the 120V wires in your house) we know that we're talking about it with reference to the voltage of the ground itself. There are 120V between the wires in your house and the ground you're standing on, there are 1000kV between the overhead power lines and the ground, etc. What this also means is that the current in your circuit now has a clear and easy return path to its source: the current can just flow into the ground near, for example, your house, and travel back through the ground to the generators in the power plant.

But isn't the system still unsafe?

You might see the new problem with that: if electrons want to go in to the ground, how do we get them there safely? If you are standing on the ground and you touch a wire with electric current moving through it, won't the current just go through you and into the ground, instead of going through the wire?

Current flows through all available pathways at once. This means that even if the ground wire path is the least resistive path for current to flow through, you could still harm yourself by adding yourself to the circuit when you touch it (making a path from the wires through you to the earth). Initially, I said that grounding was to keep your circuit safe, and I wasn't lying to you. The reason grounding keeps the circuit safe is that grounding makes up a part of your circuit's protection system.

Most circuits are designed so that when there is a fault to ground in the circuit, the majority of the current will flow through this fault, into the ground wire, and into the ground. Because we know where the current will flow, we can put protective devices along the ground path in such a way that as soon as the fault occurs, the protective devices (such as a fuse or a breaker) are tripped very abruptly. When this happens, the complete pathway for the circuit is broken, and current cannot flow. If current can't flow, we have no voltage difference, and if we have no voltage difference, we have no real danger. This is the most common way that grounding protects circuits: it forces protective devices to turn the circuit off ASAP.

In order for grounding to work in tandem with your protection system, you need to know how much current can be expected to flow through your ground wires in the event of a fault. You can figure out what amount of current to expect by performing a protection and coordination study. This will let you pick the appropriately sized fuses or breakers.

Wrap up

In summary, we can think of grounding as a circuit protection technique that doubles as an easy way to measure voltages. Electrical grounding can be:

• A common point of reference in your circuit that is used to measure all voltages
• The earth itself, when the common point of reference is stuck into the ground that you walk on
• A technique used to direct fault current along a low resistance path to protective devices that will immediately turn off your circuit
• All of the above

Hopefully you've enjoyed reading this article, and now know a little bit more about grounding.