If you’ve done any electrical work around the house, you probably know a bit about standard electrical wiring. The better you understand it, the more safely — and smartly — you’ll work. Let’s look a little more closely.
You know you have a black and a white wire connected to ordinary outlets which power most of the electrical devices in your home. The black one’s the “hot” wire and the white’s the “neutral”. Now, here’s the news. Both of these wires carry power. In an alternating current (AC) system, the electricity flows one way, then the other, changing direction dozens of times in a second.
So why is one called hot, and why is the neutral wire considered relatively safe to touch (though “just don’t do it” is good advice). When electrical systems were developed, they were connected to the earth (or grounded) in order to avoid dangerous problems and buildup of current in the system from such natural electrical activity as lightning strikes. Naturally, they had to choose only one of the wires to connect to ground, otherwise the system would short out.
That was the neutral wire, which is actually also known as the “grounded conductor”. The “grounding conductor” is the bare ground wire, and the term “neutral” is used largely in order to avoid confusion.
Once the electrical system was connected to the ground, the black wire became more dangerous. If the system wasn’t grounded, you could touch the black wire with impunity. Now, if you touch the black wire, you’ll close the circuit, since you’re standing on the ground. Boom! Thus, it’s the “hot” wire.
That white neutral wire in your house is grounded at many points in the electrical distribution system. But it’s also grounded in your circuit box. Here’s why.
The third, bare wire is the grounding conductor. It’s there to protect you from getting shocked. Appliances which have metal cases can become pretty dangerous if a wire comes loose and touches the case. The case is now “hot”, and if you touch it, once again, you’re closing the circuit. Blam!
So appliances with metal cases are intentionally connected to the bare ground wire. Now if the hot wire, say comes loose, it will be connected to ground immediately, rather than waiting for an unsuspecting person to come along.
But the process still isn’t through. In order to be really safe, it has to trip the breaker. The high power drawn by the low-resistance ground wire isn’t usually enough to trip the breaker. But by having the neutral wire also connected to ground at the box, it becomes part of the overall flow, which does draw enough power to overload the circuit and trip it. This protects you and alerts you to the problem.