I recently attended an online presentation from EC&M and Mike Holt regarding grounding and bonding. Within the first five minutes, I had two misconceptions shattered, and apparently I am not alone. My understanding of grounding came from forty years on the management side of electrical construction. What I have seen on electrical drawings, the information from electrical engineers and the demands of electrical inspectors have shaped my understanding of this subject.
As embarrassing as this may be, here are what my misconceptions were.
First, I thought electricity wants to go to ground. This is not true. The electrons in an electrical system are not trying to get to the earth (ground), they are trying to return to the source, usually a transformer. If the earth is the best return path, the electrons will use it. A very personal example happened to me in high school. I was washing the car, getting ready for a date. I was barefoot, standing on wet grass, and bent over to pick up an extension cord terminated in a 4S box with a duplex receptacle and a raised cover. There was no ground wire in the extension cord. The shock made me straighten out and fall backwards, pulling the box out of my hand, and most likely saving my life. The fault current in the box was looking for a return path to it’s source. Since there was no ground path through the extension cord, it found me, barefoot in wet grass, and returned to it’s source through me and the ground.
Second, as noted above, fault current is not trying to get to the earth. This is what I thought ground rods, counterpoise, rebar and metallic pipe grounds were for, and they are not. These systems are for induced currents, such as from lightning. When lightning strikes near a structure, it’s magnetic field induces current in the wiring systems of the structure. It is this current that needs to find it’s way to the earth.
Click on comment and let me know what you think of this. CLICK HERE to see the presentation from EC&M and Mike Holt.