Here are some real life examples. The negative battery post has a clamp around it that goes to the engine block. Imagine if you reached into the hood of your Eagle and unbolted the end of the cable at the engine block, then touched that against the hot post of the battery. You would be shorting the battery. Sparks would fly as soon as you touched the end of the wire against the post and sometimes the arc will weld the metal and get stuck there, in turn melting the insulation right off the wire and possibly making the battery explode. Just because its a cable with black insulation on it to signify that it typically provides ground to other components doesn't mean it can't create sparks and a short circuit. Likewise the car body is usually the ground to other components closer to the high voltage in the circuit. That doesn't mean it can't become a source of high voltage as soon as the ground is disconnected.
Now imagine if you car was stored with the clamp around the negative post loosened and set aside, but the headlight switch was still left on. As soon as you touched the negative clamp against the negative terminal the headlights would flicker on and a spark would fly from the clamp. Even though the clamp provides ground to other components like the headlights, it's still an extension of the positive post and positive in relation the negative post right up until the moment it makes contact with the negative post and closes the open circuit. The sparks won't be too bad because of the voltage drop across the headlight resistance, but they will still be there. If during storage you also accidently cut the wires going to the headlights and there was a dead short present in the circuit before you connected the battery, then the sparks would be pretty severe. Anyone who has connected jumper cables improperly knows how bad those sparks can be.
At any point in an open circuit, regardless if its a grounding cable or not, there can be sparks present. An open circuit behaves differently than a closed circuit. The car body will have a potential value 12 volts higher than the grounding cable if it breaks and the grounding cable will still be ground. If it swings around after breaking and makes contact with the side of the subframe sparks will fly and it could weld itself to the car body and a fire can start. It happened to me. A piece of ground strap is still welded to my subframe right now. I had a great deal of damage done to my starter and most of my wiring harness because of the fire.
I was driving the car when it happened. I noticed immediately a severe loss of power and the car started to barely run. The headlights started to flicker and flash. All the dashlights flashed together with the headlights. The engine surged alive and back to nearly dead to the exact same pattern. I had to use the clutch to restart the engine and then start overrevving it to keep it barely running after it stalled out. I suddenly realized that the song playing on the radio was the source of the surges. My entire car turned into a disco mobile, like those flashing lights at the bottom of '70s juke boxes. I figured out much later that when the ground strap burned up and destroyed my entire starting system and most of my wiring harness the only intact ground wire in the entire car was the 10 gauge wire I had run directly from the battery to the new radio. That became the only source of ground to the entire car. The headlights, ignition, and dash lights all got their ground through the radio. As the song played the entire car went to the beat of the song. It was groovy. I thought it was hilarious until I found out about the destruction in my engine bay. I was driving home from work and the car made it home. I had to drive my Eagle Station Wagon for a week after that while I repaired the starter in the SX4.
Topic: Best New ICM (Read 6386 times)