it really does nothing and doesn't need
hooked up, right?
A fire alarm system is sounding its trouble signal in a commercial building and the maintenance man has summoned me to check it. He pulls out his ring of keys, inserts them into the lock, then quite suddenly he's unable to move--he is frozen in place.
What exactly has happened here do you suppose?
Well, it's my guess that he's received an electrical shock. Luckily, this time he manages to pull away, but a few seconds more and he could have been toast. So what happened?
It appears the people who installed the fire alarm panel did not do their job properly as they forgot or simply failed to install a small green bonding jumper from the removable panel lid to the back box. This allowed electrical current to flow through the lid when the hot 120 volt wire supplying power to the panel pulled loose, coming in contact with the lid.
Now, had this jumper been bonded from lid to back box, and if the panel had been grounded properly, this event would not have taken place. The breaker would have tripped out and just the trouble signal would have sounded. The problem would have been spotted as soon as the lid was opened. Instead, because the jumper was not used, the maintenance man got a severe jolt.
Grounding and bonding of electrical equipment is critical to assuring that it operates properly. It also assures that any type of overload is cleared. It also assures that if a wire or component disintegrates and touches the metal enclosure in the system, nothing bad will happen to someone working on it.
Now, here's the kicker, not all lids need to be grounded. This is especially true if it is permanently attached to the panel box by means of a full-length piano-type hinge. In this case, code considers the lid properly bonded due to the continual path of metal touching. This is not something you have with a removable lid where only 2 small areas of the lid touch the panel box.
You say "bonding and grounding, what's the difference?"
Bonding is used to eliminate a possible hazard, like with the removable lid. The back box has a ground wire which runs the entire way back to the main service entrance for power. I know it's confusing, but follow the manufacture's guide lines and you'll do just fine. But if you fail to follow them, sooner or later something will fail and there will either be a fire or someone will get shocked.
With electricity it's just like a chain... the weakest link is where the problem will develop.