SETUP: 200 AMP Main Panel connected to a 100 AMP sub-panel via metallic nipple. A 75 AMP breaker in the main panel feeds the sub via #4 conductor. The Neutral is isolated from the ground in the sub. Bare grounding conductors are attached to the subpanel through an equipment bar attached to the metallic case of the sub.
Can someone explain to me why the NEUTRAL and GROUND must be isolated in the sub-panel since they are already bonded in the Main panel. Since the ground in the SUB is derived from the link of the two metallic enclosures, isn't the NEUTRAL in the SUB already bonded to the ground???
What happens if the NEUTRAL AND GROUND are also bonded in the subpanel?