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Is it ok to have mixed grounds and neutrals on bars in a breaker box?

I've recently opened my breaker box to connect a new line. I found that all the wires that go into the house are connected such that hot is interrupted through the breaker, and both neutral and ground are connected to the same (very long) terminal strip. I do have real ground (earth rod). The wire from the rod is just connected to the same strip.

My understanding that it's best if the ground is separate from neutral. How concerned should I be about this, i.e. how urgent is it to separate ground from neutral, if should be done at all? There is this question : Is it ok to have mixed grounds and neutrals on bars in a breaker box?, where the answer is that ground and neutral should be connected together, but only at the main panel. But I would like to understand WHY as well.

Also, I thought the latest code requires that all three lines are interrupted by the circuit breaker, how "bad" is it if only hot wire is interrupted?

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    It only has to be separated in a sub panel. In the main panel the neutral and ground bus bars should be bonded, so neutral and ground wires can be connected to either bus. – Tester101 Aug 6 '12 at 19:39
  • Take a look at this Question. – Tester101 Aug 6 '12 at 19:42
  • From my understanding of the code, all lines only need to be interrupted if they are from a separately derived system. Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.* – Tester101 Aug 6 '12 at 19:53
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    +vote @ Tester: should put your comment as the answer, it's correct. – shirlock homes Aug 6 '12 at 20:49
  • @Tester posting answers as comments breaks voting, editing, accepting, the unanswered question list... – Brad Mace Aug 6 '12 at 20:57

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