I installed a new conventional residential 200 amp service today. The service is in the house with an external main shut off (breaker) outside, as well as the main breaker at the service panel inside. I was told that I need to isolate the neutral and ground bars, which apparently (though it wasn't explained to me at the time) is because there is a remote shutoff ahead of the main panel. My question is; why would this make a difference? In other words, if I am understanding this correctly, if the feed from the meter went directly to the main breaker in the service panel, the neutral bar and ground bar would be common, or tied together. But isolated if as above. Appreciate it if I knew why. Thanks!

First off, who was supposed to explain this to you. This is something you should know before going at a big job like this. Codes are important.

It matters because the neutral conductors are current carrying. This means there is current (NOT voltage) on them any time there is a load. This is why an open neutral causes a circuit to no longer work. Thing is of this happens then the current is looking to go back to it's source. If this were tied together with the grounds then it is possible that the grounding conductors and metal parts would be carrying this current back to the source.
At the main panel/disconnect there is only one path back to the source, the service cable, which as we know is designed to carry current.

Forget that the bars are connected in the main. It's what happens to them, and their function after that that matters.

My contractor installed panel has the neutral and ground tied together. Can your above scenario not happen there?

First off, don't reply by using the "agree/disagree" ratings. That is for a different purpose. For those that care about it getting a "disagree" rating lowers their reputation number.


Second, I have no idea what you mean with your last question.
If the electrician installed a main disconnect outside you MUST keep the grounds and neutrals separate on any panels downstream of that disconnect.