This is a tough one... but to me, the electrified fence that the Warden tells Bernard and Lenina about symbolizes technology separating the "civilized" world from the "savage" one.
To me, this book really emphasizes the way in which it is technology that has changed the society and made it into the brave new world. That means that it is technology that stands between the old ways and the new ways. I think that it is important the fence is electrified -- that shows how technological it is in nature.
So I see the fence as a symbol of what it is that distinguishes the savage society from the civilization.