The Eruv is up unless otherwise noted.
Carrying is one of the 39 activities (melachot) which are prohibited on Shabbat. It is an unusual melacha in that you are allowed to carry if you are indoors. The solution is an Eruv, which broadens the definition of being "indoors" to being someplace around which there are walls. This allows people living in cities surrounded by walls to carry on Shabbat as if inside their homes. BSCTT maintains an Eruv. One may only carry in our Eruv if one checks before Shabbat to ensure that the Eruv is functioning.
What is an eruv? It is sort of a wall around the community; inside the wall, you can carry just as you would within the walls of your home, a ‘private’ domain. The 'wall' actually could be a wire, a steep hill, or a fence, and in fact the Potomac eruv is partly all these. I and others, most recently Marc Zweben, have had the opportunity to check the eruv with Rabbi Tessler over the past ~18 years, and at times it has been great fun.
Now, I'm not much of an expert on the laws of the eruv and even if I were, I don't have the space to say more than a little here. Most of the eruv around Potomac is a wire, specifically a power line or phone wire. The main rule about the wire is that there must be a structure directly under it whenever it changes direction, usually a piece of wood called a 'lechi' The lechi must be at least 40 inches long and does not need to be as tall as the wire. Fortunately, much of our eruv uses the center wire on power lines that traverse directly over the phone pole. Otherwise, we will usually designate the bottom side wire as the eruv wire. In this case, there must be a lechi attached to the phone pole directly under the side wire.
The most important fact to know about the eruv is where it is. Our eruv is sort of a triangle with the eastern edge being I-270. At the corner of I-270 and Falls road, the eruv goes south on Falls Rd, makes a left at Bells Mill, takes a right at Democracy Lane, turns left at Democracy Blvd, then left on Westlake Dr, and finally connects with the I-270 fence at the power station south of Tuckerman. If you know where the eruv is, you'll be able to appreciate how far you can actually carry in our community. More importantly, you can answer the frequent question from a friend or possible new member: 'Do I live inside the eruv?'