following up on the eruv answer, which explained that chazal created the karmelit restrictions so people would not get used to carrying and end up carrying in a real reshut harabim.
if most modern city streets are considered a karmelit and not a real reshut harabim d’Oraita, what exactly makes them a karmelit?
and once an area is a karmelit, does a simple tzurat hapetach eruv always work, or are there still situations where it would not?
Thank you Rabbi, thank you very much.
The short answer is that an area becomes a karmelit when it is not a reshut hayachid and does not meet the requirements of a true reshut harabim deOraita.
But once you ask exactly which streets qualify, how width is measured, what counts as open from end to end, what 600,000 means, what natural boundaries count, what breaches invalidate an enclosure, and when a tzurat hapetach works, you are no longer asking one simple Halachah. You are asking for a substantial part of Masechet Eruvin, the Rambam, Shulchan Aruch, and the Poskim.
There is no real shortcut to that.
In very general terms, a properly constructed tzurat hapetach can be used to enclose areas that are karmelit. But that does not mean that any wire placed around any neighborhood automatically creates a valid eruv. The underlying status of the streets and the validity of the enclosure still have to be determined.
That is about as far as I want to take it here.
This website is meant to give practical halachic answers and explain the basic concepts. If someone wants to understand exactly why one street is karmelit, another may be reshut harabim, and a particular eruv works or does not work, the proper way is to sit down and learn the sugya.
There is a whole Masechta Eruvin for a reason.