I am getting ready for yom tov and got a new shaver, a braun series 7-740s. Does anyone know if those shavers are kosher? Should I return it?
Based on Koshershaver.org, there is a very, very, good chance that your braun 7-740s, as well as any other microfilm shaver, poses a halachic issue, and you should replace it with something else. There is much debate on this topic, and you should definitely consult your personal rav. However, it is quite likely that he will advise you to get something else - not microfoil.
Using Micro-Screen Foil Shavers
The Terumas Hadeshen, quoted by the Rema, maintains that when shaving with scissors, a person should be extremely careful to either hold the bottom blade still, using only the top blade to cut his hair, or to use scissors where the bottom blade is incapable of cutting by itself. The reason for this is because extra precaution should be taken to prevent mistakenly shaving solely with the bottom blade, even a minute amount, since this is equivalent to using a razor.32
There is a possibility that in micro-screen foil shavers, the micro-screen foil itself has cutting edges that can cut by themselves. This screen is comparable to the bottom blade of scissors, since it touches one's skin. This might be problematic according to the Terumas Hadeshen’s chumrah, requiring one to take the necessary precautions to be certain that there is no possibility that the blade touching one’s face will cut by itself.33
In fact, the company that manufactures the micro-screen foil shavers claims that the foil itself has the ability to cut. The shaver is marketed as having cutting edges which are approximately twenty times thinner than a razor, and are very sharp. Their claim is that it is actually the outer screen that does the cutting by itself at skin level, and the inner mechanism just grinds the hair once it is already detached and inside the shaver. If this is indeed the case, such a shaver would be no different than a razor, since it uses one blade to cut the hair flush against the skin. Therefore, one must ascertain that his shaver has a screen that is incapable of cutting by itself.34
They also specifically mention braun shavers on their FAQ page:
What about other NON Philips Norelco (Remington, Braun, etc.) electric shavers? ... It is important to stress that ALL foil and 'Microscreen' shavers, (for example: ALL the BRAUN brand shavers, the ENTIRE Remington MicroScreen line of shavers) as well as other brand 'FOIL' shavers, may also raise serious Halachic issues. Rabbi Avrohom Blumenkrantz ZT'L writes the following in his Pesach sefer "...The new problem being discussed today is that the micro screen shields are so thin that they on their own have a cutting ability. We tested it and found out that if you bring a hair through the holes in the micro screen shield and by just moving the hair without to much pressure it could cut the hair. This is true for any brand of Micro Screen shield. It is interesting to note that Remington Dual Foil Shaver, it is advertised as such: “Microscreen CUTTING system with two ultra thin flexible foils and 112 diamond hooked Cutting edges” The problem is two-fold: 1) the outside screen can cut without the scissors action and 2) both blades are cutting and based on what we wrote above it is not good." SEE BELOW FOR OTHER HALACHIC OPINIONS