May one on Erev Shabbos, or on Shabbos, place food which is wrapped in tinfoil on top of a pot that is on a flame, Blech or electric plate?
If the wrapping is placed in order to guard the heat of the food, then it is forbidden to do so even before Shabbos. If done in order so the food does not spill or fall or get dirty, then it is permitted to do so, although one must be careful to not wrap it more than needed for this purpose. Thus one may not wrap his challah in tinfoil even on Erev Shabbos and place it on any source of heat, unless it is being wrapped to prevent it from getting dirty and the like.
 Shabbos Kehalacha Vol. 2 page 41; Piskeiy Teshuvos 258:4
 Igros Moshe 4:74 Hatmana 3; Rav Shlomo Zalman Aurbach brought in Shabbos Kehalacha ibid; See Mahdurah Basra 259; Pnei Shabbos 12
 Rav Shlomo Zalman brought in Shabbos Kehalacha ibid; Maor Hashabbos 1:9; Koveitz Mibeis Levi 6:36; However, see Igros Moshe ibid from which it can be learned that even in this situation it is prohibited.