If one wishes to leave food that is not fully cooked in an electric oven on Friday afternoon, he must do something to remind himself of Shabbat, to ensure that he will not turn up the heat on Shabbat. One option is to place a metal strip or thick aluminum foil between the pot or pan and the heating element. Since this reduces the heat reaching the food, he will remember that it is Shabbat and not raise the oven temperature. Alternatively, he may cover the oven knobs.
There is an additional problem with using an oven. If the oven has a thermostat, some maintain that one may not open the door of the oven because this may result in the heat cycling on (below 17: 8, and n. 8). In order to allow oven use according to all opinions, many ovens have a special setting for Shabbat. This setting bypasses the thermostat and guarantees that the oven maintains a constant low temperature unaffected by opening the oven door.
One may use an electric urn, but it is preferable to make sure the water boils before Shabbat. One should use an urn that does not have a thermostat. If it does have a thermostat, one should try to remove water only when the heating element is on (see below, ch. 17 n. 8). One may not use a device that automatically fills with cold water to replace the hot water that is removed, since that cold water is then cooked (section 24 below).
In principle, as long as one covers the oven knobs, he may leave raw food in the oven before Shabbat begins, and set a timer to turn on the oven an hour before the meal, so that the food will be ready in time. Similarly, one may put flour and water and other ingredients into a bread machine if its knobs are covered, and set the timer to turn on the machine, so that the bread will be kneaded and baked on Shabbat morning and finished in time for lunch. Indeed, some permit this in practice. Others prohibit it absolutely, fearing that it would lead to people actually cooking on Shabbat.
. In the time of the Mishna, ovens were very hot, and there was no permissible way to leave inadequately cooked food in them. Since a kira was not quite so hot, the Sages allowed it to be used on condition that the coals were either removed or covered (Shabbat 38b). The ovens that people have in their homes today, whether large or small, are not as hot as the ovens of old, so the laws of today’s ovens correspond to those of the kira (Rema 253:1; MB 253:28). However, the concern that one might “stoke the coals,” that is, raise the flame of the oven, still exists. To neutralize this concern, the Mishna suggests that one cover the coals with ashes in a way that will reduce their heat. In an electric oven, this is accomplished by placing a sheet of heavy foil or metal over the heating element, separating between it and the pot. This is the suggestion of Shvut Yitzĥak 2:7:3 in the name of R. Yosef Shalom Elyashiv. Az Nidberu 8:16 and Or Le-Tziyon 2:17:4 maintain that it is sufficient to place something between the bottom of the oven and the pot, and one may rely upon them (taking into account the lenient position presented above in n. 13). The second option is to cover the knobs. Even though this is not the same as covering the coals, nevertheless in practice it will remind people not to turn up the flame. (In our case one can more easily rely on covering the knobs, since in any case the fire is covered.) This also seems to be the opinion of Har Tzvi, OĤ 1:136; Yabi’a Omer 10:26:1; Shemesh U-magen 2:62; Menuĥat Ahava 1:3:7; and R. Yitzĥak Halperin. Orĥot Shabbat 2:15 permits using the oven if the knobs are taped in such a way that it is impossible to move them. We have already seen in n. 13 that some are even more lenient since we are not concerned that the fire will go out, but their opinion is not followed in practice.. The water must be boiled before Shabbat for two reasons. First, some say that an urn is considered an uncovered fire, and one may not leave water on an open fire to cook on Shabbat (SSK 1:46; Hilkhot Shabbat Be-Shabbat 1:5:26; Orĥot Shabbat 2:32). However, if it is impossible to raise the heat, then according to Minĥat Yitzĥak 5:91 and Shevet Ha-Levi 5:30 this is permitted. If one covers the knob that controls the heat, then many maintain that there is no problem of hashhaya, as explained above in n. 16. Even when one can turn up the heat, some are lenient, as mentioned in n. 13. Second, if one removes water from the urn before it is boiled up, he causes the remaining water to cook more quickly (Hilkhot Shabbat Be-Shabbat, loc. cit.; Orĥot Shabbat ch. 2 n. 39). If the urn’s spigot is at the very bottom, one may not use it. This is because when the water runs out, one might add cold water to prevent the heating element from burning out, as MB 318:68 mentions. Ĥut Shani 26:5 and Avnei Yashfeh 5:50 forbid using an urn even if the spigot is higher up, out of concern that the water might still get used up if one tilts the urn or if water evaporates on its own. But the rest of the poskim we mentioned do not feel it necessary to worry about this.
. Tzitz Eliezer 2:6 and Minĥat Yitzhak 4:26 forbid leaving fully cooked food on a plata before Shabbat when the plata has not yet turned on, because one may come to place food that is not fully cooked on the plata on Shabbat, and thus violate a Torah prohibition. In contrast, Melamed Le-ho’il §58 and SSK 1:32 permit placing fully cooked food on a plata before Shabbat if the plata is off; but they forbid putting on food that is not fully cooked, lest one place such food on the plata on Shabbat. Those who permit putting food that is not fully cooked on a plata that is off on Friday include R. Shlomo Zalman Auerbach (Minĥat Shlomo 2:34); Or Le-Tziyon 2:30:18; and R. Ovadia Yosef (Yabi’a Omer 10:26, where he mentions other Aĥaronim who rule similarly). The same law would apply to a bread machine as well. See Harĥavot.