8. Attaching one’s electric plate or oven to a timer over Shabbos:
A. General rule:
It is forbidden to place a food that contains a cooking prohibition [as explained in Halacha 1] on or even near a heat source that will eventually turn on and heat the food to Yad Soledes, even though it is not currently on.
B. Preparing the food on it from before Shabbos Erev Shabbos place food on his electric plate which is not yet on but will later turn on with the timer?
If the food contains a cooking prohibition: If the food is not cooked or not fully cooked or it contains liquid that has fully cooled down, it is forbidden to set up a Shabbos clock to turn on the electric plate on Shabbos.
If the food is fully cooked but the electric plate is not covered: Then if the electric plate has more than one heat setting and is not covered, it is forbidden to place food there before Shabbos if the food is cold. However, if the food is still hot, there is a dispute amongst the Poskim, and it is proper to be stringent.
Food is fully cooked and electric plate is covered: If the electric plate is covered, or has only one setting it is allowed to place on it before Shabbos fully cooked food [which will still be warm by the time the clock turns on if the food contains liquid].
Water urn: Practically, due to the above one may not leave a boiler attached to the Shabbos clock being that by the time the clock turns back on, the water has completely cooled off and will become re-cooked on Shabbos.
Radiators: The custom is to allow to set up a radiator from before Shabbos to turn on through a Shabbos clock on Shabbos, even if the radiator contains liquids inside its coils that will be heated as a result.
C. May one on Shabbos itself initially place food on his electric plate which is not yet on but will eventually turn on with a timer?
No. This applies even if the food is fully cooked and is a solid, and even if it was originally on the electric plate when Shabbos began and was then removed [without fulfilling the conditions of Chazara].
No, as in addition to the problem explained in the previous answer [in footnote there], here it would furthermore forbidden being that the fire is not covered and there is thus suspicion that one may come to higher the flame.
E. May one before Yom Tov set up his electric plate on a timer for it to turn on during Yom Tov to have food placed on it?
Yes. However, when doing so for Shabbos, such as when Yom Tov falls on Erev Shabbos, one must place the food on the electric plate with enough time for it to heat up before the entrance of Shabbos.
 See Admur 253:26 “[Although we rule like the stringent opinion regarding placing food directly on a heater, that it is forbidden to be done unless the Chazara restriction are fulfilled,] nevertheless prior to the Tanur being turned on the custom is to be lenient to [allow] to place [fully cooked] food [which if contains liquid is also still warm] on top of [the solid covering] without placing an [additional] separation. [Now], even though the oven will later be heated when it turns on, in which case there will then be a suspicion that one may come to stoke the coals, nevertheless one need not protest against those which do this being that they have upon whom to rely on.”
 Admur 253:27 “To place food near a heater which is not swept or covered, even if it has fire in it, is permitted in all cases, as is our custom as explained above, as long as the food is [still] a little hot if it contains liquid, if it is Yad Soledes by the area that is next to the heater. However, if it is not Yad Soledes there next to the heater then even if the food [which contains liquid] is completely cold, it is permitted to place it there in all cases, as explain above [in Halacha 21]. However, if one knows that the gentile will [eventually come to ] stoke the heater later on in order to heat up the house, which is permitted on Shabbos as will be explained in chapter 276, then if the heater will be heated up so much to the point that it will also heat up the food that was placed next to it to the point that that food will become Yad Soledes, it is forbidden to place the food there [if it is a liquid which has fully cooled down]. Rather one must remove it from near the heater before the gentile stokes it.”; Kitzur Dineiy Shehiyah and Hatmanah 259 “However, by cold food [which contains liquid], even before the oven has been lit, it is forbidden to even place the food next to the oven.”
 The reason: The reason for this prohibition is because this is like one who places a pot [over a stove] and someone else comes and lights it up under it, in which case although the former person [which placed down the pot] is not liable [to bring a sin offering, or get the death penalty], nevertheless, it is Rabbinically prohibited to do so. [Admur 253:27]
 Shabbos Kehalacha Vol. 1 pages 244-247; See SSH”K 1:26
This question is dealt with extensively by many different Poskim of the previous and current generation. Some, including the Munkatcher permit it entirely. Others, including the Minchas Yitzchak and Sheivet Haleivi rule that [Lechatchilah] this is completely forbidden, for the reason that it appears like belittling of Shabbos. Others, including the Chazon Ish, Igros Moshe, and Rav Shlomo Zalman Aurbach rule that if the food is fully cooked and the flame is covered, it is allowed. The ruling here is based on this latter opinion.
 Igros Moshe 4:60
The reason: This is forbidden due to Zilzula Deshabbosa, belittling of Shabbos, as it appears one is doing a Melacha on Shabbos. It is not similar to placing a non-fully cooked food on a hot electric plate from before Shabbos. [ibid]
 Shabbos Kehalacha p.247; Oatzros Shabbos in Piskeiy Teshuvos 2:59; See Igros Moshe ibid from which it is understood that certainly one is allowed to do so, just like one is allowed to ste uop the lights to the Shabbos clock.
The reason: The entire reason to forbid using a Shabbos clock on Shabbos to cook with is because of Zelusa Deshabbasa. The Igros Moshe ibid however explains that by those matters that some Poskim allow to be done through a gentile then certainly it may be done through a Shabbos clock, and there is no belittling of Shabbos applicable. Hence certainly here regarding a radiator of which the Sages permitted even asking a gentile to turn it on [being that the cold can lead to illness as explained in 266:15 and 253:28] it is permitted to set it up with a Shabbos clock. [Shabbos Kehalacha ibid] This is in addition to the fact that the water of the radiator has already been precooked and it is disputed as to whether we apply the prohibition of cooking towards it [see 318:9], and hence there is more room to be lenient.
 Shabbos Kehalacha Vol. 1 page 247
 The reason for this ruling, despite the fact that in Halacha 26 Admur brings that there are those which are accustomed to be lenient in this and one is not to protest, is because a) Nevertheless Admur agrees that one is to not do so initially as is evident from the terminology of simply not to protest and b) Back then it was indefinite whether the gentile will in truth turn the heater on while today with electricity the electric plate will definitely turn back on. [See Shabbos Kehalacha ibid]
 Rav Asher Lemel Kohen
 As cooking is allowed on Yom Tov and thus there is no real belittlement of Shabbos involved.