Rabbi Eli J. Mansour writes:
Use of Blech or Hotplate on Shabbat
The Gemara in Shabbat states that the Hachamim instituted a gezeira prohibiting leaving food cooking before Shabbat on the fire during Shabbat. The Hachamim were concerned that since people are hungry and want their food cooked faster, they are likely to unwittingly stoke the coals when they see them starting to dwindle. Stoking coals would violate the Torah prohibition of cooking or burning.
To overcome this problem, the Hachamim said that one has to create a reminder for himself before Shabbat so that he shouldn’t come to stoke to coals. One type of reminder is to perform ketima- to take ashes and to spread them over the coals, covering the open flame.
Cooking in Modern Times- Today, we generally do not cook with coals. Some Rabbis, therefore, argued that the gezeira does not apply to leaving food on our gas flames, because there are no coals to stoke. However, the Panim Meirot explained, if gas stoves would have existed in the time of Hazal, they would have included it in the gezeira. Using the knobs to turn up the gas is the equivalent of stoking the coals. Therefore, he ruled that the flame of our gas stovetops are not considered katum or garuf.
How, then, can we leave food on open stovetops going into Shabbat. Hacham Rav Ovadia a”h says: If one places a metal sheet, called a blech in Yiddish, over the stovetop, it is considered katum. Just like the spreading of ashes over the fire serve as a reminder because it is not the normal way to cook, so too, putting a blech over the fire serves as a reminder not to adjust the flame.
Hacham Rav Ovadia a”h says that using a blech makes the stovetop kosher to place on it any type of food before Shabbat. This is also the opinion of the Kaf HaChaim (commentary to the Shulhan Aruch, by Rabbi Yaakov Haim Sofer (1870-1939), Rav Moshe HaLevy and many other Hachamim.
Use of Shabbat Hotplate- Hacham Rav Ovadia a”h rules that an electric hotplate, a plata Shabbat, is even better than a blech. Since it doesn’t have buttons or knobs, there is no way that one might inadvertently adjust the heat on Shabbat. Although Hacham Ovadia Hedaya (1890-1969), one of the leading Sephardic authorities of 20th-century Israel) initially disagreed, he later retracted and said the custom is to permit the use the electric Shabbat plate (Yaskil Avdi 7).
This is also the opinion of Rav Moshe Feinstein (1895-1986, Posek in the United States) in the Igrot Moshe. He takes the argument one step further and says the original gezeira would not apply to turning the gas higher. Hazal were only concerned about stoking the existing coals, not about bringing additional wood to the fire. Increasing the gas is analogous to adding new wood and would therefore never have been prohibited. He concludes that it is clearly permitted to put a blech on top of the stovetop.
Although the Chazon Ish and the Shevet HaLEvy did not approve of it, Rav Ovadia a”h rejects their proof in his first volume of Chazon Ovadia.
Using a blech or a Shabbat hotplate is the recommended practice for the Jewish home. By doing so one avoids many complicated halachic issues as to which types of foods at which stages of cooking can or cannot be left on an open flame. This preferred way enables the Jewish woman to prepare for Shabbat without worry and concern.
Summary: It is permissible to keep food on the fire on Shabbat by using a blech or an electric Shabbat hotplate. Doing so solves many Halachic problems that would otherwise arise.
Lots of people have been killed or injured over the years in fires started by blechs and, especially, platas (as we recently saw in Brooklyn).
The entire reason for using one of these dangerous devices is a rabbinic enactment called a fence (gezera) meant to stop people from mistakenly violating a Shabbat prohibition by adjusting a gas flame that arguably was not covered by the original Shabbat prohibition (which was itself made up out of nearly whole cloth by earlier rabbis).
All of this argues clearly for ignoring a rabbinic ruling that is dangerous and, quite frankly, stupid, as well.
But Orthodoxy won't do that in any formal fashion and few of its members will do so on their own, even quietly.
However there is a simple solution that could end many, although not all, of the dangers involved in having a blech or plata – and induction cooktop. Unless metal comes in contact with an induction burner, there is no heat generated. Those burners can be enclosed so there's no actual open cooking element, and simple (non-flammable) reminder placed on the controls (similar to the easily removable strips of metal or plastic many people placed over their bathroom light switches just before each Shabbat) serves as a clear reminder not to adjust the temperature of the cooking element.
Of course, rabbis – especially haredi rabbis – often reject simple logical solutions because those solutions often also challenge their authority. And rabbis advance in today's world of small minds and big hats by being ever more strict than the rabbi who lives just over there. The stricter, the more pious, the more holy – perhaps better viewed as the more you remove your followers from the other rabbis and the world around you, the more cult like their allegiance becomes, and the more control over them you have. Think, for example, New Square and its rebbe.
At any rate, electricity is only itself prohibited for Shababt use by a rabbinic fiat – a fiat based on a combination of rabbinic lies and rabbinic misunderstandings of the science behind it. Two of the proponents of these lies and misunderstandings were the last to Chabad-Lubavitch rebbes. One, Yosef Yitzchok Schneersohn, did what he did to fight modernity. His son-in-law Menachem Mendel Schneerson, did what he did in part to honor his father-in-law's request and in part to save Jews from assimilating at time when so many were being murdered by Hitler.
But the reality is that, as Rabbi Shlomo Zalman Auerbach famously noted, electricity use on Shabbat is really less of a problem than violating a rabbinic prohibition. In and of itself, electricity use is permitted on Shabbat. It's only the common custom pushed into place by rabbis (against all facts and logic) that makes it forbidden now. Auerbach told medical professionals who had to work on Shabbat not to worry overmuch if they had to use electricity, but he wanted them to be careful with two aspects of electricity usage: generating light and generating heat. In the case of cooking, a covered element and a reminder placed on the controls not to adjust the heat level would completely comply.
That means Jews should be able to use an induction burner with a small reminder strip over the controls – a set up far safer than anything Jews use today.