Everybody should take fire safety courses. I, as a fire safety instructor, leave on my oven on keep warm every Shabbos and Yom Tov. Never had a problem. There might be problems with leaving flames on (blech) or hot plates on. I think a fire under a blech is probably safe just for a Shabbos. I don’t know about hot plates even for a Shabbos. If you need hot food for Yom tov -use either a oven that is kept on or a crock pot.
