According to R' Yehoshua ben Levi, one should place a candle in the Sukkoh (small) to demonstrate the disqualifying of it.
If as mentioned earlier that reason for this Psul was due to creating a danger, how could this be contemplated ?
Sholem, UK
Sholom Rav.
I had the same problem as you. Initially, I did not find an answer and was surprised that the Poskim and the Meforshim do not comment on it. So I thought that, bearing in mind that if one has Kavanah not to be Yotzei was does not transgress Bal Tosif anyway, and that placing the lamp (which can be a very small one) therefore means without actually lighting it - the idea being that one is merely stressing the fact that one does not have the intention of fulfilling the Mitzvah of Sukah.
However, the Gemara and the Rambam specifically say that one lights it (interestingly, the Shulchan Aruch does not!). Then I figured that perhaps, based on the same logic, one is only obligated to leave the lamp in the Sukah for only a short time, after which one may remove it - though admittedly, that is a Chidush which the Meforshim would have mentioned.
A short while ago, I found the Me'iri who (following in the footsteps of the Ba'al ha'Ma'or) Explains that it is not necessary to light the lamp - because the mere fact that it is in the Sukah - which, as the Gemara explains, is exactly seven by seven Tefachim, renders it Pasul by virtue of the fact that it detracts from the minimum Shi'ur Sukah, and the fact that the Gemara mentions lighting it is La'av Davka - and it does so because one normally brings a lamp into a room in order to light it.
Wishing you and yours a Kesivah va'Chasimah Tovah
Your's sincerely
Eliezer Chrysler.