An ambulance that is used on Shabbos for cases of Pakuach Nefesh has on it a camera and screen in order to see behind the ambulance to make driving safer. However this function is only sometimves needed. When the ambulance is turned on the screen doesn’t turn on by itself, one must turn it on manually. Is it better Erev Shabbos to turn on the screen in order to avoid doing more malacha on Shabbos. Even though this might be a problem of ribui shiruim since really it’s not always needed and only a sofek that it might be needed. Or better to not turn it on Erev Shabbos and avoid the issue of ribui shiurim and if it needs to be used on Shabbos then to turn it on normally (even though this malacha perhaps could have been avoided.) In short is it better to make ribui shiurim b’makom sofek in order to avoid extra malachos.
Answer:
If there is no need to turn on the screen, this must not be done, because it is an extra, non-essential melachah.
The screen should also not be turned on before Shabbos, because by igniting it will go on, and this is a non-required melechah on Shabbos.
See Shemiras Shabbos Kehilchasah 32:67.
