Question: If a time switch has been set from before Shabbos to turn the lights on and off and the lights are now on, am I allowed to extend the time switch so that the lights remain on for longer?
Introduction:
The action of extending the time switch is an action performed to maintain the current situation (i.e. the result of my action will not cause the lights to turn on earlier; rather it will only maintain the current situation).
There are two Gemoros that discuss a situation of an action that results in maintaining the current situation:
• The Gemoro in Beitzah (22a) tells us that one who adds fuel to a candle that is currently burning has transgressed the prohibition of ma’avir (setting fire). This Gemoro is ruled by the Rambam but is not quoted explicitly by the Shulchan Aruch.
The Rosh (Beitzah Perek 2 Siman 17) explains the reason for the prohibition: “…one who adds fuel to a fire is increasing the burning time; since without adding fuel to the fire the fire would have extinguished when the fuel finished, and the increase in burning time is considered as if the person adding the fuel has lit the fire”.
From this Gemoro we see clearly that one who “extends the current situation” is considered as if having transgressed a Torah prohibition of ma’avir.
• However, in Siman 277 the Beis Yosef quotes the Gemoro in Shabbos that discusses a lit candle placed in close proximity to the door, and the Gemoro writes that Rav condemned one who opens or closes the door (because this will inevitably result in stoking/extinguishing the fire). Rashi explains that the candle is behind the door and the draught resulting from opening the door may extinguish the candle.
The Beis Yosef writes that the Rif, Rosh and Rambam rule in line with the statement of Rav. The Beis Yosef also writes that the only prohibition according to Rashi is to open the door; there is no prohibition to close the door [which will prevent the candle being extinguished because of a draught from the open door]. It also appears from several other Rishonim (Tosfos, Terumas Hadeshen, Mordechai) that there is no prohibition to close a door next to a lit candle.
The Remo (Siman 277) quotes this leniency; i.e. that it is permitted to close an open door next to a candle, which will result in the absence of a draught and consequently the candle will burn for longer.
We can see from this case that to perform an action that will result in the continuation of an existing action, there is no prohibition.
Ostensibly, these two aforementioned halochos contradict one another, since they are both situations of maintaining the current situation, and adding fuel to a burning fire is a prohibition of ma’avir, whereas preventing a candle from being extinguished is completely permitted?
There is however, an obvious difference between these two situations: By adding fuel to a fire, according to the Rosh it is considered as if performing a positive action that will result in further combustion. However, by preventing a draught that will extinguish a fire, one is merely “removing the hindrance” and consequently the candle can continue to burn. “Removing the hindrance” to allow a melocho to continue on its own accord is permitted on Shabbos.
Discussion: Let us now turn our attention to adjusting time switches on Shabbos. Rav Shlomo Zalman Aurbach (in Minchas Shlomo volume 1, siman 13) writes that there appears to be no problem to extend a time switch [that is currently on] in order to prevent the lights from turning off, because the results of one’s actions only results in prevention of the flame being extinguished, and this is not a melocho.
The Shmiras Shabbos Kehilchoso (perek 13, 28) also writes that one is allowed to adjust a time switch [that is currently “on”] to cause it to switch off at a later time. He explains (in footnote 100) that this is not similar to adding fuel to a burning fire; because extending the time switch merely prevents that which will inhibit the circuit. He compares this situation to the precedent of closing a door to prevent a draught that will extinguish a candle.
Conclusion:
Various Acharonim compare extending a time switch to closing a door to prevent a draught, which is permitted on Shabbos (even if the draught would have extinguished a candle). Therefore, an adjustment to a time switch that will result in “maintaining the current situation” is permitted on Shabbos.