The basic prohibition of Shabbat is creation. Thus, it is forbidden to create something new on Shabbat, but there is no prohibition to "prolong an existing situation." The Zomet Institute uses this principle for devices which can be turned on for a short time at regular intervals. In this case, a person simply "prolongs the existing activity." In principle, such action is permitted not only for essential activities but without any limits.
It can be proven from the discussions of various Shabbat labors that the activity is permitted if a person does not take any action or create anything new and only continues the existing situation. A good example is the Mishna in Shabbat (106b) permitting the "continuation" of keeping an animal trapped (such as a deer) by closing the entrance of a room if the animal was captured by another person. Preventing the animal from going free is not capture, even if the new person arrives just when the first one leaves the room.
There are some people who open a refrigerator door on Shabbat only if the motor is on (most people are lenient about this, and they are acting in accordance with generally accepted principles), in order not to have any direct influence on the motor. But when they do this, they are prolonging the existing operation of the motor. However, as noted above, this does not present a problem.
Rabbis allow moving a mechanical setpoint of a Shabbat timer in order to extend the existing situation. (It is recommended to try to avoid doing this and certainly not to plan for it in advance so as to prevent moving the timer in the opposite direction by accident, since any action that shortens the existing situation is prohibited.)
The Zomet Institute has developed an electrical mechanism for "continuing the existing situation" which works as follows (using a refrigerator as an example): Every ten minutes, the compressor is turned on automatically for about ten seconds, no matter what the temperature is. This cycle is repeated throughout Shabbat. This short operation of the motor has no significant effect. If somebody causes the temperature to rise by opening the door, nothing happens. The only change occurs when the motor goes on. In this case, it will continue to operate until the desired temperature is reached.
Supplying Hot Water on Shabbat
, in the tap at home, in an institution, in a hotel, or in a mikveh (ritual bath). On Shabbat, the temperature is limited to 40 degrees C, so that incoming water is not considered to be cooked from the halachic point of view, since this temperature is definitely lower than that which "scalds the hand." But there is still a problem of causing the thermostat to turn on the heating element. If water is used, the water in the tank will cool down when new water is added. Thus, a person who takes water is liable to directly cause the heat to go on, or at the very least cause the heat to go on sooner than it would have. (This is a case of gramma
, which is prohibited for nonessential use.) The "continued status" mechanism solves this problem. No matter what the temperature of the water is, the heat is turned on for a few seconds in regular cycles, every ten or twenty minutes. If water is used, so that the temperature decreases, nothing happens immediately. But when the next cycle of heating arrives, the mechanism will "continue" the heat for as long as needed.
Nurse Signaling System in a Hospital:
From a halachic point of view, there is a difference between turning on an incandescent bulb, which is considered a "flame" in halachic terms, and electronic light, including a LED
, which is not included in the definition of a "flame." Call systems for a nurse usually have incandescent bulbs in the hallways, in order to mark the room of the patient who called for help. The patient pushes a gramma switch
to alert the nurse's station. But the incandescent bulbs in the hallway operate under the principle of "continuing an existing status." All during Shabbat, the lights in the hallway are turned on for a few seconds every half minute. Nothing happens immediately when a patient presses a signal button. But at the end of a cycle, when the light goes on, it will remain lit as a signal to the nurses. And then the "existing situation" is reversed. The signal light stays on for thirty seconds and goes off for a few seconds. When the nurse presses the "cancel call" button in the room, nothing happens at first. Then, when the cycle reaches its end and the bulb goes off, it will stay off, once again reversing the cycle.
Pressure Regulating Pump:
This type of pump is normally installed in tall buildings in order to maintain a high pressure in the upper floors. Known as a hydrofor, the equipment consists of a small tank, a pressurizing pump, and a pressure gauge. The water in the tank is pressurized so that it will reach the highest floors of the building. When a person uses water, for example for washing dishes on Shabbat, the pressure in the tank falls, and the pump is turned on, returning the pressure to its desired value. The Shabbat installation includes a "continuing status" mechanism. Every fifteen minutes, the pump is turned on for five seconds, no matter what the reading is on the pressure gauge. When somebody lowers the pressure by using water, nothing happens at first. But then, as the cycle reaches its end and the pump is turned on, it will "continue" running until the desired pressure is reached.
This is a pump used to raise water from a low level into the local sewage system. In houses where a toilet or a sink has been installed in a cellar, below the level of the sewage system, the waste water is collected in a tank with a level control (either electrodes or an electrical gauge). When the water reaches a predetermined level the sump pump is activated, sending the waste into the local sewage system. But what about Shabbat? In this case, the device is set to automatically go on every 5 or 10 minutes for a few seconds. When a person fills the sump tank, nothing happens at first. But when the pump goes on at the end of the current cycle it will stay on until the level of the tank is decreased.
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