A Personal Emergency Response System (PERS) is an electronic device designed to let you summon help in an emergency. If you are a disabled or an older person living alone, you may be thinking about buying a PERS (also called a Medical Emergency Response System).
How a P.E.R.S. system works
A PERS has three components: a small radio transmitter (a help button carried or worn by the user); a console connected to the user’s telephone; and an emergency response center that monitors calls.
When emergency help (medical, fire, or police) is needed, the PERS user presses 
the transmitter’s help button. It sends a radio signal to the console. The console automatically dials one or more pre-selected emergency telephone numbers. Most systems can dial out even if the phone is in use or off the hook. (This is called “seizing the line.”) Most PERS are programmed to telephone an emergency response center where the caller is identified. The center will try to determine the nature of the emergency. Center staff also may review your medical history and check to see who should be notified.
If the center cannot contact you or determine whether an emergency exists, it will alert emergency service providers to go to your home. With most systems, the center will monitor the situation until the crisis is resolved.
Transmitters
Transmitters are light-weight, battery-powered devices that are activated by
pressing one or two buttons. They can be worn on a chain around the neck or on a wrist band, or they can be carried on a belt or in a pocket. Because the transmitter is battery-powered, the batteries must be checked periodically to ensure they work. Some units have an indicator to help you know when to change batteries.
The Console
The console acts as an automatic dialing machine and sends the emergency alert through the phone lines. It works with any private telephone line and generally does not require rewiring. If you have more than one phone extension, a special jack or wiring may be required to enable the console to seize the line.