A power or voltage sag is a short duration reduction in voltage. Power sags can be caused by abrupt increases in loads such as short circuits or faults, motors starting, electric heaters turning on, or they can be caused by abrupt increases in source impedance, typically caused by a loose connection. A voltage sag occurs when the voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to as long as one minute.
Power sags can also come from utilizing an incorrect transformer that is not rated for the device that it is powering, or sags may come from using a low-cost transformer that is rated for the required power for that device. Over time, depending on the quality of the transformer, the power supply can "burn in" or decay, and, as a result, the voltage provided can diminish slightly after several years of use and underpower the device. Over an extended period of time, underpowering a device is more detrimental to a device than overpowering it slightly. Depending on the quality of the power supply, this can depreciate the amount of time that your electronic device is going to last. These are typical power supply situations in which sags may occur that you want to be aware of and watch for.
With regard to access control equipment, when a power sag occurs, it is quite possible that a strike may take longer to release; on a mag lock, it may take longer to lock. A power sag can damage a mag lock or a strike just as easily as it can any other hardware.
Refer also to the following Power Management video for additional overview information about power.