A temporary partial loss of supply voltage in an electrical system. It’s akin to the brownout, which is a (often controlled) power reduction where the utility company decreases the voltage on the power lines, so customers receive weaker electric current. Brownouts can be used if total power demand exceeds the maximum available supply, though the typical household often does not notice the difference. Voltage sag is a similar result, but can be more dramatic, of shorter duration, and is not controlled. Voltage sags are usually caused by turning on some device that requires high current to start up. In the second it takes to start up, the supply voltage to other equipment momentarily dips because the source of power either becomes momentarily overloaded or simply can’t react quickly enough to the increased power demand caused by the starting device. It is not uncommon for the supply voltage of a device or electrical system to drop when great power (and thus current) demands are place on it. This is what you are observing when you notice the lights briefly dim as you turn on your vacuum cleaner. Turning on a large power amp often causes the same behavior. Minor voltage sags are usually harmless to most equipment, but if severe enough can cause problems, particularly with delicate equipment.