To many of us, the utility power grid is a vast system of
unknowns. Its performance can make unprotected electronic
equipment useless. Why? Because grid voltage values, higher or
lower than guaranteed nominal values, have an effect on
electronic intelligence processing equipment. Incoming grid
power “sees” the equipment's DC power supply, which bears the
brunt of any AC grid voltage variation.  The impact of voltage
sag  We can hardly assume our electronic hardware operates from
a distribution network with zero internal impedance, receives a
pure undistorted sine wave, and never sees line voltage
variations of 55% from nominal. Yet that's exactly what many
electronic system manufacturers think when designing their power
supplies.  The combination of utility- and locally generated
disturbances results in no such modest limits. Most utilities
are permitted line voltage reductions (brownouts) to cope with
seasonal demands. In addition, large motors accelerating high
inertia loads, spot welding, and other loads a