The primary function of a surge protector is to limit let-through voltages
to acceptable levels. What is acceptable?
The CBEMA guideline, created by the Computer Business
Equipment Manufacturers Association
is a realistic, at-the-equipment, maximum allowable voltage that equipment can withstand,
without damage or upset.
The Federal Information Processing Standards (FIPS) publication,
"Guidelines on Electrical Power for Automatic Data Processing Installations"
(FIPS Pub. DU294) includes a susceptibility profile (CBEMA
Chart) thats considered to be a typical design objective for computer hardware
designers. The profile shows the relationship between clamp voltage, system voltage and
The CBEMA standard essentially defines the threshold computer-based
equipment is designed to be able to withstand for short duration, < 200 microseconds ,
transient voltages if these voltages do not exceed a level that is five times the sine
wave peak voltage.
The goal is to protect electrical and electronic equipment
connected to the AC power line, limit transient voltage amplitudes to less than the CBEMA
guidelines of 5 x E rms x 1.414 volts, for <200us duration impulses. For commercial
power lines this works out to be: