Common Mode Protection

AC power surges and interference can enter electronic systems through two different pathways or "modes”. Power conditioning devices are designed to be comprehensive solutions to such problems while single purpose surge protectors such as series mode devices are designed to be functional for a limited set of AC power problems.

Normal Mode power disturbances enter the computer using the path provided by the line (hot) and neutral conductors. Normal mode disturbances are most often associated with lightning, utility grid switching, facility load cycling, and similar events. Large normal mode events are relatively rare but can cause power supply or other hardware failures.

Common Mode power disturbances occur between ground and either line or neutral. Such events occur frequently. They may not be as dramatic or destructive as normal mode surges. But, their affect on electronic equipment operation can be significant.

Juice Goose RX Series power conditioning products protect against major voltage spikes and surges as well as more frequent and less visible ground line (common mode) disturbances. Ground line protection is important because very low levels of common mode interference can disrupt the operation of a digital processor based electronic equipment such as communication equipment, digital signal processors (DSPs) and computers. Unreliable operation, lockups, and "soft” failures occur far more frequently because of ground line interference than hardware damage from normal mode surges.

Power problems affect your system in three different ways:


A large power disturbance literally "blows up” or destroys a trace on a circuit board or a semiconductor device (e.g. transistor or integrated circuit). Destructive disturbances can enter an electronic system via either normal or common mode, but are most often associated with normal mode pathways. Destructive power disturbances are the easiest to identify since they leave visible evidence of their occurrence – smoke, soot, and charred components. Establishing cause and effect is easy with destructive events.


Lower amplitude power disturbances enter an electronic system and affect a semiconductor at a microscopic level. Solid state junctions are designed to operate at small voltages. Degrading power disturbances often exceed the voltage tolerances of the material. Erosion of the semiconductor material occurs (much like rust damages metal) leaving less of the semiconductor material to handle the current needs of the circuit. Higher operating temperatures are the result. Damage from degrading events tends to be cumulative. Eventually so much damage occurs that the semiconductor junction experiences "thermal runaway” and fails. Establishing cause and effect may be difficult or impossible since there are no visible failure indications.


Binary logic of digital processors operates on strings of 1s and 0s based on very rapid fluctuations of low voltage. Very small common mode power disturbances can mimic actual logic signals causing the computer system to make incorrect decisions. Disruption also occurs when neutral to ground voltages become excessive.

The semiconductor industry published recognized guidelines on the maximum levels of normal mode and common mode (ground) power disturbances that should be present in the semiconductor environment. Their guidelines clearly state that power disturbances up to those described in ANSI-IEEE C62.41 (maximum 6000 volt) must be reduced to less than 10.0 (ten) volts for normal mode surges and 0.50 (one-half) volt for common mode surges.

The only way to limit power disturbances on normal and common mode to the levels recommended by the semiconductor industry is through the use of a specialized power conditioner. Ordinary surge protection devices, even so-called "series mode" devices, do not have that capability.