Mode 2 Suppressor Dangers Exposed!

Ordinary powerline surge suppressors can slow up networks, disrupt data flow, add noise to audio/video signals, trip GFCI circuits and even damage the very equipment they are supposed to be protecting!

This page exposes these dangers.

Powerline surge suppressors operate in one of two modes (U.S. Government definitions)1:

  • Mode 1:  Normal Mode (Line to Neutral suppression) or
  • Mode 2:  All Modes (L-N, L-G, N-G).

To the novice, “all modes of protection” may sound preferable to “only one mode of protection,” but beware:

Mode 2 products (all modes) have several sinister side effects which can degrade and endanger your system. Two of these sinister modes (L-G and N-G) divert surges to the ground wire, supposedly protecting from “common mode” surges.

These “common mode” circuits are the root cause of system problems, however!

  • There are basically two types of surges, “normal mode” and “common mode.”

To claim “common mode” protection, surges are diverted to the ground wire, which can have disastrous results. Fortunately, only normal mode protection is needed because:

  • Only normal mode surges enter buildings in the U.S. since the neutral and ground wires are bonded at the service entrance. These surges may be as large as 6,000 Volts/3,000 Amps with 90 Joules2 of energy. Energy, not voltage, causes damage.
  • Common mode surges exist only at extremely low energy levels well within a building (0.17 Joule for worst case surges according to American National Standard ANSI C62.41 — formerly IEEE 587, page 47)2,3.
  • Modern equipment is inherently immune to common mode surges.
  • Mode 2 (all mode) products divert surges to the ground wire, where experts know the diverted surges can disrupt audio, video, data and communications signals and also damage interconnected equipment4. Slow network operation can result as well as “lock-ups” and generally poor system reliability, which results when low level circuits are bombarded with surges diverted to the ground wiring.

Before risking performance degradation and endangering data and signal ports by using “all modes” circuits, how can we tell if common mode surges pose any real danger? To evaluate the common mode risk, we can look at extensive field experience.

First, there are many computers in North America running on 208 Volts, where neither power line is grounded. There are ships at sea, and the experience in Europe, where diverting surges to safety ground is considered too dangerous and is prohibited.

All these ungrounded applications have vastly greater “common mode” surge exposure2 than the grounded 120 Volt systems in the USA, yet they do not experience “common mode” surge damage. Why?

  • For safety reasons, UL5 requires power supplies have a 2,000 Volt minimum “dielectric withstand.” This means they must withstand a common mode voltage of 2,000 volts or more with no ill effects. Safety concerns dictate very high common mode immunity levels be built into power supply designs.
  • Modern power supplies simply ignore common mode surges, as demonstrated in APC technical note #T14: “Therefore, the total noise and transient attenuation from input to output (of a modern power supply) must be on the order of 10,000,000,000.” Therefore, the worst common mode surge (in the U.S. according to American Standard ANSI C62.412) will be reduced to microvolts at the power supply output! Microvolts will do no harm!

While tiny common mode surges can occur (only well within a building), the WORST CASE common mode surge (ANSI C62.41, page 47) is an extremely low energy Ring Wave of only 100 Amps (only 0.17 Joule3 into a 200 volt load). This surge is so weak it poses absolutely no risk to equipment!

Evaluating the common mode risk:

  • Common mode powerline surges do not enter buildings in the U.S.
  • Internal common mode surges in the U.S. have very low energy and pose no threat.
  • Electronic equipment is inherently immune to common mode surges.

We can readily see the common mode surge risk is not just low, it is nonexistent. Diverting surges to the ground wire, however (as with all mode suppression), poses many real dangers:

APC Technical Note #T3 states, “One unfortunate side effect of most (Mode 2) surge suppressors is that they act by diverting surges into the building ground system. …. (Mode 2) Surge suppressors can create intersystem ground noise and therefore interfere with or damage data communication equipment.”

Diverting surges to the ground wire to claim “all mode” protection can slow up (interfere with) network operations and damage interconnected equipment!

Since surges only enter a building in normal mode because of neutral and ground wire bonding, only Mode 1 protection is needed.

Normal mode surges can be as large as 90 Joules of energy, 6kV and 3kA inside a building, but even worst case common mode surges are a tiny 0.17 Joules2,3, 530 times smaller!

Evaluating the risks of “all mode” products:

  • Disrupts data and communications.
  • Introduces noise into audio / video signals.
  • Degrades or damages data ports.
  • Can fail to ground, posing a safety risk.

Added benefits of “all mode” products:

  • No added benefits over normal mode; just added risks.

A choice of modes must be made; choose wisely:

Mode 1 (Line / Neutral) suppression (required).
Mode 2 (All Mode) suppression (risky).

A Rochester University Electrical Engineering professor asked: Why would anyone make a mode 2 product?

While puzzling to an engineer, a clever marketer can say mode 1 products “have only one protection mode,” while mode 2 products have “all three protection modes.” This sounds compelling to the novice – until they learn there are no benefits – only dangers!

Until the disruption, degradation, safety concerns and damage from mode 2 products is common knowledge, “all modes of protection” will continue, particularly at the novice or consumer level!

When mode 2 products cause data port damage, a salesman can also sell data port protectors!

Mode 2 is a win-win for the marketers.

Mode 1 is a win-win for the user!


  1. US Government CID (specification) #A-A-55818
  2. American Standard ANSI C62.41: call 908-981-0060.
  3. The equation for Ring Wave energy delivered into a load is: E=KVIt, where K = 0.86, V= the load voltage, I = surge current and t= the Ring Wave cycle period.
  4. Noise Susceptibility in Analog and Digital Signal processing Systems, by N.A. Muncy, J. Audio Eng. Soc., Vol. 43, No. 6, 1995 June.
  5. Underwriters Laboratories (UL): call 516-271-6200

Return to

For 120V Surge Protection click here