Ordinary power line 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!

Power line 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 C62.4--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 equipment. 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 micro
  volts at the power supply output! Micro volts 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 power line 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!

References:

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