Normal Mode vs.
Common Mode Suppression

An explanation.

When researching methods of surge protection, one concept that you may find is the idea of surges or noise that can follow one of two paths:

chevronThe normal path (i.e. Normal or Differential Mode) along the Line (Hot) to Neutral circuit

chevronThe common path (i.e. Common Mode) which is a path connecting the Line to Ground or the Neutral to Ground.

The US Government1 defines the operation of surge suppressors in one of two modes:

chevronMode 1: Normal Mode only (Line to Neutral suppression)

chevronMode 2: All Modes (Both Common and Normal Modes)

Zero Surge Series Mode Filter Technology operates on Normal Mode surges only. Common Mode protection is not required to protect equipment from surge damage. Therefore, looking for a product that claims “All Modes Protection” or “Mode 2 Protection” is not required and actually can cause more harm than good.

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

chevronThe Neutral line and Ground line are bonded at the service entrance which prevents external Common Mode surges.

chevronCommon 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.

chevronModern equipment is inherently immune to Common Mode surges.

chevronBy sending surges to the ground line, the voltage rise on the ground can disrupt audio, video, data and communications signals and also damage interconnected equipment.4

Practical Evidence

Some of this information may seem like it’s too technical and you are not sure are based on real world experience. Consider that there are many computer systems today running on 208 Volt three phase systems which are not grounded. European power systems and Maritime power systems also exist where sending any surges to the ground line as too dangerous and is prohibited.

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

chevronFor 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 damage. Safety concerns already dictate very high Common Mode immunity levels be built into power supply designs.

chevronModern 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!

What about Isolation Transformers?

Many claims made for the need of Common Mode protection is so someone can sell you an Isolation Transformer or other such power conditioner. These type of devices can reduce the signal of a common mode surge but you now know the Common Mode surge risk is not just low, it is nonexistent.

For Normal Mode surges, an Isolation Transformer will let all of the surge energy pass with little or no attenuation. This happens because Isolation Transformers are designed to pass through the power frequencies that most of the surge energy contains. An Isolation Transformer will only provide effective protection against Common Mode surges provided the peak voltage does not exceed its insulation rating.

chevronFor 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 damage. Safety concerns already dictate very high Common Mode immunity levels be built into power supply designs.

chevronModern 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!

Fortunately, only Normal Mode protection is needed because:

Only Normal Mode surges enter buildings in the North America 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. Since surges only enter a building in Normal Mode because of neutral and ground wire bonding, only Mode 1 protection is needed. A choice of modes must be made; choose wisely:

chevronChoose – Mode 1 (Line / Neutral) Suppression Products!

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