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Oct 1, 2004

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What to Know About Common Mode Surges and GroundContamination

Not knowing the difference between ground noise and common mode surges and

noise on power lines can cause you problems

Rudy Harford, Zero Surge, Inc. | Electrical Construction and Maintenance

According to IEEE/ANSI Standard C62.41-1991 (formerly IEEE Standard 587), Recommended Practice on

Surge Voltages in Low-Voltage AC Power Circuits, the worst normal mode surge expected inside a building

is a 6,000V, 3,000A, 90 joule (J) combination wave. A normal mode surge is one that occurs between the

hot (line) and neutral wires. This standard also states that the worst common mode surge referred

According to IEEE/ANSI Standard C62.41-1991 (formerly

IEEE Standard 587), Recommended Practice on Surge

Voltages in Low-Voltage AC Power Circuits, the worst

normal mode surge expected inside a building is a 6,000V,

3,000A, 90 joule (J) combination wave. A normal mode

surge is one that occurs between the hot (line) and neutral

wires. This standard also states that the worst common

mode surge — referred to in the standard as neutral-ground

mode — is only found far from the service entrance and is a very weak 3,000V, 100A,

0.645J ring wave. This worst-case common-mode surge is already about nine times

smaller than the suppressed surge left over after typical suppression, also referred to as

the let-through surge. See the Sidebar below for joule calculations.

How can this worst-case common mode surge, which is 140 times weaker than the

worst-case normal mode surge (90J vs. 0.645J) and weaker than the let-through surge,

be a threat? The answer is that either this surge isn't a threat or the typical suppression

isn't adequate.

Surge diversion: which path is best?. The most severe power line surges are

lightning induced. In such instances, the surge voltage is seeking an earth ground return

path. The only truly reliable return path within a building is the neutral wire.

While most modern buildings in the United States have neutral and ground wires bonded

together at the service entrance, many older buildings lack a ground wire. So when older

buildings are re-wired, compromises may occur, such as bonding neutrals at metallic

enclosures. Also, if the neutral wire connection is bad, there will be no power delivered to

the connected load.

If you have the option of diverting surges and noise to the ground or neutral wires, both

of which should return to the earth ground, you should know how much surge and noise

voltage those wires can handle in sample situations so you can make an informed choice.

Ground wire noise tolerance. Component audio and video systems are interconnected

by cables that, for safety reasons, are often connected in some way to the electrical

ground wire circuit. Their signals can be degraded by even millivolts of noise. On the

other hand, computer systems may be able to tolerate several volts of noise on the ground

system and continue to operate without problems.

So what happens when a shunt surge suppressor diverts a 6,000V surge to the ground

wire? Simple math shows the voltage will divide between the connected wires, and nearly

half the surge voltage can appear on the ground wire momentarily. This puts audio,

video, and computer circuits, which can be disrupted by as little as 1V, in the path of

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2,000V. Although this is an extreme case, it demonstrates the severe susceptibility of

these systems to electrical noise and highlights the importance of protecting them against

even the lower levels of electrical noise that may be present on the system.

Neutral wire noise tolerance. How much surge and noise voltage can the neutral

wire tolerate (common mode)? Since the neutral wire carries power and the neutral and

hot wires may inadvertently become reversed, Underwriters Laboratories (UL) instituted

the dielectric withstand, or Hi-Pot, test. For safety reasons, UL requires that both the

neutral and hot wires on electrical equipment be able to withstand at least 2,000V with

respect to ground (common mode). This mandated neutral wire tolerance (common

mode tolerance) is significantly greater than the millivolts of tolerance on the ground

wire that can degrade or disrupt audio and video signals.

A suppressor manufacturer must choose how to return the 6,000V surge back to earth

ground. Should it use the building ground wire — if the building has one — which has a

tolerance to surges and noise of only a few volts, or should it use the neutral wire, which

has a tolerance to surges and noise of at least several thousand volts? If you're trying to

protect audio, video, or computing equipment, it seems obvious that ground wire surge

diversion is less desirable than diverting the surge to the neutral conductor.

Is common-mode protection necessary?. A major UPS manufacturer has published

a 4-page technical note that shows switch-mode power supplies, as used in most modern

equipment, to be inherently immune to common-mode surges and noise, due principally

to the high-frequency transformers and filters used in these supplies. However, due to

their relatively weak nature, common-mode surges may not deserve the attention. With

split-bobbin transformer designs, it's very easy to achieve a 6,000V dielectric withstand

capability, which more than easily meets the 2,000V common-mode immunity required

by UL.

Since the ground and neutral wires are bonded together at the service entrance, common

mode (N-G) surges don't enter a building and can only develop far from the service

entrance. As previously stated, the worst-case common-mode surge at this distant

location is only 0.645J, which is 1/140th the size of the 90J worst-case normal mode

surge. As the UPS manufacturer's report shows and as mandated, modern equipment is

inherently immune to common-mode surges and noise up to 2,000V, principally for

safety reasons.

To claim “protection” against common-mode surge (neutral-to-ground surge),

manufacturers divert all surges to the ground wire. However, as stated earlier, this

procedure can lead to audio, video, and computer noise and reliability problems. This

procedure also has no benefit, since common mode surges are a non-problem due to their

extremely low energy and the fact that equipment immune to them.

What to look for in a surge suppressor. The U.S. government recognizes the

importance of using the proper mode for surge suppressors. In its power line surge

suppressor specification, CID A-A-55818, the government makes the following

definitions:

If you're looking to protect audio, video, or interconnected computing equipment, then

use a U.S. government Mode 1 product (line-to-neutral suppression only). Stand-alone

products like microwave ovens can use a Mode 2 (all modes) product, provided that no

audio, video, or computing equipment is connected to the same circuit.

If the application is important, such as the protection of expensive and/or critical

electronic equipment, you should focus on the endurance rating of the surge suppressor.

For example, a suppressor with a certified endurance rating of 1,000 worst-case surges

should last at least 10 years.

UL offers a service they call “UL 1449 adjunct classification,” whereby the firm will

certify a manufacturer's claimed performance and endurance rating for its product.

While available to any equipment manufacturer, only the manufacturers of higher

performance and endurance products have elected to use this service.

Beware the ground wire. The building ground wire is very sensitive to surges and

noise if audio, video, or networked computers are in use on the electrical system.

Therefore, it's undesirable to divert surges to the ground wire if you're looking to protect

this equipment. On the other hand, the building neutral wire isn't sensitive to surge and

noise voltage, which makes it a good choice for returning surges back to the earth

JUL 2 1 , 2 01 5

JUL 1 4 , 2 01 5

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ground.

If you're trying to protect audio, video, or networked equipment, use surge protection

products that provide “Mode 1” (line-to-neutral only) protection devices. If you choose

the alternative, excessive noise or damage to the respective equipment data ports may

result.

Harford is president and chief engineer at Zero Surge, Inc., Frenchtown, N.J.

Sidebar: Joule Calculations

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The matched impedance energy delivered to a load for a 6,000V, 3,000A, 20μs

surge is: 6,000V×3,000A×0.25×20μs=90J.

The matched impedance energy delivered to a load for a 3,000V, 100A, 10μs ring

wave is: 0.86×3,000V×100A×0.25×10μs=0.645J.

The matched impedance surge energy let through from a sample “good” shunt

suppressor for a dangerous surge is: 400V×3,000A×0.25×20μs=6J.

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Larry West (not verif ied)on A u g 2 6 , 2 01 3

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The last paragraph is misleading. Amplitude is less of a factor than rate of occurrence of

surges.

"Surge voltages occurring in low-voltage ac power circuits originate from two major

sources: lightning effects (direct or indirect) on the power sy stem and sy stem switching

transients. The rate of occurrence of surges varies over wide limits, depending upon the

particular power sy stem. Prediction of the rate for a particular sy stem is alway s difficult

and frequently impossible. The rate is related to the level of the surges; low-level surges

are more prevalent than high-level surges. The frequency of surges is a key indicator of

their damage potential."