Forum Subscribe About Us Magazine Issues Newsletters Advertise Shop
Oct 1, 2004
Tweet COMMENTS 1
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
Hot Topics Comments New Topics
Tip of the Week: The Importance of NEC
Definitions and General Requirements
Watch Out for that Voltage Drop
Electrical Troubleshooting Quiz - July 21, 2015
Price Versus Cost Versus Value
Tip of the Week: When it Comes to Safety,
Maintenance is Not an Option
Jo in the Discussion on EC&M Talk
Polarization Index Test
la st r eply by m a h m ou ddia a in T est &
Mea su rem en t
HOME > CONTENT > WHAT TO KNOW ABOUT COMMON MODE SURGES AND GROUND CONTAMINATION
SHARE 0Recommend
Photo Galleries
Test Your ElectricalSymbols IQ
Are you ready? It’s time
for another challenge on
the symbols front. If you
think you’re an electrical symbols master, then now is
your chance to prove it.
EC&M White Papers
VIEW MORE WHITE PAPERS
Know Your TCO: A
Look at MediumVoltage VFDs
Learn about the intricacies
involved in calculating TCO
and the elements that
affect it....
More
NEC Design Ops & Maintenance Contractor Safety Power Quality T raining Basics
Products
REGISTER LOG IN
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
EC&M Webinars
SPONSORED
ON DEMAND:Mitigation Methods forArc Flash Hazards -
Enhancing PersonalSafety
SPONSORED
ON DEMAND:Understanding the2015 Edition of NFPA
70E and the Arc FlashHazard
VIEW MORE WEBINARS
EC&M Learning Center
Understanding the 2014 NEC,
Volume 1 (Articles 90 to 480)
This resource has proven itself in the
field and in the exam room. This latest
edition w ill provide you w ith a rock-solid
foundation...
Understanding NEC Requirements
for Grounding vs Bonding
One of the most confusing areas of the
trade continues to be grounding and
bonding, w hich is w hy this book needs
to be in the hands...
Changes to The NEC 2014
Don't let the scale of the code changes
intimidate you, this book w ill get you up
to speed on the most essential...
BROWSE ALL TITLES
Mode 1: Normal mode (line-to-neutral suppression).
Mode 2: All modes (line-to-neutral, line-to-ground, and neutral-to-ground).
EC&M TV
BROWSE ALL VIDEOS
New CertificationAnnouncement from
NFPA
...
More
Site Features
Author Guidelines
RSS
Sitem ap
Site Archive
Subscribe
View Mobile Site
EC&M Corporate
Privacy Policy
T erm s of Service
About Us
Advertise
Contact Us
Follow Us
Search ecmweb.com
Subscribe to the print magazine
Ecmweb.com
NEC Design Ops & Maintenance Contractor Safety Power Quality T raining Basics Products
Tweet
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
Discuss this Article 1
Please Log In or Register to post comments.
Related Articles
How to Avoid Common Mistakes in Power Conditioning
Intermatic Inc. Offers F20 Series of Surge Protection Devices
Diagnosing Power Problems at the Receptacle 6
What's the Difference Between Surge Arresters and TVSSs?
The Pros and Cons of IG Wiring
COUNTRY*
E-MAIL*
Sign-up to receive our free newsletters
eTrain - (Bi-Weekly) View Sample
Product NewsWire - (Monthly) ViewSample
ElectricalZone - (Weekly) View Sample
MRO Insider - (Bi-Weekly) View Sample
PQ NewsBeat - (Bi-Weekly) View
Sample
Illumination Insider - (Bi-Weekly) View
Sample
CodeWatch - (Bi-Weekly) View Sample
G-Biz - (Varies) View Sample
Electrical Testing - (Monthly) ViewSample
United States
Enter your email above to receive messages about
offerings by Penton, its brands, aff iliates and/or third-
party partners, consistent w ith Penton’s Privacy Policy.
Connect With Us
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.
SHARE 0Recommend
Larry West (not verif ied)on A u g 2 6 , 2 01 3
Log In or Regist er t o post com m en t s
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."
Top Related