CURRENT ENGINEERING
Establish Surge Current Capacity Levels:
Multiple Benefits for Today’s Suppression Filter System Specifier
A publication of the Current Technology
Applications Engineering Group
As the transient voltage surge suppression industry progresses toward
PART I: SERVICE ENTRANCE
APPLICATIONS
maturity, performance standards for
surge suppression products are begin-
At electrical service entrances, two
ning to take shape. Regulatory agencies
separate issues must be addressed to
such as NEMA (National Electrical
ensure reliable protection: survivability of
Manufacturers Association),Underwriters
large (“catastrophic”) transients and sur-
Laboratories (UL) and IEEE (Institute of
vivability of the much more frequent
Electrical and Electronics Engineers) are
lower-magnitude transients that occur on
furthering the establishment of concise
a daily basis in most facilities.
performance standards by publishing
product parameters in a more uniform
manner.
Typically, a building’s degree of large
magnitude transient disturbance exposure is higher at service entrance than at
A one-performance criterion that has
any other location in the facility. It is an
begun to gel is the single pulse surge cur-
established fact that lightning is the most
rent capacity. NEMA has provided a defi-
damaging of high exposure transients to
nition for single pulse surge current
threaten service entrances. The IEEE
capacity that includes tested, rather than
Emerald Book graph in Fig. 1 denotes the
calculated, surge current values pub-
statistical probability of a single lightning
lished per mode (L-N, L-G, N-G, etc.)
stroke occurring above a specific current
Improved product technology and the
level and also illustrates the following
desire of surge suppression manufactur-
probabilities and associated current.
ers to provide offerings capable of surviving the most catastrophic surges have
prompted single pulse surge current
capacities per mode to climb higher and
higher over the last several years. In
response, a number of manufacturers of
lower-rated surge suppression devices
have questioned the benefit of high surge
current capacity at high exposure service
entrance locations as well as in lower
exposure panel applications. This article
details two primary reasons to conservatively rate single pulse surge current
capacities in both service entrance and
distribution system applications.
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CURRENT ENGINEERING
% GREATER THAN ORDINATE
Fig. 1
Distribution of Lightning Stroke Current
(Reproduced from IEEE Std. 1100)
•20% of primary strokes are greater than
40 KA in magnitude
Keep in mind that a single lightning
strike is composed of multiple strokes.
•10% of primary strokes are greater than
65 KA in magnitude
Each year, 100 million lightning flashes
•5% of primary strokes are greater than
100 KA in magnitude
strikes hit the ground. Obviously, differ-
•2% of primary strokes are greater than
140 KA in magnitude
rience a statistically higher number of
•1% of primary strokes are greater than
180 KA in magnitude
occur in the United States; 20% of those
ent geographic areas of the country expelightning strikes than others; an area’s
isokraunic (frequency of lightning) rating
is one of several factors involved in the
probability of a specific facility incurring
a damaging lightning episode.
2
CURRENT ENGINEERING
SURVIVAL OF THE FITTEST
50,000 amp single pulse surge current
100KA N-G will provide protection from
capacity rating. Since 19% of primary
99% of initial direct lightning strokes.
Regardless of building or utility
lightning strokes and 5% of secondary
integrity, a lightning strike that reaches
strokes are greater than 50,000 amps in
HIGHER SURGE CURRENT CAPACITY =
the facility or travels very near the build-
magnitude, and since the role of surge
HIGHER RELIABILITY
ing or to nearby ground can force cata-
suppression devices at service entrance is
strophic lightning currents through the
to protect sensitive loads before, during
electrical distribution system. Lightning
and after catastrophic transient episodes,
cal factor for properly selecting surge
can produce potentially catastrophic
these products do not offer complete or
suppression with sufficient single pulse
surge current levels to a building’s distri-
even adequate protection. The recom-
surge current capacity is the relationship
bution system not only by directly strik-
mended minimum single pulse surge
of surge current capacity to lower ampli-
ing the facility but also by striking an
current capacity for surge suppression
tude transient suppression reliability.
incoming utility feed, by reaching the
products in high-lightning locations is
The surge current/reliability correlation
surrounding earth or even by occurring
150KA L-N, 150KA L-G and 100KA N-G.
applies not only to service entrance pro-
cloud-to-cloud above a facility. With the
very real chance of the facility being
tection that is constantly bombarded by
A PENNY SAVED?
exposed to a direct or close-proximity
lightning strike, today’s service entrance
The second and possibly most criti-
less catastrophic transients: this relationship is also the foundation for reliability
Today’s cost-conscious purchasers
of lower exposure surge suppression
surge suppression must be capable of
would do well to remember that lightning
within a facility distribution system.
surviving a large magnitude occurrence.
strikes are comprised of multiple strokes.
Although valid at all exposure levels, the
Surge suppression stressed to maximum
surge current/reliability connection is
capacity has a much greater potential for
best highlighted by studying a surge sup-
fied into a building and installed under
failure during the return strokes and
pression device designed for branch
the auspices of ideal facility and utility
therefore increases the chances of expos-
panel application.
distribution system conditions. With
ing critical electronics to residual light-
shining new grounding, recently con-
ning pulses. These same individuals (so
nected bonds and unblemished utility
anxious to shave a few dollars from a
PART II: DISTRIBUTION SYSTEM
integrity, the expected utility lightning
project) would be outraged if their per-
APPLICATIONS
strike magnitude conducted into the
sonal insurance agent suggested coverage
building is diminished. However, as the
for only 80% of their homes or automo-
current and reliability, two metal oxide
building and utility infrastructure age,
biles.
varistor-based (MOV) panelboard level
Often, suppression devices are speci-
surge suppression devices will be com-
facility electrical conditions may cease to
be ideal. Common distribution system
To explore the correlation of surge
For geographical areas less prone to
pared with single pulse surge current
capacities as listed below:
aging problems include building ground-
lightning, the minimum single pulse
ing increasing in impedance, N-G bond-
surge current capacity should be 125KA
ing with significant impedance or no N-G
L-N, 100KA L-G and 100KA N-G. This
bonding at all and deteriorated utility
level of suppression is sufficient to pro-
L-N
80,000A
40,000A
conditions. All of these factors further
tect critical equipment from the highest
L-G
80,000A
40,000A
increase an environment’s susceptibility
probability of expected lightning magni-
N-G
80,000A
40,000A
to higher current lightning transients.
tudes. As the lightning density increases
or a facility’s propensity to conduct light-
PRODUCT A
PRODUCT B
To simplify this example, only one
ning becomes greater (high-rise struc-
mode of suppression utilizing a multi-
anxious to cut corners on project price,
tures, overhead utility feeds, etc.), the
tude of MOV components will be exam-
have been snared by unscrupulous surge
single pulse surge current capacity
ined, but analysis will apply to all modes
suppression manufacturers offering so-
should increase as well. A surge suppres-
as well as to MOVs and silicon avalanche
called “equal” high-exposure service
sion device with a minimum rating per
diodes.
entrance products that carry only a
mode of 150KA L-N, 150KA L-G and
Many budget-minded customers,
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CURRENT ENGINEERING
Fig. 2
MOV Manufacturer Component Data Graph
What is the probability of a medium-to-
2KA impulse may then be easily deter-
varistors. The proportions of the basic
low exposure branch panelboard
mined.
relationship of a single MOV hold true for
encountering an 80,000A transient? To
EXPONENTIAL LIFE EXPECTANCY
a multitude of current sharing MOVs.
create a real-world scenario, these branch
The Harris 20mm MOV data graph in
panel suppressors will be exposed to
Fig. 2 provides information for a single
more frequent lower level events. To
MOV. The graph depicts Impulse
20mm MOV pulsed with 20 usec 400 amp
keep calculations simple, a lower current
Duration vs. Surge Current with each
transients should be able to withstand
magnitude transient of 2KA with an 8/20-
graph line representing expected pulse
1000 pulses. Halving the current to 200
usec standard transient current wave-
life. The area circled represents sample
amps extends the MOV life from 1,000
form is used as a lower level transient
readings to be used for discussion. A 20
hits to 10,000 hits. Similarly, halving the
magnitude. Long-term reliability of each
usec pulse (as read from the bottom axis)
stress on a product utilizing multiple
device will be analyzed using the 2KA
with a current magnitudes 400A (as read
MOVs not only doubles product life
transient current and MOV manufactur-
from the vertical axis) has a pulse life of
expectancy but increases product life
er’s component data. By reviewing the
1000 (10E3) (as read from the intersect-
expectancy by a factor of 10 as illustrated
MOV data sheet, the relationship of cur-
ing line of the graph). Although the
in Fig. 2. As shown, halving the stress on
rent to life expectancy will be discerned.
graph shows the response of a single
MOVs not only doubles the life of the
The same relational proportions to a
20mm MOV, keep in mind the Products A
MOVs but increases the life by a factor
device with multiple MOVs exposed to a
and B utilize a multitude of metal oxide
of 10.
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Review of the graph reveals that a
CURRENT ENGINEERING
The single pulse surge current capac-
Product A
80KA
ities of Products A and B are 80KA and
40KA respectively, and Fig. 3 illustrates
these capacities along with a 2KA
80,000A
impulse indicating the fraction of each
Product B
40KA
device’s rated maximum. This graph
shows that the 2KA transient is 1/20th of
1/40
Capacity
40,000A
the single pulse surge current rating of
Product B and 1/40th of the rated capacity of Product A. If the relationship
1/20
between an MOV’s number of pulses
Capacity
before failure and the surge current were
linear, the life expectancy of Product A
2,000A
would be only twice that of Product B.
However, surge suppression devices
Fig. 3
Product A and Product B Surge Current
using non-linear solid state suppression
such as MOVs or silicon avalanche diodes
exhibit logarithmic impulse current vs.
life expectancy relationships. Therefore,
halving the current stress on the surge
suppression provides a life expectancy
extended by an exponential factor 10.
The principal for exponential
improvement of life expectancy is applicable to service entrances as well. Rather
than utilize two devices rated for 40KA
Product C
150KA
150,000A
and 80KA, service entrance surge suppression, Product C carries a surge current capacity of 150KA in any single
mode, compared to Product D with 75KA.
Instead of using a 2KA surge current to
Product D
75KA
75,000A
extrapolate the device’s percent of capac-
1/15
Capacity
ity, a 10KA service entrance current magnitude is used to relate to utility power
factor correction, utility transformer
1/7.5
reactance or reclosure switching.
Capacity
10,000A
Fig. 4 shows the single pulse surge
current capacities of service entrance
Products C and D along with a 10KA
Fig. 4
Product C and Product D Surge Current
impulse that depicts the fraction of each
product’s rated maximum. As does internal suppression, some service entrance
products use only non-linear solid state
suppression such as multiple MOVs or
silicon avalanche diodes and therefore
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CURRENT ENGINEERING
exhibit logarithmic impulse currents vs.
tribution system, the catastrophic tran-
nitude does not stress the capacity of the
life expectancy relationships. Once
sient is still present but less likely to be
internal components. In other words,
again, Product C will last 10 minutes
critical criteria. Instead, the single pulse
daily electrical transient activity will have
longer than Product D.
surge current capacity internally applied
less detrimental effect on a more power-
suppression devices will directly correlate
ful surge suppression device than on a
to the life expectancy of the suppressors.
less powerful one. And just as critical sit-
rent handled by the MOVs to increase life
In both service entrance and distribution
uations may call for an automobile driver
expectancy, some manufacturers includ-
system applications, this is an exponen-
to accelerate beyond the speed limit,
ed an additional surge current path with-
tial relationship rather than a line at one.
surge suppression devices may call
in the product. If Product C features a
Devices with higher surge current capaci-
encounter extraordinary transient cur-
selenium cell that shares current with the
ties last exponentially longer than lower
rent magnitudes. Better to be equipped
MOVs and Product D relies solely on
rated devices.
and be prepared than to face the results
In addition to halving the surge cur-
MOV technology, the component life
of a catastrophe that could have been
expectancy of Product C will be further
HORSEPOWER
extended. In this situation, Product D is
To compare surge capacity ratings,
stressed to almost 1/8 of the MOV capacity while Product C implements MOV
consider the wide range of horsepower
available in today’s automobiles. For
technology current sharing with a seleni-
many individuals, a major consideration
um cell. Should Product C contain MOV
in the purchase of a new car is the size of
technology only, the current stress on the
MOVs would be half that as encountered
the motor under the hood. Although
often more expensive, cars with greater
in Product D; therefore, Product C would
horsepower maintain a higher resale
have a life expectancy 10 times greater
value. Since driving a car faster than 65
than that of Product D. Adding selenium
mph is legally prohibited, why is higher
and further reducing the current con-
horsepower desirable?
ducted by the MOVs will further expo-
CONCLUSION
As detailed in this article, selection of
proper single pulse surge current capacity hinges on two critical factors: exposure
of the device to high magnitude transients and the desired reliability through
lower magnitude transients. Single pulse
surge current capacities relate not only to
a product’s ability to electrically function
following severe transient episodes but
are also directly related to the expected
nentially extend the expected life of the
Horsepower has more performance
device.
benefits that top speed, such as quicker
acceleration, less motor stress over the
At service entrance, single pulse
duration of the car’s life and better
surge current capacity is both a measure
responsiveness. Routine acceleration
of protection from catastrophic external
during city driving is less stressful on a
transients and a measure of surge sup-
higher horsepower motor than a smaller,
pressor life expectancy when subjected to
the daily bombardment of lower level
lower rated engine. Similarly, a surge
suppressor with greater surge current
external transients. Within a facility dis-
life of the product from low level transients as well. As the surge suppression
industry continues to design increasingly
higher capacity products, the greatest
improvement will come in long term
reliability. Meanwhile, specification of
high-rated surge current capacity offerings provides the maximum available
protection for today’s sensitive loads.
capacity lasts longer if the transient mag-
®
© 2007, Thomas & Betts Power Solutions
prevented.
By Thomas & Betts Power Solutions
5900 Eastport Boulevard
Richmond, VA 23231-4453 USA
Tel: 804.236.3300
Fax: 804.236.4047
All Rights Reserved. Printed in U.S.A. KK/3M/08.07
C-1505
800.238.5000
www.currenttechnology.com
info@currenttechnology.com
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