Putting 10/350 Under the
Microscope
By Glenn J. Clifford, MCG Surge Protection
Nov 26, 2003 12:00 PM
Editor’s note: The following article
is based on a white paper written by Dion Neri and Bruce Glushakow that is
currently in review as an IEEE transactions paper.
For decades, American transient voltage surge
suppressor (TVSS) products have been tested using the 8/20µsec waveform
according to ANSI/IEEE C62.41 guidelines. The products designed and
manufactured under these guidelines have performed well in the field.
Until 1995, the 8/20µsec waveform was also
accepted in virtually all other countries, including those that use the
International Electrical Code (IEC). At that time, however, the IEC 61643
family introduced a “Class 1” test for surge protectors installed at service
entry locations. In order to meet the impulse current (Iimp)
requirements of IEC 61643-1, testing organizations now had to use the
10/350µsec test waveform. The change was based upon the assumption that the
10/350µsec waveform better simulated the parameters of a direct lightning
strike, hence making it more relevant in the determination of the
effectiveness of surge protective devices (SPDs).
Based upon its proven track record, the IEEE
saw no reason to change its SPD testing standards. However, the introduction
of the IEC 61643-1 Class 1 test opened a window of opportunity and European
manufacturers engage upon a campaign to legitimize the use of this long
waveform. Their message was simple: “If direct lightning strikes could only
be simulated by a 10/350µsec waveform, then the SPDs produced according to
ANSI/IEEE guidelines were an inadequate defense against lightning.”
According to the new testing requirements,
the 10/350µsec waveform is to be used for protection against direct
lightning currents while the 8/20µsec waveform is useful only for protection
against indirect lightning effects.
One manufacturer produced a diagram (at
right) to show that a 100kA 10/350µsec impulse delivers 125 times as much
discharge in amps as a 20kA 8/20µsec pulse. And use of an MOV protector for
a 100kA 10/350µsec pulse would require about 2,500kA 8/20µsec MOV
capability.
In reality, there’s no ANSI/IEEE-tested,
low-voltage SPD on the market rated anywhere near 2,500kA. It’s understood
that a TVSS rated at 400kA 8/20µsec is more than sufficient for even the
most extreme lightning environments.
Formulas were even presented in a textbook
that allegedly proved that an SPD tested with a 20kA 10/350µsec waveform was
as much as 20 times more capable than one tested with a 20kA 8/20µsec
waveform.
However, in the midst of this debate, SPDs
tested with the 8/20µsec waveform continued to be successful in protecting
sensitive electronic equipment against the effects of direct lightning
strikes all over the world.
The controversy.
The 10/350µsec waveform made its debut in IEC 61312-1. Controversy
surrounded it even before IEC 61312-1 was adopted. Attempts to forge a
consensus for the 10/350µsec parameter in the 1995 TC 81 committee resulted
in 78% of 18 voting countries voting in favor of the document. When the
lightning parameter was reissued in IEC 61312-3:2000, the percentage dropped
to 68% of 19 voting countries.
IEEE C62.41.2-2002 assessed the parameters
of first-stroke lightning, including the 350µsec “time to 0.5 peak” defined
in IEC 61312 documents, and made the following conclusion:
“The case for ‘high-energy’ surge
requirements rests on a consensus based on limited data, a matter of some
concern when comparing these ‘requirements’ with the field performance of
SPDs designed on the basis of the standards of the IEEE C62 family.”
The following three areas of research
prompted investigation:
- Who or what was the source of the
10/350µsec waveform?
- Upon what technical data was this
waveform based?
- How well did the 10/350µsec waveform
represent the parameters of a direct lightning strike?
What is the 10/350µsec waveform?
The 10/350µsec waveform describes two parameters of an impulse of energy.
The “10” denotes the amount of time, in microseconds, it takes to achieve
90% of its rise to peak amplitude. The “350” refers to the duration, in
microseconds, it takes for the trailing edge to diminish down to 50% of that
peak. It’s illustrated in the diagram at right. However, a duration of
350µsec along with a high Ipeak will cause damage in almost all
semiconductor based protection devices.
As it turns out, it was the authors of IEC
61312-1 who made the decision to adopt this waveform. IEC Technical
Committee TC81 (Lightning Protection) asserted in that standard that the
“time to half value” of a typical lightning strike was 350µsec.
Chain of events.
The chain of events that led to the widespread acceptance of the 10/350µsec
waveform warrants reiterating here. The following facts come from the white
paper upon which this article is based:
- IEC countries use the 10/350µsec waveform
for testing SPDs to be installed at service entrance locations in order to
meet the impulse current (Iimp) requirements of IEC 61643-1.
- IEC 61643-1 offers IEC 61312-1 as the
only reference for its lightning test parameters. See Annex A on page 143
of 61643-1.
- IEC 61312-1 bases its lightning current
parameters solely on the results of studies published in two issues of an
international electrical journal in 1975 and 1980.
The International Council on Large Electric
Systems (CIGRE) is a non-governmental international association based in
France. It was founded in 1921 to develop the exchange of engineering
knowledge and information regarding generation and high-voltage
transmission, between engineering personnel and technical specialists around
the world.
Despite IEC’s claims to the contrary,
CIGRE’s bilingual magazine Electra didn’t include any observations or
contributions whatsoever concerning the duration of lightning strokes in
1980.
An issue published in 1975 included a paper
authored by Swiss lightning researcher K. Berger that documented results
from a lightning measurement station on Mount San Salvatore above Lake
Lugano in Switzerland. Particularly relevant is the parameter the author
termed the “lightning stroke duration,” which he defined as the time
interval between the 2kA point on the front and the point on the tail where
the current amplitude has fallen to 50% of its peak value.”
IEC 61312-1 focused specifically on the
parameters offered in the article for positive first strokes. TC81 explained
their reasoning for this in Annex A of 61312-1: “As a first approach it is
assumed that 10% of all flashes are positive and 90% are negative. Despite
this low ratio of positive to negative flashes, the positive ones,
consisting only of a first stroke and a long duration stroke, determine the
maximum values of the parameters I, Q and W/R to be considered. If according
to protection level 1, about 99% of all flashes are to be covered, the
positive flashes with probabilities below 10% determine the maximum values
of peak current I, the charge of the flash Qf, the charge of the
short duration strokes Qs, and the specific energy W/R. The
values of the corresponding 1% of probabilities of negative flashes are much
lower than the values of the 10% probabilities of the positive flashes and
may be disregarded for that reason.”
In other words, the authors of IEC 61312-1
determined that they would be “safe” if they used the parameters found by
Berger to correspond to the much rarer but nominally longer positive first
strokes. It should be noted, however, that the author deemed those results
inconclusive.
Studies conducted in the past 30 years have
brought those results into question. To begin with, the positive
cloud-to-ground flashes included in the study originated from tall towers,
not from any mountain peaks without tall structures. That places them into
the category of “triggered” lightning. All but one of the flashes noted in
the study had an upward propagating leader followed by a downward moving
return stroke. IEC 61321-1 falsely stated that the study’s data was
representative of the 10% naturally occurring positive first strokes.
Triggered lightning of this nature accounts for considerably less than 1% of
all lightning.
These findings led the authors of IEC
61312-1 to believe that the Ipeak of positive return strokes was
much higher than that of its negative counterparts. However, when the
National Lightning Detection Network (NLDN) completed its census of 1.46
million large peak current cloud-to-ground flashes (LPCCGs) of both
polarities in the late 90s, which represent 2.46% of the more than 60
million measured flashes, it found “for all values of Imax>75kA,
the large negative cloud-to-ground events outnumbered the large positive
cloud-to-ground events by considerable margins.”
IEC 61312-1 inferred from Berger that the
return-stroke waveform of positive flashes was very different from that of
negative flashes. Subsequent research established that the two wave shapes
were in fact comparable.
What’s the duration of a lightning
stroke? Lightning is perhaps
nature’s most elusive phenomena. Despite inherent difficulties involved in
studying lightning, recent researchers more or less agree on its parameters:
- In 2001, J.R. Lucas, author of High
Voltage Engineering, noted that for the purpose of surge calculations,
only the heavy current flow during the return stroke is important. During
this period it has been found that the waveform can be represented by a
double exponential of the form: i = I(e-et – e-Bt)
with wavefront times of 0.5µsec to 10µsec and wavetail times of 30µsec to
200µsec. An average lightning current waveform would have a wavefront of
the order of 6µsec and a wavetail of 25µsec.
- A 5-year study completed by the Korea
Electrical Power Corp. that used an LPATS manufactured by Atmospheric
Research Systems of USA found that of the strokes measured for the study,
95% had a time to half peak of less than 22µsec. The average time to half
peak was 10.82µsec.
- A study in Japan in 1977 found the
average time to half peak to be 40µsec.
- The National Oceanic & Atmospheric
Administration (NOAA) reports that “Each return stroke exhibits a peak
current of 5 to 300 kilo amps and has a nominal duration of 20 to 50µsec.”
Where does the IEEE stand?
Those who support the concept of the 10/350µsec waveform now believe that
the IEEE is moving in the direction of accepting the 10/350µsec waveform.
The IEEE publishes the standards that are in
used in environments that contain the largest and most sensitive
concentration of electronic devices in the world. C62.45-2002 together with
C62.41.1-2002 and C62.41.2-2002 have been issued as the IEEE SPD Trilogy,
which comprises 292 pages of technical materials.
After a survey of testing waveforms and
procedures that portray the lightning surge environment, the IEEE concluded
that the two standard waveforms recommended by IEEE Std C62.41.2-2002 are
the 100 kHz ring wave and the 1.2/50µsec to 8/20µsec combination wave, the
latter involving two waveforms—one for voltage and the other for current.
Glenn Clifford is a design engineer at MCG
Surge Protection in Deer Park, N.Y. |
