10/350 - Marketing ploy or useful Test Waveshape?
Following a comment by one of the members participating in the Task Force setup to review references to the 10/350 waveshape within C62.45, it seems appropriate to attempt to correct some of the misinformation being propagated and clarity when and how this waveshape is used in IEC standards.
1. The 10/350 waveshape replicated the form of lightning discharge?
As mentioned during the IEEE 10/350 Forum (October, 2006), this waveshape is not intended to represent the lightning discharge per se, nor is this claim made within the IEC standards community. Indeed, the vicarious nature of lightning does not allow it to be modeled by any one waveshape, as has been borne out by the many studies of captured lightning waveforms which show different rise times of the “front edge” and decay times of the “tail”. Within a typical C-G lightning event, which may contain several re-strikes, the front and tail times of these successive shots follow no one pattern, nor any absolutes. Generally the first strike is the highest in magnitude (kA), and subsequent re-strikes have faster rise times and shorter tails, but not always. There is also the chance of subsequent long duration follow currents which can see a few hundred amps flowing for duration up to several hundred milliseconds.
2. So what is its purpose?
The purpose of this waveshape is to evaluate the ability of a device (SPD, conductor, component within a LPS etc) to handle large energy depositions, comprising the elements of: charge, peak current and time. The area under this curve is sometimes called the action integral and the energy referred to as the specific energy. The 10/350 has gained acceptance as being one such waveshape which replicates the longer tail of the lightning discharge (than the 8/20 does) and more importantly, which can be reproduced in a laboratory via a surge generator. Again, it makes no claim to being an absolute replica of the lightning discharge, rather it comes closer to replicating the energy deposition associated with a partial or direct discharge.
3. The 10/350 waveshape is used within IEC standards when a conductor (or SPD), is expected to carry direct or partial lightning currents.
It is not intended that this waveshape be used to characterize the protection level of an SPD. The 8/20 waveshape is used for this purpose. All coordination of SPDs is carried out using their Up (voltage protection level) obtained using the 8/20 waveshape.
The 10/350 waveshape is used when there is a need to characterize the SPD’s ability to handle partial or direct lightning currents as may flow when it serves as an equi-potential bonding device. In other words, when such a device (whether SPD or conductor) is likely to be subject to the passage of partial or direct lightning currents either seeking ground, or sourcing from ground and traveling along service conductors to equalize potential gradients with some remote point (referred to as exit path currents).
4. IEC has a number of standards which characterize the electrical environments due to different strike scenarios. It also describe various classes of tests use to characterize SPDs for these environments.
The IEC 62305 series of standards under the control of TC81, deal with Lightning Protection, much like the NFPA780 standard. For the most part this series of standards deals with components found in a typical Lightning Protection System (LPS). Part 4 of the series deals with the protection of Electronic Equipment within a structure.
If the only interest is to avoid dangerous voltages (step and touch potentials, flash-overs etc) then the standard only requires that equi-potential bonding SPDs be installed, and that these are tested to class I.
If however there is a requirement to protect the electrical equipment as well, then a “Coordinated SPD System” consisting of equi-potential bonding SPDs (Class I) and over-voltage protection SPDs (Class II and III) is required.
The IEC 61643 series of standards under the control of SC37A deal with LV Surge Protection, much like our IEEE C62 series. IEC 61643-1 describes three test classes used to classify SPD (for use in other IEC standards) – test Class I, II and II.
- Test Class I is characterized by an Iimp and Up (Iimp means 10/350)
- Test Class II is characterized by an (Imax), In and Up (Imax, In mean 8/20)
- Test Class III is characterized by a Uoc, (Uoc means driving voltage set on 2 ohm combination generator – essentially what SPDC refers to as “let-it-rip”).
Like the C62 Trilogy, the IEC 62305 documents describe scenarios, and addresses the different electrical environments which each present to personal, equipment and structural LP components:
- S1 : flashes to the structure,
- S2 : flashes near the structure,
- S3 : flashes to the services entering the structure,
- S4 : flashes near the services entering the structure.
5. The 8/20 and 10/350 waveshapes can be equated via a multiplier?
Whilst it is true that the area under the curve of the 8/20 and 10/350 waveshapes can be computed and shown to be roughly in the ratio of 1:16 this does not mean that an SPD capable of handling 40kA 8/20 can be rated to 2.5kA 10/350! Both waveshapes test (exercise) the SPDs in very different ways. Indeed, the 40kA 8/20 waveshape is much more sever on the SPD’s mechanical construction than the 2.5kA 10/350 – simply looking at the di/dt will make this evident (as force is a function of rate-of-change of current).
WG3.6.4 agreed under the direction of the former chair to issue a corrigendum to remove the table where an equivalency of 10:1 was suggested. The reader is referred to the paper “Reality Check Initiative #1 8/20 vs.10/350” by Francois Martzloff on this issue.
6. IEC TC81 has it right as far as assigning surge ratings to SPDs?
At the 10/350 forum it was mentioned by Hubert Bachl and myself, that the 62305 standard has adopted a worst case approach to assign magnitudes to the likely surge current that SPDs will encounter under the different scenarios of direct or nearby strikes (S1 to S4). In fairness to the standard, it also provides comprehensive methods to handle current sharing, but due to the overt complexity involved, many choose to adopt the simple current sharing ratio of 50% to the ground, 50% to the service SPDs, thereby arriving at abnormally large kA ratings. A maintenance team is currently working to provide a better balance to this aspect in the documents and the IEEE response to this aspect has been useful to support this position.
7. The Trilogy needs all reference to 10/350 removed from the body of the document?
Any decisions taken with respect to such a proposal should be based on sound technical merit and not on emotive rhetoric alone. To claim justification on the basis that the 10/350 is simply a marketing ploy, is just not adequate nor acceptable. The importance of retaining significance of these standards in the international arena and within a global economy should not be under estimated. The IEC standards are gaining dominance in Asia - evidenced in part by the fact that for the first time SC37A working groups responsible for the development of the 61643-11 and -12 standards will meet in Shanghai at the invite of the Chinese standards association. As the SPD industry within the continues to consolidate into fewer but larger companies, the need to design and certify our products to not only IEEE but IEC and other recognized standards and test waveshapes, will become more important.
C62.45 provides a balanced discussion on the 10/350 waveshape under Section: 5.2. Surge Generators and 9. Standard Surge Generators. Our document will be the poorer for its removal. Indeed if it comes to removing reference to 10/350 I trust we will have an answer to why we do not also remove references to the 10/250, 10/1000 waveshapes etc which appear within these documents.
8. Can we live with an 8/20 waveshape and a 10/350 waveshape?
Both 8/20 and 10/350 are useful and important in the characterization of different aspects of an SPD. 8/20 is used to characterize the voltage protection level of the device and its mechanical properties (higher dv/dt), while 10/350 looks to characterize the behavior of the device in conducting large charges which flow for longer durations. In summary: one is not more important than the other, they are just useful in different ways.
9 Replies & 18 Comments
Michael Maytum Aug 28, 2007
The title of this discussion "10/350: Marketing ploy or useful Test Waveshape?" reads more like a sales closing question than an carefully considered discussion topic. It is along the lines of "Jim's rope: red or long?". Engineers would what to cover more options and ask "Jim's rope: red or long or both or neither?". Even then, one would need to clarify what the relative "long" term means - long compared to what? - and if some of the stakeholders were colour-blind.
Testing the first bit of the question "10/350: Marketing ploy?" here is my history.
Many years ago, when I attended my first UK National Committee (NC) meeting it was clear that the NC members were incensed by the activities of certain members of the IEC TC 81 (Lightning protection). It was related to me that a certain manufacturer had developed a self-extinguishing air-gap of enormous capability. This manufacturer together with certain people from the academic world had effectively hijacked the standards process to justify such a protector. Certainly the first documents drafts I saw had a surge requirement of 200 kA.
Fortunately reality intervened, currents like that would have caused side effects and these effects had not been observed. So, like a speeding driver approaching a speed check camera area, the peak current was progressively reduced over time. The 1994 meeting of SC 28A, SC 37A, TC 64, TC 77B and TC 81 reported that the current level was "hastily" reduced and that several committee members even then considered the (TC 81) proposals extreme. One report suggested that if a single waveshape was used, something in the 100 microseconds duration would be much more reflective of field damage.
Talking with various people around at the time, they concur that it was a well executed IEC standards hijack for a product looking for a solution.
The second-hand evidence I have seen and been told leads me to believe the answer to "10/350: Marketing ploy?" is yes or TRUE. The marketing (ploy) was to establish an International (standard) requirement for a manufacturer's product, but the others joined the bandwagon for academic kudos reasons as well. The second part of the question "10/350: useful Test Waveshape?" is the alternative TRUE option.
As I believe "10/350: Marketing ploy?" is TRUE, the second option becomes FALSE and the 10/350 isn't a useful Test Waveshape.
Michael Maytum Aug 30, 2007
10/350 - The legacy of a marketing ploy
Some aspects of the commercial and academic hijacking of the IEC process were quite brilliant and other parts were incredibly dumb.
It was brilliant to shift the goal posts from the well-known 8/20 surge area to the TC 81 blind spot of long duration impulses. Had TC 81 had any real telecommunications expertise that could come through, this would not have been a blind spot as telecommunications have been using long duration impulses since 1961! A 1000 kA 8/20 SPD would not have seen to be useful - a 200 kA 10/350 SPD was an unknown.
However, it was dumb to expect ALL the lightning current to channel through a single SPD. Hence the rapid reduction in specified SPD peak current level.
The peak current reduction then allowed other protection technologies to complete with the product being promoted. Having established a much lower level of peak current, there was no reason for the perpetrators to shift the goal posts again by a revalidation the surge waveshape.
The fact that this didn't happen is a great shame, as a reformulation would have resulted in a waveshape that in more in line with the most damaging lightning waveforms. System reliability is better served by using impulse test generators that produce stress levels that reflect the most prevalent surges causing damage.
10/350 the sweet-spot that isn't
IEC TC 42 has revisited the 10/350 to some extent and has proposed a maximum rise time of 50 microseconds (<50/350). (An 8/20 surge is over and done by the time the <50/350 reaches its peak.) Correctly TC 42 classified the 10/350 as a current generator. That is a generator that produces a specified current, but doesn't have a defined open-circuit voltage waveform.
People mix generator waveshape types; oblivious they are comparing apples, pears and apricots e.g. a recent comparison used 10/350 and 10/1000 waveshapes.
These produce a defined current waveform, but don't have a defined open-circuit voltage waveform. Examples are the 10/350 and the 8/20 (current) generators.
These produce a defined voltage waveform, but don't have a defined short-circuit current waveform. An example is the 1.2/50 generator for insulation testing.
GENERATORS WITH A DEFINED FICTIVE IMPEDANCE
These produce a defined short-circuit current waveform and a defined open-circuit voltage waveform. Examples are the 1.2/50-8/20 combination wave generator, the "10/700" circuit defined generator, the 10/1000 generator and the 10/250 generator. People often mix-up the 8/20 (current) generator and the 1.2/50-8/20 combination wave generator - these are very different animals!
Why's this important?
Well current source generators are only appropriate for testing component parts, as they can only be applied to two terminals of the item. One cannot rely on individual device terminal pairs being independent of each other.
Voltage source generators can be used to multiple ports provided the ports test good and don't try to draw substantial current.
Generators having defined fictive impedance can be used to test multiple sets of terminals. Some may remember at the 10/350 Forum I pointed out the 10/350 generator is totally inappropriate for testing many telecommunications systems as twisted pairs are used, which require at least two dependent outputs from the generator to surge both wires.
To be meaningful any surge testing discussion should not just be on waveshapes, but also the test generators themselves and their applicability.
François has requested enlightenment on "SPD response to an imposed current versus response to an imposed voltage". This is a case where defining the generator type and configuration used for testing is most important.
Michael Maytum Sep 18, 2007
10/350 – what do other people do?
Although the 10/350 has bulldozed it’s way into many IEC standards, one has to wonder what people did before the 10/350.
TRANSMISSION LINE ARRESTERS
These are the people who should be concerned about the diverse nature of direct strikes. Doesn’t the 10/350 greatly enhance the arrester survival?
No is the opinion of GOEDDE, G.L., KOJOVIC, Lj.A., and WOODWORTH, J.J. in “Surge Arrester Characteristics that Provide Reliable Overvoltage Protection in Distribution and Low-Voltage Systems,” Proceedings, IEEE-PSE Summer Meeting, Seattle WA, July 2000. The paper describes field experience of arresters designed in accordance with IEEE Std C62.11-1999 and concludes that a 100 kA, 10/350 test is not necessary for distribution and low-voltage systems.
The heavy-duty arresters in the paper were rated at 100 kA, 4/10, 10 kA, 8/20 and 250 A, 2 ms.
By reading the paper you find that the arresters covered have a long duration rating – the standard 2 ms pulse many manufactures use for MOV verification. Arrester manufacturers already comprehend long duration pulses by the 2 ms test. The heavy-duty arrester is likely to have a 10/350 current rating in the 0.5 kA to1 kA region based on charge or i^2.t equivalence.
Similarly for small MOVs you will find a 2 ms test in IEC 61051-1 Ed. 2.0, Varistors for use in electronic equipment - Part 1: Generic specification as a type 2 pulse current.
The intent of this mode of transport is that the plane does have to leave the ground. Hence the plane is in series with the lightning stroke. Apparently commercial aircraft expect to have one lightning strike per year.
MIL-STD464 appears to be the main standard here. The simulated lightning stroke is not a simple double exponential, but in components A, B, C and D. An approximate description of the lightning impulse is: after an initial high current strike (component A: 200 kA, 6.4/69, 2 MA^2.s), there are two periods of lower current (component B: 5 ms, 2 kA, 10 C and component C: 400 A for 0.5 s) ending with another strike (component D: 100 kA, 3/33, 0.25 MA^2.s)
The 200 kA peak current, component A has an i^2.t value of 2 MA^2.s and no charge constraint (unlike the 10/350). For a 200kA peak current, the 10/350 has an i^2.t value of 10 MA^2.s, five times higher than is needed for aircraft proving. A 200 kA double exponential pulse with an i^2.t value of 2 MA^2.s has a duration of 70 microseconds. Alternatively a 10/350 with an i^2.t value of 2 MA^2.s would have a peak current of 90 kA.
For MOV type components and devices the existing standard 2 ms rectangular test pulse obviates the need for a 10/350 test.
Michael Maytum Mar 19, 2008
The reference of Cianos, N., and E. T. Pierce, "A Ground-Lightning Environment for Engineering Usage," Stanford Research Institute, Menlo Park, California, August 1972 often occurs in documents on direct lightning strikes. This document can be obtained from the National Technical Information Service (http://www.ntis.gov/).
Professor M. A. Uman was involved and the work was carried out for Bell Telephone Laboratories and the U.S. Army Safeguard System Command. This comprehensive piece of work resulted in partial rejection of previously accepted values. Summarized below are the author’s thoughts on Berger’s earlier 1955, 1961 and 1969 work.
“Positive currents get a special mention because of the widely held misconception that the current in positive strokes is usually of very high value. Examination of all Berger's data for positive currents shows there is an apparent separation into a large number of quite small currents and a few instances of high currents.
Tall structures experience more positive flashes than does open ground and, the taller the structure, the greater the chance of high-current positive strokes. Thus, Berger’s measurements made at a tall structure are likely to be especially misleading as regards positive flashes.
Berger et al return stroke current waveform measurements have been widely quoted and used. However, many lightning experts have reservations as to measurement validity, particularly for the first stroke. Subsequent Fisher and Uman measurements appear to justify the mistrust of Berger's results.
Optical observations on positive strokes show upward leaders of nearly I km in length. Such lengthy upward leaders from open ground seem unlikely. Thus, any application of the San Salvatore (Berger) results, on positive strokes, to normal lightning environments is dubious.
Readers are encouraged to obtain a copy of the 156 page report to understand the contexts of these comments. Cianos and Pierce arrived at maximum lightning values of 250 kA peak, half value time 250 µs and 70 C - Table 3 page 59 (these values are not necessarily correlated). The applied severe three-stroke lightning model has peak currents of 200 kA, 100 kA, 100 kA, time to half value (all 3) = 40 µs, i^2.t = 1.2E6 J, Qs = 40 C and Qf = 200C. Because this is a three stroke simulation the equivalent i^2t time to half value is 40 µs and the equivalent Qs time to half value is 140 µs.
In later years, there were further warnings that Bergers positive lightning data should not be relied on and some rejection of other Cianos and Pierce material.
The 1998 book “The Electrical Nature of Storms” by D. R. MacGorman, W. D. Rust, gives the 1982 measurement results of Garbagnati and Lo Piparo. They found two categories of positive flash. Berger put all his 26 measurements into one category and the book authors are unclear as to why Berger allocated a single category to his positive lightning measurements.
The document “Analysis and Assessment of Peak Lightning Current Probabilities at the NASA Kennedy Space Center, by D.L. Johnson, W. W. Vaughan, May 1999 uses the lightning statistics and procedures from five published reports dealing with lightning probabilities and the Kennedy Space Center pad area. The document Figure 1 shows the distribution of peak currents for first return stroke and subsequent strokes.
A six years study of Cloud-Ground lightning for Cape Canaveral space launch complex #40 was analyzed and published by Chai ("Survey of CGLSS/SLC40 Lightning Data and Retest Criteria," IEEE 1997 International Symposium on Electromagnetic Compatibility, Austin, TX, pp. 391-396, August ! 8-22, 1997). His paper covers 6200 Cloud-Ground events. The measured absolute maximum peak currents were -284 kA and +144 kA and the mean values were -30.9 kA and +23.3 kA. Some 94.5 % of flashes were negative and 5.5 % positive. The probability for natural lightning current >200 kA to occur within 5 miles of complex each year is estimated to be 0.051 % (l event in 950 years).
The NASA report states that much disagreement exists as to which lightning peak current probability curve (of Figure 1) to use. It states that, with exception of Uman's positive stroke curve and the Cianos plots, the more recent probability curves parallel each other. Bergers plots (not in Figure 1) run parallel with the Cianos curves for negative lightning and parallel with Uman's curve for positive lightning.
Perhaps the most damaging criticism comes from Professor V. A. Rakov (University of Florida). At ICLP 2000, Rakov states in the “Positive And Bipolar Lightning Discharges: A Review” paper:
“A reliable distribution of positive-lightning peak currents is presently unavailable. The sample of 26 directly measured positive-lightning currents analyzed by Berger et al. (1975) [K. Berger, R.B. Anderson, and H. Kroninger, "Parameters of lightning flashes" Electra, vol. 80, 23-37, 1975] is apparently based on a mix of
(1) discharges initiated as a result of junction between a descending positive leader and an upward connecting negative leader within some tens of meters of the tower tip and
(2) discharges initiated as a result of a very long upward negative leader from the tower making contact with an oppositely charged channel inside the cloud.”
So there you have it. TC 81 formulated its positive (10/350) lighting stroke from Berger’s Electra number 41 (1975) and 69 (1980) article data, which is now widely regarded as suspect. Not only that, the variation of positive lightning that occurs around the World, such as between Japan and South Africa, were ignored in the formulation of an International standard.
Michael Maytum Dec 3, 2009
It is interesting to see how various disciplines handle lightning currents. Aircraft testing has been mentioned previously. Today it's DC traction.
One of my jobs is to review CENELEC surge protection documents for UK publication. CLC/FprTS 50544:2009 covers (un-gapped) MOVs designed to limit voltage surges on d.c. traction systems. The document contains some discussion on lightning values and gives a typical value for lightning as 30 kA, 5,5/75. A single feed line surge impedance is of the order of 500 ohms. Even the typical lightning current of 30 kA to two lines would theoretically generate 7.5 MV (yes it is volts with a capital "M"). Such voltage levels can't be sustained and one or more insulators would flashover.
Test waveshapes of 4/10, 8/20, 30/60 and 10/350 are discussed. The recommended waveshape for classifying these surge arresters was 8/20. The reasoning being that flashover at a pole truncates the waveform and the propaging waveshape duration after a few spans would be short. For those who remember it, shades Martzloffs "More Begets Less" paper here (http://www.eeel.nist.gov/817/pubs/spd-anthology/files/More%20begets%20Le...).
F Martzloff Aug 29, 2007
There is some validity in the proposal of 10/350 as a test wave (sometimes cited as "a solution in search of a problem" for some marketing approaches) but if WG 3.6.4 would -- instead of arguing about "duplicating lightning" versus "show me a recording of 10/350" -- propose and accept the arbitrary 10/350 as a useful SPD characterization test (actually the scope of 3.6.9), we might succeed in reconciliation of test waves with reality. We have done that for almost a century of acceptance of the equally arbitrary 8/20, 4/10 and long transmission-line type characterization of arresters.
The challenge for C62.41.1 and C62.41.2 updates will be to mention lightning in Scenario II and then suggest that 10/350 would be a useful characterization of an SPD impacted by a lightning flash while not directly "representing lightning" as a single surge. I trust that following the "Reality Check" paper, the C62.41.2-2002 attempt to propose an equivalency of the two test waves by applying a multiplier is now dead. That would also clarify the unsettled issue of multiple strokes in a flash. 8/20 would be a test wave aimed at determination of the protective level of an SPD during the first stroke while 10/350 would be an assessment of the durability of the SPD against severe flashes. Explaining how we got from describing the environment (C62.41.1) to accepting (recommending) 8/20 (C62.41.2) and do the same for a few paragraphs introducing 10/350 should be acceptable?
3.6 organized the 10/350 Forum a year ago, and it is a pity that it did not deliver specific recommendations accepted by a consensus of attendees as time ran out and the agenda timing was not respected.
On a related issue, I am waiting with great expectations for the development by 3.6.4 as well as by 3.6.6 and 3.6.9 of a discussion of SPD response to an imposed current versus response to an imposed voltage, a topic that is only briefly discussed in the narrative associated with Figure 4 of C62.41.1-2002
Michael Maytum Sep 18, 2007
The preamble of the starting post raises one’s hopes for a balanced review.
Following a comment by one of the members participating in the Task Force setup to review references to the 10/350 waveshape within C62.45, it seems appropriate to attempt to correct some of the misinformation being propagated and clarity when and how this waveshape is used in IEC standards.
However, clause 1 could be misinterpreted to read, “The 10/350 does not represent a lightning discharge and the IEC has never said it does” – which is untrue. It is the difference between generalisation (per se) and specifics.
1.The 10/350 waveshape replicated the form of lightning discharge?
As mentioned during the IEEE 10/350 Forum (October, 2006), this waveshape is not intended to represent the lightning discharge per se, nor is this claim made within the IEC standards community. Indeed, the vicarious nature of lightning does not allow it to be modelled by any one waveshape, as has been borne out by the many studies of captured lightning waveforms which show different rise times of the “front edge” and decay times of the “tail”. Within a typical C-G lightning event, which may contain several re-strikes, the front and tail times of these successive shots follow no one pattern, nor any absolutes. Generally the first strike is the highest in magnitude (kA), and subsequent re-strikes have faster rise times and shorter tails, but not always. There is also the chance of subsequent long duration follow currents which can see a few hundred amps flowing for duration up to several hundred milliseconds.
If you pull the 8.8 MB 2001 PEG presentation “Characteristics of direct strike lightning events and risk assessment” by Mr “10/350” Peter Hasse, http://www.atis.org/peg/docs/peg2001/hasse.pdf, you will find it covers the 10/350 evolution, much as described at the first IEEE 10/350 meeting.
There’s a fair bit of junk science in the presentation and from people who quote on the 10/350. For example, the 10/350 is termed the “first stroke” with subsequent strokes of 0,25/100. This is rubbish. The 10/350 is the result of TC 81 trying to model an extreme positive lightning stroke. It isn’t the first stroke of all lightning. Negative first strokes are more common than positive ones and it’s normally negative flashes that have the subsequent strokes.
The first clause would have been better served by referencing the 10/350 specific.
1.The 10/350 waveform and lightning strokes
Lightning strokes can take many forms. Negative lightning flashes are typified by a high-amplitude short-duration stroke followed by a succession of lower amplitude, but faster rising, short-duration strokes. Positive lightning flashes typically are a single high-amplitude, long-duration stroke. The 10/350 waveform is the result of TC 81 trying to model an extreme positive lightning stroke.
Jody Levine May 12, 2008
I'll confess to not having read everything, but I'll throw my 2 cents in anyway (from my own experience as a lab tester), I'll try to keep it short, and I'll be there in person for flaming tomorrow at the hospitality suite (probably my last meeting, so this is the chance).
I don't think the 10/350 wave simulates lightning well, but I do think it's better suited than 8/20 to an energy handling horsepower race (i.e. marketing ploy). Has there ever been 200 kA recorded at a service entrance (maybe if the mast gets a direct strike? in the winter)? But long tails that have had a couple of current splits already could propagate in from somewhere else. It's also a reasonable simulator of total energy handling capability from multiple smaller strokes.
The 8/20 test at ridiculous levels is essentially about mechanical shock. It favours devices that have multiple current paths, and drives MOVs into the upturn region where even poorly matched parallel MOVs share current well. And since the rise isn't fast enough to simulate a big direct hit anyway (particularly subsequent stroke), it doesn't show if the lead inductance would be equal enough for the set to share properly anyway. The 10/350 test also has the disadvantage of a slow rise (although when I did it I had enough fast ringing on the front to help with that :-) ), but at least it keeps the current down in the clamping area where they're meant to operate. The 10/350 test favours large, single element arresters, which is frankly what I want now that I'm buying them rather than testing them.
Although it's not as important for 10/350 (where it's easier to get a high source impedance) as it is for 8/20, I'm also a strong proponent of imposing a current waveform, rather than some sort of supply impedance. That way you absolutely know what test has been imposed, and it doesn't make any difference what the source impedance of the supply is. Getting the OC voltage wave right is a waste of a lab's time (easier for me to say now that I don't work for one), when the whole point of the same test for everyone is to have the same current. I think the fixed impedance has become a way for some manufacturers to use higher impedance arresters with inadequate protective level that don't actually take the full current. This is not the intention of the test, but I was asked to do it many times! Go to required current and the whole matter goes away. It also eliminates this business of the open circuit voltage wave, which is a source of great confusion for a lot of people.
Regards and have fun,
now with Hydro One
416 371 0747
Michael Maytum May 12, 2008
Thanks for your balanced posting to this often contentious issue - its refreshing! As your posting indicates, the 10/350 waveshape has its benefits in certain situations, just as 8/20 does.
....Although it's not as important for 10/350 (where it's easier to get a high source impedance) as it is for 8/20, I'm also a strong proponent of imposing a current waveform, rather than some sort of supply impedance...
Yes, and this is supported if one considers that the lightning discharge is a current impressed discharge (constant current source). In the "IEC world" 10/350 is used to exercise an SPD in such a way as to better characterise behaviour under this condition. In essence it seeks to explore an SPD's ability to handle bulk energy deposition, such as it would encounter under what IEC terms "partial or direct lightning discharges" (IEC S1 and S2 events - see IEC 62305-1, or IEEE Scenario II events - see C62.41.2).
Celio Barbosa Oct 25, 2009
I have just joined the SPDC and got interested in the discussion on 10/350 waveshape, which I believe is far from getting over. The most remarkable aspect of this waveshape is, as long as I know, the lack of support from measurements of surge currents flowing through the low-voltage SPD installed at the entry point of buildings. However, if we look for data of surges flowing through SPD installed in medium-voltage lines, we may find many field survey results. My favorite is the one carried out by Barker et al. ("Characteristics of lightning surges measured at metal oxide distribution arresters", IEEE Trans. on Power Delivery, Vol.8, N.1, Jan. 1993). This survey registered voltages and currents in SPD (arresters) in four US power utilities for an equivalent 100 arrester-years of monitoring. A total of 357 surge currents were measured, and about 95% of discharge peak currents were less than 2 kA. Only 3 surge currents were above 10 kA, and the maximum recorded value was 28 kA. In general, the arrester discharge currents had short duration decay times; nearly all decayed to 50% of crest magnitude within 50 us. The median arrester discharge current decay time was 22 us and 9.8 us for the first and subsequent strokes, respectively. The longest measured decay time was 110 us. These results are in good agreement with the requirements for this kind of arrester, which are tested with 8/20 us waveshape and 5-10 kA peak.
It is clear that these results can not be applied to the SPD installed at buildings, as they may have been influenced by the insulation level of the lines (BIL) and by the effect of electrical power apparatus. However, it is also true that the "collection area" of an aerial medium-voltage line is much greater than the collection area of a building, so that the probability of a high-current direct strike is much higher for the line. In other words, it is surprising that the SPD installed in the "exposed" lines experience low and short current surges, while the SPD installed in the "unexposed" buildings would experience high and long currents (10/350 us).
Another aspect to take into account is the field experience with building's SPD tested for 8/20 us. During about 10 years I was in the engineering office for electrical protection of the then Brazilian nation-wide telecommunication operator (Telebras). The operator had thousands of radio-base stations, which comprises a small building with a nearby tower ranging from 30-120 m high, often installed in the top of mountains. These stations were used as relay stations for micro-wave links or access to mobile communication. The standard protection used MOV SPD at the power line entranced, rated 40-80 kA, 8/20 us, and we had never received information from the field that these SPD were performing badly. Recently, I was informed that China Telecom has a similar experience.
Some years ago I was involved in a research project dealing with rocket-triggered lightning (RTL), and I took the opportunity to investigate this issue. We built a radio-base station, triggered flashes to its tower and measured the currents through the SPD. The results can be seen in the paper (C. F. Barbosa et al., “Current distribution on power conductors of an installation struck by rocket-triggered lightning”, Proc. VIII SIPDA, São Paulo, Nov. 2005). The results show that the currents through the phase conductors (and SPD) are much shorter than the current through the neutral conductor (for TN systems), which leads to a relatively short current through the SPD. This result is in line with the field experience using MOV SPD rated for 8/20 us, as mentioned before.
All considered, I have the opinion that the 10/350 waveshape lacks support from field data, which should be the basis of good engineering. On the contrary, the available data (up to my knowledge) indicates that such waveshape leads to an over-specification of the SPD.
Andrea Haa Nov 1, 2009
Thank you for your perspective on this matter. As a member of WG 3.6.4, this is a continuing critical issue. Would it be possible to get a copy of your paper referenced (Nov. 2005) as well as the Baker paper? Thank you again.
Celio Barbosa Nov 2, 2009
Dear Andrea Haa,
I can't find a way to attach a file to this message. If you send me an e-mail to email@example.com I'll attach the SIPDA 2005 paper to the reply. Regarding Barker's paper, you can pick it from the IEEExplorer, using the Digital Object Identifier: 10.1109/61.180350.
Michael Maytum Nov 2, 2009
Barker et al. "Characteristics of lightning surges measured at metal oxide distribution arresters", IEEE Trans. on Power Delivery, Vol.8, N.1, Jan. 1993 is now posted in the 10/350 reference papers folder.
Jo Ann Neumaier Nov 30, 2009
How do I get to the 10/350 reference folder?
Michael Maytum Nov 30, 2009
Apparently I am blacklisted on your e-mail server so you wouldn't have received the following message from me.
. I note that you joined the On-line Community in March 2005.
The 10/350 Conference was a special event at one SPDC meeting. A lot of work went into this and we had special dispensation from the IEEE to share papers with the attendees. To comply with the IEEE security requirements the papers were put in a special access restricted folder. Only listed members of the "10/350 Group" can see the existence of the "10/350" folder and access it's contents.
If you feel you can contribute to the 10/350 discussion, please make your case to Ray Hill (Ray.Hill@neetrac.gatech.edu). The 10/350 discussion is under the LV SPD group of the SPDC and Ray is the chair of that group. Ray can then authorise me to give you 10/350 folder access.
Michael Maytum Nov 3, 2009
The following paper has been posted in the 10/350 Folder:
Current Distribution On Power Conductors Of An Installation Struck By Rocket-Triggered Lightning, C.F.Barbosa, F.Nallin, V.Cardinalli, N.Carnetta, J.Ribeiro, S.Person, A.Zeddam. VIII International Symposium on Lightning Protection; 21st-25th November 2005 – São Paulo, Brazil
Abstract - This paper presents the results of simultaneous measurements of the lightning current and the current conducted by the power line conductors of a radio base station struck by rocket-triggered lightning. The experimental data is compared with the results of computer simulations, showing a good agreement. It’s also presented an analysis of the current distribution between the phase and neutral conductors, highlighting the effect of clamping type SPD. The energy dissipated by the SPD is also investigated. Finally, the paper addresses the evaluation of the current rate of rise at the SPD connections, leading to a simple equation to evaluate this parameter.
Michael Maytum Nov 3, 2009
One can divide up the customer premises telecommunications twisted-pair cabling into three diameters; 0.4 mm, 0.3 mm and 0.2 mm. The 0.4 mm cable is used for feed wiring and the 0.3 and 0.2 mm cables are the flexible cords that connect to the equipment. The 0.2 mm cord is formed by four 0.1 mm tinsel conductors and represents the smallest conductor sized that can be used.
Telecommunication cables don’t need to use large diameter conductors, as the any powering current is low. On POTS you have about 50 V and 50 mA and for a 100 VA capability the voltage is raised to +200 V and –200 V, making the conductor current about 250 mA.
IEC TC108, like TC 81 lacks telecommunication experts who speak out and hold their ground. A spin-off of investigating a TC 108 activity was the short-term i^2*t ratings of these cables for three conditions: no damage (ICEA standardP-32-382), 50 % insulation voltage loss (Middendorf) and conductor melting (Onderdonk). This gives the following format matrix:
Cable size, i^2*t value for no damage, i^2*t value for 50 % insulation voltage loss and i^2*t value for conductor melting.
0.4 mm cable, 218 A^2s no damage, 587 A^2s 50 % insulation voltage loss and 1586 A^2s conductor melting.
0.3 mm cord, 86.3 A^2s no damage, 232 A^2s 50 % insulation voltage loss and 627 A^2s conductor melting.
0.2 mm cord, 13.5 A^2s no damage, 36 A^2s 50 % insulation voltage loss and 98 A^2s conductor melting.
The Exponential Impulse waveforms Annex of the ATIS 0600338 Electrical Coordination of Primary and Secondary Surge Protection for Use in Telecommunications Circuits states the i^2*t of an exponentially decaying current impulse of amplitude I and 50% decay time Td is 0.72*I^2*Td. If you make Td =350 µs, then i^2t = 2.5*10^-4*I^2
The previous i^2t values can be converted to peak currant 350 µs values:
0.4 mm cable, 900 A no damage, 1500 A 50 % insulation voltage loss and 2500 A conductor melting.
0.3 mm cord, 590 A no damage, 1000 A 50 % insulation voltage loss and 1600 A conductor melting.
0.2 mm cord, 230 A no damage, 380 A 50 % insulation voltage loss and 600 A conductor melting.
The 8/20 current is not so easy to calculate, as the 8 µs rise time is comparable with the 20 µs decay time. Using Ronald B. Standlers (Protection of Electronic circuits from Overvoltages) cubic equation for the 8/20 waveshape gives i^2t = 1.22*10^-5*I^2.
The previous i^2t values can be converted to peak currant 8/20 values:
0.4 mm cable, 4200 A no damage, 6900 A 50 % insulation voltage loss and 11000 A conductor melting.
0.3 mm cord, 2700 A no damage, 4400 A 50 % insulation voltage loss and 7200 A conductor melting.
0.2 mm cord, 1000 A no damage, 1700 A 50 % insulation voltage loss and 2800 A conductor melting.
I haven’t seen copious reports of cable damage, so it is reasonable to assume that actual lightning currents don't exceed the “no damage” values. If anyone wants to test at higher current values than the “no damage” values ask to see the cables from the field that took these currents or, failing that, ask them to test using the standard wiring cables – that should be a good reality check.
That's not the complete story as the cords connect via a plug and socket. The plug and socket arrangement does't look like a high pulsed current design and could be the weakest link in the chain. This would further reduce the maximum realistic current that could flow.
Phil Day Nov 8, 2009
As the Rapporteur of ITU-T Q4/5 (resistibility of equipment) and as a staff member of a network operator (Telstra Corporation, Australia) I would like to make the following comments.
1) Mick has pointed out some real world facts about cables connecting equipment. It is of course important to ensure that the current in these cables is kept below that which can damage the cables. Recommendation K.21 has a coordination requirement which keeps the current below 100 A 10/310 µs. The only exception is a test for equipment which has a protection level equivalent to primary protection. If the enhanced requirement is specified it may preclude the use of 0.2 mm cord (5 kA 8/20 µs requirement) for equipment without a coordination element.
2) Telstra experience with protection modules is that a 5 kA 8/20 µs requirement significantly reduces the frequency of lightning damage to modules. The same experience occurred with cable jointing connectors. This 5 kA requirement is included in the enhanced requirements of Recommendations K.55 and K.65.
3) Recommendation K.67 "Expected surges on telecommunications and signalling networks due to lightning" states that the maximum surge current in an underground cable due to a direct strike to the cable is limited to the breakdown voltage of the cable divided by the surge impedance of the cable. Assuming a 100 kV breakdown voltage and a surge impedance of 100 ohm for underground cables results in a maximum current of 1000 A. This only applies outside the earth potential rise (EPR) zone. At the point of the strike there will be an EPR. This EPR will extend along the cable route for some 10's to 100's of metres. At the SPD point the current may double due to reflections. However to obtain the current per conductor it is necessary to divide by the number of conductors in the outdoor cable. Taking a worst case of a single pair cable, the maximum current per SPD is 1000 A 10/350 µs.
4) In my opinion, up to and including the primary protector, and up to and including equipment without a a coordination element, it would be useful to specify a 1000 A 10/350 µs requirement for the wiring, the SPD and SPD holder. This would coordinate with the 900 A "no damage" to 0.4 mm conductor. It would also ensure no damage to the wiring wiring, SPD and Holder for direct strikes to the external plant provided the structure is outside the EPR zone along the cable. The 5 kA 8/20 µs requirement in K.21 for equipment without a coordination element, K.55 and K.55 could be equivalent to 1 kA 10/350 µs. It would be safer to test at both 8/20 and the 10/350 test currents.
The ITU recommendations can be downloaded from http://www.itu.int/ITU-T/recommendations/index_sg.aspx?sg=5
Michael Maytum Nov 22, 2009
Item 3) - the stepping-stone to item 4) justification - interests me because it is at variance with other accepted values.
Books like Lightning Physics and Effects (Rakov & Uman) states that the initial potential difference between the cloud charge region and ground is estimated to be between 50 MV and 500 MV. Any voltage developed by a lightning struck object is going to be insignificant compared the cloud ground voltage. Thus the cloud to ground discharge can be thought of as a current source. Once the lightning current enters a network of multiple paths then the surge source impedance becomes that of the path or network.
So what is the impedance of a twisted pair? Telcordia GR-1089-CORE gives the twisted pair impedance for two frequency bands. For 4 kHz to 12 kHz it is 300 ohm metallic and 500 ohms longitudinal (a "Y" network of two 150 ohm resistors and 425 ohm to ground). For above 12 kHz to 6 MHz it is 135 ohm metallic and 90 ohms longitudinal (a "Y" network of two 67.5 ohm resistors and 56.3 ohm to ground).
That in turn begs another question - what frequency spectrum of lightning? Rakov & Uman state that electromagnetic signal levels from lightning peak in the 5 to 10 kHz region, then fall inversely with frequency up to 10 MHz and then fall inversely with the square root of frequency to 10 GHz. Ron Strandler (Protection of Electronic circuits from overvoltages) is even more helpful by describing the spectrum of actual simulated lightning generators, such as the 1.2/50 and 10/1000. The -6 decibel HF spectrum fall-off points are variously 300 Hz 10/1000, 5 kHz 1.2/50, 30 kHz 8/20 and 200 kHz for the 100 kHz ring wave. Thus a waveshape like 10/350 really only has a significant frequency spectrum below 1 kHz. Under these conditions we should be using impedance values of 300 ohm metallic and 500 ohms longitudinal. The item 3) 100 ohm value is about right for xDSL signals but not for lightning.
Re-work the item 3) values gives "Assuming a 100 kV breakdown voltage and a surge impedance of 500 ohm for underground cables results in a maximum current of 200 A". As this is longitudinal, and assuming you don't live on planet TC 81 where current can exist in only one wire of a twisted pair and not the other, then the current flow is 2 x 100 A. This is five times lower than was calculated originally.
Michael Maytum Nov 22, 2009
Before leaping off of stepping stone item 3), as it sinks into the waters of incredibility consider the statement "At the SPD point the current may double due to reflections." Again this interested me as I have previously got round a British Telecom coordination specification using transmission line principles.
"How do you coordinate a GDT primary protector operation and a load that is effectively a short circuit". Sounds impossible, but remember the primary and equipment are in different places connected by a transmission line. The test surge of 10/700 was applied at the primary and it takes the line transmission time, t, to reach the equipment. At the equipment, the rising surge voltage finds a short and reflects back along the line to the primary. The entire round trip takes 2 x t, If during this time the applied surge voltage has risen to or above the primary sparkover voltage the GDT sparks over and is in conduction before the reflected surge edge returns. The waveform propagation speed in cable is about 9 ns/m (slower than the speed of light in air). The primary would receive the reflected waveform from the equipment, the return would be delayed by 18 ns/m. Note that this design works on the rising edge of the surge not on the decaying part. OK it took 30 m of cable (0.54 µs round trip) to realise sufficient delay, but work it did!
Those who want to brush up on basic transmission line principles should look at something like http://web.cecs.pdx.edu/~greenwd/xmsnLine_notes.pdf and Figure Zo-2 Transmission lines: (a) short-circuited; (b) open-circuited. This shows how the voltage doubles at the source end for an open-circuit termination.
The referenced ITU-T Rec. K.67 (02/2006) has something on this in clause 7.3 "Direct lightning to the telecommunication or signalling lines". It states:
"In situation a, the total peak current in the line will be given, in the worst case, by twice the line to earth breakdown voltage divided by the line surge impedance (e.g., 2 × 100 kV/400 ohms = 500 A);"
Attentive readers will notice that 400 ohms is used for the surge impedance (not 100 ohms), very close to the (Telcordia) 500 ohms of the previous message! This factor of two in 2 × 100 kV works well for pulses with an open-circuit termination, but for voltage ramps it looks far too simplistic. For a start, if an SPD is connected then the line will be effectively shorted once the voltage exceeds the limiting voltage of the SPD. Secondly if a wave of 200 kV is propagating then won't there be other cable breakdowns and waveshape truncations? Yes there will be reflections, but an increase of two in current needs more justification.
Celio Barbosa Nov 23, 2009
Reading the previous post, I was tempted to getting into the models to represent real power and telecommunication lines, as it seems to have a misinterpretation of ITU-T K.67. However, I think that the real problem lies on the root of the calculations, i.e., the waveshape considered for the lightning current and how it propagates. The real phenomenon (lightning flash) is complex and generally composed of a first stroke, none to several subsequent strokes and a low-amplitude continuous current. The 10/350 waveshape was originally proposed to represent the stress imposed by the flash current on a lightning rod (air termination). However, when IEC62305 took this waveshape and used it to model the current flowing through lightning down-conductors, earthing systems, lines and SPDs, it did a mistake (in my opinion). To make things worse, the standard considers that the current in each part of the distributed system retains its original waveshape and divides itself accordingly to arbitrary rules.
This leads to some absurd situations. For instance, if we consider a radio-base station (RBS) having an antenna tower and a power line, the IEC standard says that half of the lightning current striking the tower will flow to the earth and the other half will flow through the power line conductors. As the waveshape is retained, for LPL I a subsequent stroke with 25 kA peak and 0.25 us front-time flows through the power line, which leads to a current rate-of-rise of 100 kA/us. The SPD used to protect the AC powered equipment have to have leads to connect them to the power conductors and to the main earthing bar. To make the calculation easier, let us assume an usual case where a single conductor is used to connect the SPDs to the earthing bar. It can be shown that the inductance of a single conductor is close to 1 uH/m. If 100 kA/us flows through this conductor, a voltage equal to 100 kV/m (or 1 kV/cm!) will develop in series with the SPD. The resistibility level of AC powered equipment is usually in the order of a few kV (e.g., ITU-T K.20 gives 2.5 kV for basic and 6.0 kV for enhanced resistibility). Therefore, even neglecting the SPD residual voltage, the SPD connecting leads shall be shorter than 6 cm! This doesn't make sense, as the SPD itself is larger than that. On the other hand, thousands of RBS are doing pretty well against lightning, many of them with a few meters of SPD connecting leads. It is obvious that the current flowing through the power line (and SPD) has a waveshape that is completely different from the lightning stroke waveshape.
This is just one example of the paradoxes created by replacing the real lightning current by the waveshapes from the IEC standard and its crude propagation models. Fortunately, mother nature doesn't have to follow standards ...
Michael Hopkins Nov 24, 2009
This is a great discussion but I think we need to keep things in perspective. From what I can see, that currents observed from direct strikes in the power distribution system are different in both magnitude and waveshape than those seen at the service entrance to a building. The 10/350 seems to be appropriate for testing SPD's used in the HV power distribution grid, but not at the service entrance of a facility. I'm convinced that any lightning current observed from a direct strike won't look anything like what appears some distance down a cable, which again won't look like what's observed at some building service entrance -- or the wall socket, to take it to the extreme. Waveshapes and magnitudes aside, I'd like to see some data on real SPD performance at installations that would indicate a need for new test waveforms at the service entrance and/or inside a building. I think that any discussion of adding a 10/350 waveform test for devices at an installation has to be data driven. Are there failures of installed SPD's that might be due to longer (10/350) type waveforms? I don't remember seeing anything myself. Is there evidence that testing with a new waveform will provide a benefit to industry and/or consumers? Perhaps by increasing the level of protection and/or decreasing the likelihood of damage? Again, some data about SPD performance at the service entrance would be necessary to convince me...
Michael Maytum Nov 25, 2009
I have provided some details of lightning surges in telecommunication networks by posting the 2001 SIPDA paper “Determination of lightning stress levels by thyristor SPD fault signatures” in the 10/350 folder.
After a disastrous CPE deployment of CO thyristor primary protectors, Telcordia organised a field trial to find out what went wrong. Clearly the CPE location stress levels were much higher than the CO location, where thyristor primaries gave excellent service. Thyristors fail when their capability is exceeded. GDTs can cope with overloads and this typically only manifests itself as a shortening of service life. Thus a thyristor failure indicates its capability was exceeded for that particular impulse. Even better, the thyristor failure signature clearly indicates the parameters of the impulse that killed it (the paper covers this in depth).
The installation environment was severe, so the values derived should be reduced for urban and suburban deployment.
Some 10 % of failures were caused by an extremely fast di/dt – we don’t have the standard generators to emulate such fast di/dt rates. Failure is likely to occurred during the subsequent strokes of negative lightning, which have the highest di/dt values.
Some 90 % of the failures were caused by an equivalent decaying waveshape of 180 to 200 microseconds. The projected withstand level required was 600 A. Parts were made with this capability and these survived.
A common myth is that you can accumulate multiple impulses into one single big one. This is definitely not true for thryristor devices as they can thermally recover in the 30 ms or so period between the subsequent negative strokes. (This is described in the paper “Lightning Surge Voltage Limiting and Survival Properties of telecommunication Thyristor-Based Protectors” presented at the 16th Annual Electrical Overstress/ElectroStatic Discharge Symposium, September 1994)
There you have it - two parts of the lightning spectrum in the telecommunication network are killers for thyristor technology: very fast di/dts and decay times of about 200 microseconds of sufficient amplitudes. Our existing 10/700 combination generators provide longitudinal current surges of 250 microseconds decay time and are in the right region for testing field conditions.
Robert Schlesinger Nov 25, 2009
Before commenting, can you direct me to where I can read the 2001 SIPDA paper “Determination of lightning stress levels by thyristor SPD fault signatures” in the 10/350 folder. https://www.ieeecommunities.org/spd?go=2269801
I have not been able to access the link.
Robert Schlesinger, P.E., M.B.A.
Robert Schlesinger Consulting
28801 Shady Lane, Suite B
Laguna Beach, CA 92651
Tel: +1 949.376.4960
Fax: +1 949.376.8660
Michael Maytum Nov 25, 2009
The URL link given only works for the original attendees of the 10/350 meeting. These attendees have access to the materials in the 10/350 folder. I have separately sent you a copy of the paper " Determination of lightning stress levels by thyristor SPD fault
Bruce Glushakow Oct 9, 2011
The last post on this forum was over two years ago, but the subject has, if anything, become more relevant in the intervening period. I just returned from XI SIPDA where I presented a paper on this subject: "10/350 Test Waveform In Focus." I was astounded at the data I was told there by people who were around at the time to personally witness the hijack of the standard (their word, not mine.)
But leaving the emotionalism out of it, my paper documents the fact that the 10/350 waveform has no basis in science. There's nothing wrong with using it as a testing waveform only so long as standards clearly state that it has no special relevance, significance or seniority to other waveforms, such as the 8/20. For the last 20 years TC 81 has promoted the idea that only 10/350 waveform tested SPDs could be installed at service entrance locations because the Class I test "represented direct lightning." TC 81 continues to say that right up to this month: See the most recent Committee Draft on PV Surge Protection (64/1799/CD) which says: "If protection against direct lightning strokes is specified....Class I test SPDs shall be used." (pp 21 and 28 of the Committee Draft.) This approach is based on myths concerning the true significance of the 10/350 waveform, which is what I show in my paper. The time has come for North American stakeholders to take the lead in getting this false science corrected.
Michael Maytum Oct 10, 2011
A copy of the paper
10/350 LIGHTNING TEST WAVEFORM IN FOCUS by Bruce Glushakow, presented at the XI International Symposium on Lightning Protection, October 2011
has been posted in the SPD Forum 10/350 folder, which is in the Main Committee 3.0 folder.
Many thanks to Bruce for supplying the copy.
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