Dr.-Ing. Peter Hasse,Dehn + Söhne GmbH + Co. KG,

Hans-Dehn-Str. 1, 92318 Neumarkt/Opf., Germanye-mail: hasse@dehn.de

Abstract—There is the risk of fire due to flashovers between electrical and metalinstallations if a building will be hitten by lightning. This risk will be avoided by alightning protection system (LPS) with carefully carried out equipotential bonding.Lightning current arresters (LCAs) are important components to ensure the potentialequalisation between the lightning protection systems (LPS) and the electricalinstallation. Their application is required in IEC62305-3 [1] and their selection andcoordination is described in IEC62305-4 [2]. The steadily increasing number ofelectronic devices and their growing sensitivity against overvoltages require surgeprotective devices (SPDs), with a high lightning current carrying capability as well as alow protection level.The low protection level may cause a huge number of possible operations of the LCAswith possible follow currents driven by the low-voltage mains generally. The number ofincoming surges, their energy content as well as the number and the amplitude ofpower-frequency follow currents determine the life cycle of the SPD.Today the assessment of risk becomes more and more important for the design ofelectrical and electronic systems also in the field of lightning protection (IEC62305-2[3]). The assessment of risk demands the specification of an expected life time of theinstalled devices. Also manufactures of SPDs have to specify the expected life time oftheir devices since an increasing number of projects are placed under the aspect ofrisk assessment.

1 INTRODUCTION

The increasing electromagnetic sensitivity of electronic devices installed in low-voltagesystems requires a reliable mains-voltage supply. The proven concept of a co-ordinatedsurge protection by SPDs with downstream decreasing protection levels and decoupling ele-ments is sometimes not applicable: Restrictions in the application of such a stepped surgeprotection concept are given, e.g. in applications with limited space or where high ratedcurrents are required.

This led to the design of a new generation of modern spark-gap based LCAs with a lowprotection level and high surge current carrying capability. The new generation ofcombination type SPDs is able to act without downstream installed class II SPDs and

LIGHTNING CURRENT ARRESTERS WITH LOW PROTECTION LEVEL –CHANCE AND CHALLENGE

therefore without further decoupling elements and offers an adapted protection level. This isthe Chance of this special type of arresters as a combination of Class I and Class II SPDsand offers the user an improved protection behaviour in conjunction with a space savinginstallation. In [4] the design of such a combination type SPD is described.

The advantage of a low protection level arises the question: Is the coordination between thiscombination type SPD, other downstream installed SPDs (e.g. class III SPDs) and customerdevices really ensured?

The combination type SPD [4] can be directly installed in parallel to the fine protectionelements inside of a customers device to protect.

With the low protection level also an other question arises: Does the low protection levelcause a huge number of possible operations of the LCAs with follow currents driven by thelow-voltage mains?

To give a proper answer to this question, it is necessary to investigate the factors of influenceon the life cycle of LCAs. Therefore the occurring stresses like surges and overvoltagescoming from the mains, have to be compared with the number of surges without im-permissible degradation declared by the manufacturer of the LCA.

The requirement for a high durability together with a low protection level and the possibility tobe coordinated with the customer devices to protect is the Challenge for the combinationtype SPD.

2 MAIN REQUIREMENTS ON THE DESIGN OF MODERN LCA´s

During the lightning event and in case of other overvoltages LCAs have to ensure thepotential equalization between the LPS and the electrical installation and the interruption ofpossible follow currents. An ideal LCA would have the high surge current carrying capabilityof a heavy-duty spark gap and the follow current behaviour of a varistor.

Reliable surge protection measures require a certain protection level UP, which is not onlylower than the withstand capability of the insulation of the electrical installation, but also lowenough to protect sensitive electronic equipment. In some cases only a low protection levelallows the co-ordination with other downstream installed surge protective devices or SPDsintegrated in customer devices. This leads to a new technology of spark-gap based LCA´swith a low protection level activated by an adjustable energy controlled monitoring unit.

The introduction of this technology causes 2 major questions:• Does the low protection level cause a huge number of operations of the LCAs with

possible follow currents driven by the low-voltage mains?• What are the consequences for the life cycle of these LCAs?

For a proper answer the occurring stresses (like surges and overvoltages coming from themains), have to be compared with the permitted number of surges, which cause no loss offunctionality of the LCAs?

Main factors of influence on the life cycle of LCAs are:• technology of the spark gap• numbers, amplitudes and energies of incoming surges or overvoltages• numbers and amplitudes of power-frequency mains follow currents

Mains follow currents and surges are mainly responsible for the numbers of events which canbe handled by the LCA without a loss of functionality. It is obviously that LCAs with lowprotection levels have to operate more frequently. This may lead to a huge number of followcurrents and may cause an accelerated degradation. On the other hand it will be shown laterthat not in any case of LCA-operation a mains follow current will be initiated.

3 OVERVOLTAGES IN LOW-VOLTAGE MAINS

A complete description of overvoltages in low-voltage mains is not possible. Overvoltages inlow-voltage mains originate from different sources, e.g. direct and nearby lightning strikes orindirect lightning effects, switching operations of inductive loads or capacitor banks or duringfault situations in the power system

3.1 Peak Amplitude vs. Rate of Occurrence

To estimate the probability that a certain voltage level will be exceeded, differentmeasurements have been made through the years [5-14] (Figure 1).

p/1/

year

10000

1000

100

10

1

0.1

01 2 4 1053 u/kV

AB

C

D

E

F

G

H

IA, B, C [5]D [10]E [7]F, G [13]H, I [14]

Fig. 1 - Rate of surge occurrences vs. voltage levels

Taking into account that surge amplitudes within buildings may be limited by installed surgeprotective devices or by flashover of clearances or by the propagation effects in the wiringsystem, a basic dependency for the peak amplitude as a function of number of surges peryear can be found.

3.2 Duration and Energy Contents of Incoming Surges

The time of interference (duration of a surge or an overvoltage) is significant for the energycontents of the incoming surges.

The measurements made with different equipment and with different time resolutions show abroad scattered range of surge durations from a few ns up to some 100 µs. The investigationof [7] shows clearly that the energy content of the incoming overvoltages is constant in arange up to 500 V. This is a clear indication that overvoltages with lower crest values haveshorter durations. The energy contents of the measured overvoltages above 500V increasewith V2 (as to be expected).

The wide range of pulses makes it difficult to determine a typical source impedance. Usuallylong duration surges (in the range of some µs) can be considered as surges with lowersource impedances as fast and short pulses.

Short and fast events as they occur during switching operations of small inductive loads arecalled burst-pulses of Electrical Fast Transients ( EFTs). For testing purposes a typicalwaveshape of a single burst pulse is given by the time parameters 5/50ns. These very shortburst-pulses can be assumed as electromagnetic waves travelling through the low-voltagesystem.The behaviour of different types of LCAs during burst stress was tested to check, if the lowprotection level UP can also be kept during burst pulses. E.g. burst pulses with a crest valueof uOC = 4 kV were applied to a spark-gap based LCA with an adjustable energy controlledmonitoring (Figure 2).

mains itotal load

Trigger-Unit

Wtrigger

iprim. isec.

ULCAMain-

spark gap

Trigger-sparkgap

Fig. 2 - Spark-gap based LCA (combination type SPD) withan adjustable energy monitoring unit [4]

Figure 3 shows that due to the special trigger unit this energy controlled LCA can limit theseburst pulses to values of UP ≤ 1,5 kV.

This LCA was also connected to a power frequency source with a maximum continuousoperating voltage Uc. Burst-pulses with a crest value of Uoc = 4 kV were applied to determine

the number of occurring power frequency follow currents. It could be shown that (due to thelow energy content of the burst pulses) no follow currents occured.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

u [kV]

0 100 200 300 400 500 600 t [ns]

Input : Burst-pulse (UOC = 4 kV)

LCA output voltage

3.5

Fig. 3 - Protection behaviour of the combination type LCA (Figure 2) during burst pulseapplication

These investigations have shown, that short-term pulses, even if they are repetitive, are notable to initiate mains follow currents through an energy-controlled LCA (Figure 4).

-600

-2000

200

u/V

050

-50

i/A

0 5 10 15 20 25 30 35 t/ms

15 ms/150 Impulses

LCA voltage

LCA current

Fig. 4 - Time behaviour of current and voltage during the application of burst pulses to theenergy controlled LCA (Figure 2)

Therefore burst pulses as an additional stress can be excluded for these kind of energy-

controlled LCAs with low protection level. This allows the following assumption: Due to thelow energy contents of burst pulses (which are not able to initiate mains follow currents) infurther considerations only overvoltages with a duration above 1 µs have to be taken intoaccount.

The basis for the further considerations are the results of investigations [10,11] which wereperformed with the goal to detect low-voltage insulations endangering overvoltages.Therefore the measuring equipment was adjusted to detect only overvoltage events with atime duration tmin > 1µs.

3.3 Assessment of Threat to SPDs with Low-protection Levels

Modern spark-gap based LCAs with low protection levels are generally SPDs with a triggercircuit. Several conditions for the ignition of a power frequency mains follow current can bedefined depending on the trigger circuit design as well as on the spark gap characteristicsthemselves.

To assess the threat by overvoltages to a SPD the boundaries of follow current initiation haveto be defined. Independend from the LCA-technology two boundaries can be found (Figure5).

• Duration of the surges: If the time of interference is smaller than a given limit nofollow current will occur (boundary b1 or b2).

• Energy contents of the incomming surges: If the surge current remains smallerthan a defined value, the LCA is not activated independent from the duration of thesurge (boundary a1 or a2).

isurge/kA

50

2010

t / µs200100

5

21

0.5

0.20.1

2010 5021 5

LCA with energy-controllingtrigger circuit[2]

LCA without energy-controlling

trigger circuit

a1)

b1)

b2)

a2)

Fig. 5 - Boundaries of a follow current initiation in LCAs

From a very conservative point of view, all measured surges above a certain peak value andlonger than a certain duration can be considered as surges generated from a combinationwave generator with a fictive impedance of Zf = 2 Ω. They are able to initiate follow currents.

These considerations allow a correlation between the measured overvoltages and theresulting surge currents.

3.4 Application to an Energy Controlled LCA

The minimum specific energy to initiate a mains follow current in a LCA with energycontrolling trigger circuit [4] was determined with W/R = 16 A2s. This value corresponds to a8/20µs current i8/20 = 1 kA or a 10/350µs pulse current of Ipeak = 240 A (Figure 6).

0

0.4

0.8

1.2u/kV

0

400

800

1200i/A

0 10 20 30 40 t/µs

total current

LCA-voltage

main spark gap current

a)

0.0

0.4

0.8

1.2

u/kV

0

60

120

180

i/A

0.0 0.2 0.4 0.6 0.8 t/ms

b)

total current

LCA-voltage

main spark gap current

Fig. 6 - Behaviour of the LCA with energy controlling trigger circuit at different surge currentsa) LCA operates in the 8/20µs-pulse tail (imax = 1 kA) onlyb) LCA operates already in the 10/350µs pulse-front (ipeak = 240 A)

Acc. to the previous assumption of a representative source impedance of Zf = 2 Ω followcurrents may occur if the overvoltages exceed a value of û = 2 kV. The rate of occurrence ofthis overvoltage level is (acc. to [10]) p = 1..3 events per year.

This rate of occurrence describes only the number of overvoltages which are able to initiatemains follow currents and is not the total number of overvoltage per year. The total number ofovervoltages may activate the energy-controlled LCA, but only a few of them have sufficient

energy contents to initiate follow currents. The number of overvoltages in connection withmains follow currents increases rapidly with a lower crest value boundary (Figure 5 a1) or alower duration boundary (Figure 5 b1): This is the case at LCAs without an energy controllingtrigger unit.

4 LIFE CYCLE TEST

The durability of a LCA installed in the low-voltage system depends on the number ofovervoltages with high energy content, on the number of follow currents and their peakvalues. In [15] prospective short- circuits at different points of installations were investigated(household and smaller industrial installations). It could be shown that approximately 95% ofall measured values of the prospective short-circuit currents were smaller than ip ≤ 5 kAeff.

Modern LCAs with their high follow current interrupt rating ifi are able to limit mains followcurrents to low values and to extinguish them (independent from the prospective short-circuitcurrent at the point of installation).

It is also reasonable, that overvoltages with an energy content high enough to cause followcurrents, are shared equally: This means, that in a life cycle test all synchronization angleswith respect to the mains voltage should be considered.

4.1 Life cycle test procedure

To test the durability of a LCA to be installed at the mains the test procedure taken from thepreconditioning test procedure described in [16] was applied to the energy-controlled LCA.With a power-frequency source of Uc and prospective short-circuit currents from ip = 5 kA toip = 25 kA 8/20µs-surges (Imax = 5kA) starting from a synchronisation angle of 0° in steps of30 electrical degrees were applied to inititate possible follow currents. The test was repeatedas long as the pass criteria of [16] could be fulfilled by the LCA. The pass-through energyprovided from the mains was measured.

Furthermore a 35 A fuse was included into the test circuit. The test was finished if the fuseoperates. This additional pass criteria is not equivalent to a general end-of-life of the LCA. Itindicates only, that the LCA is not longer selective with an 35 A fuse. These tests wereperformed with different types of LCAs of the DEHNventil -family [2]. The results aresummarized in Figure 7.

Also in case of high prospective short circuit currents (ip > 5 kA) where the follow currentfrequency increases, the mains follow current can be limited to negligible values (comparableto those ones of MOVs).

Figure 8 shows the behaviour of the LCA (DEHNventil ) during the application of the mainsvoltage UC = 230 V with a prospective short circuit current iP = 25 kA. The mains followcurrent if was limited to 500 A peak or 1.5 % of iP. The let-through integral of this follow currentdoesn’t exceed the value of the melting integral of a 35 A NH-fuse.

This remarkable limitation of follow currents is the result of the immediate increase of the arc-voltage within the spark gap. As faster this arc-voltage reaches the present value of themains voltage as lower the peak values of follow currents will become. The property of thiskind of spark gaps to reach high arc voltages within a short time leads furthermore to thebehaviour that only for some few synchronisation angles a high energy surge causes anoticeable follow current (Figure 8).

If the arc-voltage within the spark gap is in the order of the mains voltage (Figure 8) theinfluence on other connected devices is small. This is comparable with the limiting behavioura MOV based SPD.

0

1,0

2,0

3,0

4,0

20 - 23

32 - 36

40

44 - 4852

56 - 59

63 - 65

68 - 71

767680 - 81

++ –– ++ – ++ ++

–– –– ++ –– ++ ––

0° 60° 120°180°240°300° 0° 60° 120°180°240°300° 0° 60° 120°180°240°300° 0° 60° 120°180°

180°240°300° 0° 60° 120°180°240°300° 0° 60° 120°180°240°300° 0° 60° 120°180°240°300°360°

Surge at ????????electrelectr..

W/R [

kA²

*s]

++

––0

1,0

2,0

3,0

4,0

8 - 11

iP = 25 kAiP = 5 kA

iP = 25 kAiP = 5 kA

44 - 4852 - 53

56 - 60

64

68 - 72

80 - 84

34 - 36

20 - 248 - 12

Fig. 7 - Pass through-energy of the LCA DEHNventil® during alife cycle test at ip = 5 kA and ip = 25 kA

ip = 25 kA

-100

20

40i / kA

u / V

-400

0

400

10 15 20

0

1

0 5 1510 20 t / ms

t / ms

current flowfor ∆t ≈ 7 ms

spark gap voltage

mains voltage

prospective shortcircuit current

follow current ofthe LCA

i / kA

Fig. 8 - Limitation of the mains follow current of the LCA (DEHNventil ) at 10 kA (8/20µs) anda prospective short circuit current iP = 25 kA r.m.s.

Figure 7 shows clearly, that only at a few synchronisation angles a mains follow current isinitiated. The given numbers correspond to synchronisation angles in the positive andnegative half-wave of the mains voltage where follow currents occur. During the testprocedure the pass through-energy increased evenly and not rapidly. At a prospective short-circuit current of ip = 25 kA the pass through-energy reaches the defined pass criteria at 85surges and at a prospective short-circuit current of ip = 5 kA even at 117 surges.

The claimed protection level of up = 1,5 kV and a sufficient insulation have always been keptduring these test.

With these test sequences the number of stresses was determined where no inadmissiblechange of functionality of the LCA occurs.

With the above mentioned rate of occurrence of overvoltages (causing follow currents) ofp = 1..3 per year a life time of the investigated LCA (DEHNventil ) of tLCA ≥ 33 years can beextrapolated.

The measured pass through-energies of the tested LCAs are always lower then the meltingenergy of a 35 A fuse, which means that the LCAs of the DEHNventil -family keep theirclaimed selectivity to a 35 A fuse during their whole determined life time. This is remarkablewith respect to the fact, that the SPDs of the DEHNventil -family are encapsulated LCAs,without blowing any hot gases or particle emission during operation.

5 CONCLUSIONS

The design of a modern spark-gap based LCA with a low protection level and high surgecurrent carrying capability has to consider a possible increase of operations and a possibleincrease of mains follow currents. The number of overvoltages, their energy contents as wellas the number and r.m.s.-values of prospective short-circuit currents at the points ofinstallation are influencing the life time of spark-gap based LCA.s.Only surges with sufficient energy contents are able to initiate mains follow currents. Due totheir short time of interference burst-pulses or EFT are not able to initiate follow currents inthe investigated LCA. With the measured behaviour, that only surges with a sufficient energycontent and a certain duration can initiate follow currents, a rate of occurrence of theseovervoltages (p = 1..3 per year) could be found.

The investigations were performed with LCAs with a high follow current interrupt rating andhave been done an addition to the existing operating test [16]. The measured number ofsurges up to n > 80 without a loss of functionality delivers for the investigated LCAs (with theirhigh follow current limiting behaviour) a prospected life time of tLCA ≥ 33 years even at veryhigh prospective short circuit currents (e.g. ip = 25 kA)..

6 REFERENCES

[1] IEC 62305-3, Ed. 1: Protection against lightning - Part 3: Physical damage to structuresand life hazard, IEC 81/214/CD

[2] IEC 62305-4, Ed. 1: Protection against lightning - Part 4: Electrical and electronic systemswithin structures IEC 81/212/CD

[3] IEC 62305-2, Ed. 1: Protection against lightning - Part 2: Risk management IEC81/213/CD

[4] R. Brocke, P. Hasse, F. Noack, P. Zahlmann, “Spark gap based lightning current arresterswithout mains follow currents ”, Proceeding of the 24th International Conference onLightning Protection, pp. 654-659, Rhodes-Greece 2000.

[5] ANSI/IEEE C62.41-1991, “IEEE Recommended Practice on Surge Voltage in Low-Voltage AC Power Circuits.”

[6] Goedbloed, J. J. “Transients in Low-Voltage Supply Networks” IEEE Transactions, EMC-29, No. 2, May 1987, pp. 104-115.

[7] Meissen, W. “Überspannungen in Niederspannungsnetzen.” (Overvoltages in low-voltagenetworks) Elektrotechnische Zeitschrift(etz), vol. 104, 1983, pp 343-345

[8] Meissen, W. “Transiente Netzüberspannungen (Transient overvoltages in Networks)”:Elektrotechnische Zeitschrift(etz), vol. 107, 1986, pp 50-55

[9] Standler, R. B. “Transients on the Mains in a Residential Area”, IEEE Transactions, EMC-31, May 1998, pp 170-176

[10] Ackermann, G.; Hudasch, M.; Schwetz, S.; Stimper, K.: “Überspannungen inNiederspannungsanlagen (Overvoltages in low-voltage installations)” ElektrotechnischeZeitschrift (etz), Vol. 114 (1993), pp 218-223

[11] Ackermann, G.; Scheibe, K.; Stimper, K.: “Isolationsgefährdende Überspannungen imNiederspannsbereich” (Overvoltages hazardous to insulation in low-voltage systems)”Elektrotechnische Zeitschrift (etz), Vol. 116 (1997), pp 36-40

[12] Martzloff, F. D. and Gruzs, T. S. “Power Quality Site Surveys: Facts, Fiction, andFallacies“, IEEE Transaction, IA-24, No. 6, Nov./Dec. 1988, pp 100-1018

[13] Harich, H.; Enders, W.: “Transiente Überspannungen in Niederspannungsanlagen(Transient overvoltages in low-voltage installations)”, Bericht der Bundesversuchs- undForschungsanstalt Arsenal, Vienna 1982

[14] IEC-Contribution 28A (France) 25 juin 1986: Contribution du Comité national francaispour la révision du rapport 664 de la CEI

[15] F. Noack, J. Pospiech, "Kurzschluß-Kenngrößen von Nieder-spannungsnetzen(Characteristics of short-circuit currents in low-voltage mains)", ElektrotechnischeZeitschrift (etz),Vol. 116(1995), No. 5, pp 218-223

[16] IEC 61643-1:1998: “Low-voltage surge protective devices - Part 1: Surge protectivedevices connected to low-voltage power distribution systems - Performance requirementsand testing methods.”