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2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

Factors Influencing the Selection and Installation of

Surge Protective Devices for Low Voltage Systems

T. Kisielewicz, F. Fiamingo Z. Flisowski, B. Kuca G.B. Lo Piparo C. MazzettiUniv.of Rome La Sapienza Warsaw Univ.of Technology RAI Italy Telev. Broadcast Univ. of Rome La Sapienza

E-mail: [email protected]

Abstract The present paper aims to illustrate the influence ofthe main factors which affect the selection and installation of anSPD for the protection of electrical and electronic systems against

overcurrent and overvoltage due to direct lightning flash to astructure. The two most typical SPDs are considered, namelyswitching and limiting type. Simple rules are established for the

selection of effective SPD with regard to the discharge current, itsprotection level and location.

Keywords- Lightning protection; Surge Protective Device;SPD protection level

I. INTRODUCTIONElectrical and electronic systems are subjected to damage

from a lightning current and in order to reduce the risk offailure, protection measures have to be properly designed.Failure of such systems are mainly due to [1]:

- surges by lightning flashes to the structure resulting fromresistive and inductive coupling (source of damage S1),

- surges by lightning flashes near the structure resulting frominductive coupling (source of damage S2),

- surges transmitted by lines connected to the structure due toflashes to or near the lines (sources of damages S3 and S4).

To limit transient overvoltages below the rated impulsewithstand voltage of the system to be protected and to divertsurge currents, surge protective devices (SPDs) as suitableprotection measures are to be applied. The rules for theselection and installation of SPD are considered byinternational standards in several relevant documents [2- 7].

For a proper selection and installation of SPDs, it is of

essential importance to know the stress, which an SPD willexperience under surge conditions; such stress, as underlinedby the Standard [1,2], is a function of many complex andinterrelated factors. These include: the location of the SPD(s)within the structure; the method of coupling of the lightningstrike to the facility (resistive or inductive); the routing ofinternal circuits and their distance from inducing lightningcurrents; the sharing of lightning currents within the structureto the earthing system and to the connected services, mainly thecurrent peak value and waveshape, which the SPD will have toconduct under surge conditions.

Furthermore the different behaviour of the SPD containingspark gaps (switching type SPD) and SPD containing metal-oxide varistors (limiting type SPD) should be considered.

In the present paper only surges due to flashes to thestructure (source S1), protected by a lightning protection

system (LPS), are considered. The influence of the mainfactors and parameters which affect the selection andinstallation of an SPD of both types (switching and limiting)installed at entry point of line in the structure are discussed.Investigation relevant to SPD installed downstream areconsidered in other paper [8]. By several computer simulations,simple rules are established for the selection of effective SPDwith regard to the discharge current and its protection level.

Comments and comparison with the requirements of theinternational standard IEC/ EN 62305-4 are presented.

II. CASE STUDY UNDER CONSIDERATIONThe analysed arrangement is shown in Fig. 1.

The investigation is focused on the influence of the installedprotection devices, namely switching and limiting SPD, on thefollowing parameters:

peak value and shape of currents flowing through the SPD charges associated to currents flowing through the SPD

The analyses have been performed by means of thetransient software EMTP-RV.

The lightning stroke has been simulated as an ideal currentgenerator by means of the following equation:

( )

( ) ( )2101

10

1

/exp/1

/

tt

t

k

I

I

P

+= (1)

where

k is the correction factor for the peak current

t is the time

IP is the peak value of the lightning current

1 is the front time constant

2 is the tail time constant


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Three shapes of lightning current, namely representative of

positive stroke (10/350 s), first negative stroke (1/200 s),subsequent negative stroke (0,25/100 s) have beenconsidered. The relative equation parameters to obtain thementioned wave shapes are reported in Tab. I.

TABLE I. EQUATION PARAMETERS

Parameters 10/350 s 1/200 s 0,25/100 s

k 0,93 0,986 0,993

1(s) 19 1,82 0,454

2(s) 485 285 143

The typical low-voltage limiting type SPDs have beensimulated in order to match the U - Icharacteristic derived byactual voltage-current measurements. SPD switching typesimulation takes into account the volt-time characteristic of thespark gap and the arc resistance. The leads/connections of SPDhave been simulated by means of concentrated inductance.

Supply line as well as internal circuit conductors have beensimulated by means of transmission line model where theparameters typical for the usual LV cable types are inserted.

The LV/MV transformer has been simulated in transientconditions by a network representing the winding capacitance,inductance and resistance.

The transient behaviour of earth termination systems of boththe structure and the transformer has been simulated by meansof a network of elements consisting of a capacitanceC, aninductance L and a resistance R. Different types of earthingarrangement, namely rod (type A according to standardIEC/EN 62305-3), ring and mesh (type B of standard IEC/EN62305-3) have been considered.

Results of preliminary investigation on the validity of thesimulation models by a comparison with experimental resultsobtained in HV laboratories are summarized in [9].

III. SELECTION WITH REGARD TO DISCHARGE CURRENTThe selection of the discharge current of an SPD requires

the evaluation of the peak value and wave shape of the possiblecurrent flowing through the SPD [10, 11, 12]. In fact as generalrule, SPD shall withstand the energy relevant to the perspectivecurrent at the installation point and, at this value, the protectionlevel Up shall be lower than the required one. Moreover, theselection of such current has an essential influence on the agingof SPD. In particular, this aspect is more important in an SPD

limiting type, such as a varistor, where a degradation isregistered due to the effect of single or multiple current pulsesthat are able to produce microstructural changes in the material.

As shown in the diagram of Fig. 1, the lightning currentstriking the LPS, as result of resistive coupling, is sharedbetween the earth arrangement of the structure and the supplyline. The currentISPDflowing through the SPD to the incomingline depends, in amplitude and shape, on the conventionalimpedance of the earth arrangement (Z) and on the impedanceof the supply line (mainly on line length) and on the waveshape of the lightning current. An example of the currents

distribution of the considered arrangement is presented in Fig.2 and in Fig. 3 for lightning current shape 10/350 s and0,25/100 s respectively.

SPD

Z

PHASE

EBB

SERVICE

ENTRANCE PE

Z

SPD

LVMV

APPARATUS

TRANSFORMER

LPS

Figure 1. Diagram of circuit to supply an apparatus and the loop formed by

PE, phase conductor and an SPD bonded to a concentrated earth termination

arrangement (type A according to IEC/EN 62305-3).

Figure 2. Current distribution in the arrangement considered in Fig. 1;

lightning current wave shape 10/350 s; conventional earth impedance

Z = 10 ; supply line length L = 100 m with two conductors; no external

connected services. I: lightning current; IG: current to earth electrode;IN: current to the neutral;ISPD: current to the phase conductor.

Figure 3. Current distribution in the arrangement considered in Fig. 1;

lightning current wave shape 0,25/100 s; earth impedanceZ = 10 ; supply

line length L= 100 m with two conductors; no external connected services. I:

lightning current; IG: current to earth electrode; IN: current to the neutral;ISPD: current to the phase conductor.

Indeed shape of lightning current and line length scarcelyaffect the amplitude of ISPD, while the rise time T1 of ISPDincreases with the length of the supply line as reported inFig. 4; this directly affects the voltage drop V on theconnection conductors of SPD (see Section IV).


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0 100 200 300 400 500

0

10

20

30

40

50

60

T1(

s)

10/350s 1/200s 0,25/100s

L(m) Figure 4. Rise timeT1of the currentISPDas a function of supply line lengthL

for threeshapes of lightning current.

The highest values of current flowing through the SPD

occur where a single line with two conductors is connected tothe structure, with no other connected external service and forhigh values of the conventional impedance of the eartharrangement (Z), as shown in Fig. 5.

0 10 20 30 40 50 60

0

10

20

30

40

50

ISPD

(%)

Z() Figure 5. Percentage of the currentISPDas a function of conventional earth

impedance for the positive stroke current 10/350 s. Length of supply line

L= 50 m; no external connected services.

The Z value of an earthing arrangement, dimensionedaccording to IEC/EN 62305-3, increases with the lightningprotection level (LPL), i.e. Z = 10 for LPL I and Z = 15

for LPL II; therefore the currentISPDis increasing with LPL.

As an example, ifZ = 10 is assumed and a single supplyline with two conductors is connected to the structure, with anyother connected external service, the current ISPD ranges from27% to 21% for a positive flash (standard shape 10/350 s)and from 23% to 18% for subsequent stroke of negative flash(standard shape 0,25/100 s), as the length of the line Lvariesfrom 50 m to 500 m. These values are in the range of theapproximate value (25%) suggested by the IEC/EN 62305-1independently from the wave shape of lightning current, thelength of the line and the protection level LPL.

The currentISPD is not practically influenced by :

- the type of earthing arrangement, namely rod type (type Aaccording to standard IEC/EN 62305-3) or ring or meshtype (type B of standard IEC/EN 62305-3);

- the type of electrical system , TT or TN;- the type of SPD, switching or limiting .

For a properly designed SPD, the charge for unit of current,associated to the currentISPD, is increasing with the line lengthbut at least for the line length values L< 1000 m, is alwaysless than 0,5 C/kA, even in the worst case of positive flashes(shape 10/350 s), as shown in Fig. 6.

Note that the charge 0,5 C/kA is the value associated to thecurrent with the standard shape 10/350 s, so that indeed suchshape appear adequate for SPD class I testing.

0 200 400 600 800 1000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

QSPD

/IS

PD

(As/kA)

10/350s 1/200s 0,25/100s

L(m)

Figure 6. Charge per unit of current flowing through SPD as a function ofsupply line lengthLfor three shapes of lightning current.

IV. SPDSELECTION WITH REGARD TO ITS PROTECTIONLEVEL

The selection with regard to overvoltage protection level Upof an SPD has a direct influence on the values of the voltage atthe terminals of the apparatus to be protected Ul.

The voltage Uldepends on:

- the protection level Upof SPD,

- the inductive voltage drop V of the leads/connections ofSPD,

- the effects of surge travelling along the protected circuit,

- the overvoltages Ui induced by lightning current in theprotected circuit.

The inductive voltage drop Von the leads/connections ofSPD should be combined with the protection level Upin orderto obtain the so-called effective protection level Up/f of theSPD [2]. The voltage drop depends on the length of theconnecting leads and on the steepness of the current flowingthrough the SPD, which, depends, as mentioned at the


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preceding section, on the impedance of the earth arrangementand on the line length.

Moreover, the propagating effect of fast transients along theprotected circuit shall be taken into account. Due to oscillationphenomena of fast surges, in the time interval of two times thetravel time of surge along the circuit, the voltage Ul at theapparatus terminals is [13,14]:

Ul= Up+k l if l < l2 (2)

Ul= 2Up if l l2 (3)

where:

v

sk

=2 increment of the voltage per unit of the circuit length

(V/m);

s front steepness of the voltage at SPD terminals (V/s);

v the speed of voltage surge along the circuit (m/s);

l the distance between SPD and apparatus to be protected,

l2 the distance at which the voltage is doubled on apparatusterminals.

For the SPD of switching type the voltage Ul at the SPDterminals may be approximate by a triangular shape reachingthe maximum value Upat switching time of the SPD.

The distance l2 at which the voltage is doubled on apparatusterminals decreases with:

- increasing of the front steepnesss;- increasing of protection level Upof SPD;- increasing of the amplitude of earth-termination voltage

Uo=ZIGstressing the SPD, as shown in Fig. 7;

- decreasing of the rise time of the stressing overvoltageUo.

0 100 200 300 400 500

1

10

100

1000

l2(m)

UO(kV)

10/350s 1/200s 0,25/100s

Figure 7. Protected circuit length l2 as function of earth-termination voltage

Uofor three lightning current shapes; protection level of switching type SPD:

1,25 kV.

For the SPD of limiting type the voltage Ul at the SPDterminals may be approximated by a rectangular shape and themaximum value Upis reached in a short time from 0,1 s forwave shape 0,25/100 s to 3 s for shape 10/350 s of thelightning current ; such time is scarcely affected by the linelength and by the value of conventional earth impedanceZ.

The distance at which the voltage is doubled on apparatus

terminals (l2) increases as the rise time of the lightning currentIis increasing. In Fig. 7, 8 and 9 a summary of the obtainedresults is reported for negative subsequent strokes current0,25/100 s, negative first stroke current 1/200 s and positiveflash current 10/350 s.

0 10 20 30 40 50

0.0

0.5

1.0

1.5

2.0

2.5

Ul

/U

p

limiting type SPD switching type SPD

l(m)

Figure 8. Voltage ratio Ul/ Upfor SPD switiching and limiting type as a

function of the protected circuit length l. Protection level of the SPD: 1,25 kV;

lightning current 0,25/100 s; earth termination voltageUo= 80 kV; supply

line lengthL = 50 1000 m.

0 20 40 60 80 100

0.0

0.5

1.0

1.5

2.0

2.5

Ul

/U

p

10/350s

1/200s

0,25/100s

l(m)

Figure 9. Voltage ratio Ul/ Upas a function of the protected circuit length lfor three lightning current shapes.

For the fastest lightning current (0,25/100 s), usually thedistance l2 does not overcome a value of 20 m, but at thedistance of about 10 m from the SPD the voltage already is

Ul 1,9Up, as demonstrated in Fig. 9. Therefore for practicalpurposes k = 0,1Up may be assumed. Furthermore, the


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overvoltage Ui induced by lightning current in the protected

circuit, has to be also considered, and its influence on theprotection distance must be taken in account. Simple methodsto evaluate the induced voltage Ui are reported in [2].

Lastly it is noted that the voltage measured at the apparatusterminals may overshoot the SPD nominal protection level [15]even by a factor higher than 2. Such result should be furtherconsidered and discussed with particular attention to successivereflections from the LV power line and to the capacitivecoupling to earth of the line as well as of the circuit betweenSPD and apparatus, which cannot be ignored and mayinfluence the voltage at the apparatus terminals.

V. INDUCTIVE VOLTAGE DROP VON THELEADS/CONNECTIONS OF SPD

The inductive voltage drop Von the leads/connections ofSPD depends on the inductance Lc of such leads and on thesteepness of the current ISPD flowing through the SPD to theincoming lines, and then on its amplitude and rise time.

As discussed in Section I, amplitude and rise time of thecurrent ISPD depend on the earthing arrangement and on theline length. Because the amplitude of the current ISPD isdecreasing and the rise time T1is increasing with increasing ofthe line length, consequently the voltage drop V is decreasingwith such length.

The highest value of the voltage drop is due to the flowingof the current of negative subsequent stroke.

For a properly designed SPD, the values of Vare in mostthe cases higher than V= 1 kV/m fixed by the IEC/ EN62305-4, independently from the length of the connected line,from the sharing of the current among the conductors of the

incoming lines and external services as well as from thelightning protection level LPL.

At assumption of Lc = 1H/m, the voltage drop Vassumes the values reported in Tab. II according to theprotection level LPL.

TABLE II. VOLTAGE DROP V IN THE SPDBRANCH;EARTHIMPEDANCEZ=10;MONOPHASE SUPPLY LINE LENGTH RANGING

BETWEEN 50 m AND 1000m.

Source typeLPL I

(kV/m)

LPL II

(kV/m)

LPL III-IV

(kV/m)

First positivestroke

(10/350 s)

6,5 1,5 5 1 3,3 0,7

First negative

stroke

(1/200 s)

7 1,6 5,5 1,2 3,5 0,8

Subsequent

negative stroke

(0,25/100 s)

8 3,5 6 2,6 4 1,7

VI. SPDDIMENSIONINGA. Selection of protection level Up

For the SPD1 installed at the entry point of the line(main switch board) the selection of protection level Up should

take into account:a) the inductive voltage drop V of the

leads/connections of SPD,

b) the effects of surge travelling along the protectedcircuit,

c) the overvoltages Uiinduced by lightning current in theprotected circuit formed by the SPD and the apparatusto be protected.

In all the three cases reference should be made to thesubsequent strokes of negative flashes, which represent themore severe case, as discussed in Sections III.

Following the IEC 62305-4 [2], if an effective protection

level Upf is defined as the voltage at the output of the SPD

resulting from its protection level Up and the voltage drop V,namely:

Upf= Up + V for limiting type SPD (4)

Upf= max (Up, V) for switching type SPD (5)

the following formulas may help in the selection of Upaccording to the length l of the protected circuit:

Upf Uw for l = 0 m (6)

Upf (Uw Ui ) / (1+ 0,1l) for 0 < l 10 m (7)

Upf (Uw Ui ) / 2 for l > 10 m (8)

being Uw the rated impulse withstand voltage of the apparatus

to be protected.

It is to note that the value Upf 0,8 Uw suggested by the

IEC/ EN 62305-4 for 0 < l 10 m is not on safety side infront of the value given by formula (7).

Due to high values of V, even if Ui= 0 is assumed, thedistance between SPD1 and apparatus (or the value of Up)should be kept so low that practically in all cases installation ofa downstream SPD2 is required.

Once SPD2 is provided, the fulfillment of conditions a, b)and c) are no longer required for SPD1: only the energy

coordination between SPD1 and SPD2 should be considered.

B. Selection of SPD discharge currentThe impulse current Iimp of a class I test SPD and the

nominal current Inof a class II test SPD should be selected insuch way that :

a) the value of Up at the current ISPD expected at the point ofSPD installation, does not overcome the rated impulsewithstand voltage level Uw of the equipment to beprotected;


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b) the energy associated to the discharge current does notovercome the value tolerated by the SPD.

If the shape of the ISPD would be the same of the SPDstandard test current (10/350 s for SPD of class I test, 8/20 sof SPD of class II) both the conditions a) and b) could beverified if:

IimpISPD for SPD of class I test (9) InISPD for SPD of class II test (10)

Usually the wave shape of expected ISPD differs from the

standardized shape of SPD test current. In this case, conditionb) may be considered satisfied as a first approximation if thecharge QSPDassociated toISPDdoes not overcome the tolerableone for the SPD.

If we consider that the charge QSPD for unit of currentassociated to the standard current 10/350 s is Qimp= 0,5 C/kA

and to the standard current 8/20 s is Qn = 0,027 C/kA, therelations to be respected for SPD dimensioning are thefollowing:

for SPD class I test

IimpISPD (11)

Qimp QSPDor numericallyIimp 2 QSPD (12)

for SPD class II test

InISPD (13)

Qn QSPD or numericallyIn 37 QSPD (14)

It should be noted (see Fig. 6) that an SPD1 class I test isable to withstand the expected charge QSPD so that its

dimensioning is determined by the currentISPD.

VII.CONCLUSIONS The selection of a proper protection level of SPD is affected

by the length and the characteristics of the circuit betweenthe SPD and the apparatus and by the shape of lightningcurrent;

the voltage Ulat apparatus terminals increases with circuitlength and may reach the double of SPD effectiveprotection level Up/f in ten meters for the negativesubsequent stroke (worst case);

the inductive voltage drop Von the leads/connections ofSPD may reach several hundred volts even for shortconnection length (0,1 0,2 m);

due to high values of V, even if overvoltage Uiinducedin the circuit is negligible, the distance between SPD1 and

apparatus (or the value of Up) should be kept so low thatpractically in all cases installation of a downstream SPD2is required;

the rise time of the currentISPDflowing through the SPD tothe incoming lines is longer than the rise time of thelightning current and is increasing as the length of the line

increases, while its amplitude decreases with the line

length;

an SPD1 class I test is able to withstand the expectedcharge QSPD so that its dimensioning is determined by thecurrentISPD;

the standard lightning current shape 10/350 s appearsadequate for SPD testing.

ACKNOWLEDGMENT

The paper has been prepared in the frame of internationalcooperation between Warsaw University of Technology andUniversity of Rome La Sapienza. The Authors wish toexpress their gratefulness to the Authorities of bothUniversities.

REFERENCES

[1] IEC 62305-1, Ed. 2,0 2010-12, "Protection against lightning Part 1:General principles.

[2] IEC 62305-4, Ed. 2,0 2010-12, "Protection against lightning Part 4:Electrical and Electronic Systems within structures.

[3] IEC 62066: 2000: "General basic information regarding surgeovervoltages and surge protection in low-voltage a.c. power circuits".

[4] IEEE Std. PC62.41.1-2002 IEEE Guide on the Surge Environment inLow-Voltage (1000 V and Less) AC power circuits.

[5] IEC 60364, Low-voltage electrical installations Part 1: Fundamentalprinciples, assessment of general characteristics.

[6] IEC 61643-11: Low-voltage surge protective devices Part 1:Performance requirements and testing methods.

[7] IEC 61643-12, Ed. 2,0 2008-8, Low-voltage surge protective devices -Part 12: Surge protective devices connected to low-voltage power

distribution systems - Selection and application principles

[8] T. Kisielewicz, C. Mazzetti, G.B. Lo Piparo, B. Kuca, Z. Flisowski,

Electronic Apparatus Protection Against LEMP: Surge Threat for theSPD Selection, Paper ID 460, Paper accepted for presentation to EMC

Europe 2012, Rome, September 2012.[9] T. Kisielewicz, F. Fiamingo, C. Mazzetti, D. Krasowski, P. Sul, B.

Kuca, Simulated and tested protection effects on electrical equipment

terminals at overvoltages incoming through distant SPD, InternationalYouth Conference on Energetics 2011 IEEE Conference, Leiria,

Portugal, 7-9 July 2011

[10] J. Birkl, P. Hasse and P. Zahlmann, Investigation of the interaction oflightning currents with low-voltage installations and their related

lightning threat parameters, in Proc. 23rd ICLP, Florence, Italy, 1996.

[11] J. Birkl and P. Zahlmann, Lightning currents in low-voltageinstallations, in Proc. 29th ICLP, Uppsalla, Sweden, 2008.

[12] J. Birkl, C.F. Barbosa, Modeling the current through the power

conductors of an installation struck by lightning, SIPDA, Fortaleza,Brasil, 2011.

[13] G.L. Amicucci, F. Fiamingo, M. Marzinotto, C. Mazzetti, G. B. Lo

Piparo, Z. Flisowski, Protection against lightning overvoltages ofelectrical and electronic systems: evaluation of the protection distance

of an SPD, ICLP 2004, September 2004, Avignon, France.

[14] G. L. Amicucci, F. Fiamingo, Z. Flisowski, G.B. Lo Piparo, C. Mazzetti,

Surge protective devices for low voltage systems: practical approachfor the protection distance evaluation, Paper ID 160, IEEE Power Tech

Conference, Lausanne 2005.

[15] Ibrahim A. Metwally, Fridolin H. Heidler, Enhancement of the SPDResidual Voltage at Apparatus Terminals in Low-Voltage Power

Systems, in IEEE Transactions on Power Delivery, Vol. 22, No. 4,October 2007.

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