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This annex provides information on the selection and installation of a coordinated SPD system. Additional information may be found in IEC 61643-12 and IEC 60364-5-53 which deal with protection against overcurrent and the consequences in the case of an SPD failure.

The failure due to surges exceeding the immunity level of electronic equipment is not covered within the scope of the IEC 62305 series. The reader is referred to IEC 61000-4-5 for treatment of this subject.

However, lightning surges frequently cause failure of electrical and electronic systems due to insulation breakdown or when overvoltages exceed the equipment’s common mode insulation level.

Equipment is protected if its rated impulse withstand voltage U_w at its terminals (common mode withstand voltage) is greater than the surge overvoltage between the live conductors and earth. If not, an SPD must be installed.

Such an SPD will protect the equipment if its effective voltage protection level L/_p/F (the protection level Up obtained when the nominal discharge current /_n flows added to the inductive voltage drop AL/ of the connecting conductors) is lower than (J_w It should be noted that if the discharge current which occurs at the point of installation of the SPD exceeds the designated /_n of the SPD, the protection level U_p will be higher, and t/_p/F may exceed the equipment’s withstand level U_w. In this case the equipment is no longer protected. It follows that the nominal current /_n of the SPD should be selected to be equal to, or higher, than the discharge lightning current which can be expected at this point of installation.

The probability that an SPD with U_p/F < U_w does not adequately protect the equipment for which it is intended, is equal to the probability that the discharge current at the point of installation of this SPD exceeds the current at which U_p was determined.

Evaluation of the currents expected at various points in the installation is given in Annex E of IEC 62305-1:2010, and is based on the LPL determined using IEC 62305-2. A complete analysis of current sharing is required when considering the S1 event. Annex D of this standard provides additional information.

It should also be noted, that selecting an SPD with a lower value U_p (compared to the equipment’s (J_w) results in a lower stress to the equipment that may result not just in a lower probability of damage, but also a longer operating life.

Values of the probability P_SPD as a function of the LPL are given in Table B.3 of IEC 62305-1:2010.

NOTE Values of P_SPD for SPDs providing better protection characteristics can be determined if the voltage vs current characteristic of the SPD is available.

Finally, the importance of applying SPD protection to both power and signal circuits is essential if an effective coordinated SPD system is to result.

C.2 Selection of SPDs

C.2.1 Selection with regard to voltage protection level

Selection of the proper voltage protection level of the SPD depends on

• the impulse withstand voltage t/_w of the equipment to be protected,

• the length of the connecting conductors to the SPD,

• the length and the routing of the circuit between the SPD and the equipment.

The impulse withstand voltage L/_w of the equipment to be protected should be defined for

• equipment connected to power lines in accordance with IEC 60664-1 and IEC 61643-12,

• equipment connected to telecom lines in accordance with IEC 61643-22, ITU-T K.20 ^[3], K.21^[4] and K45 ^[5],

• other lines and equipment terminals in accordance with information obtained from the manufacturer.

NOTE 1 The protective level U_p of an SPD is related to the residual voltage at a defined nominal current /_n. For higher or lower currents passing through the SPD, the value of voltage at the SPD’s terminals will change accordingly.

NOTE 2 The voltage protective level U_p should be compared with the impulse withstand voltage U_m of the equipment, tested under the same conditions as the SPD (over voltage and over current waveform and energy, energized equipment, etc.). This matter is under consideration.

NOTE 3 Equipment may contain internal SPD components. The characteristics of these internal SPDs may affect the coordination.

When an SPD is connected to equipment to be protected, the inductive voltage drop At/ of the connecting conductors will add to the protection level U_p of the SPD. The resulting effective protection level L/_p/F, defined as the voltage at the output of the SPD resulting from the protection level and the wiring voltage drop in the leads/connections (see Figure C.1), can be assumed as being:

U_p/F= Up + &U for voltage limiting type SPD(s);

t/_p/F = max (Up, &U) for voltage switching type SPD(s).

NOTE 4 For some switching type SPDs it may be required to add the arc voltage to ДІЛ This arc voltage may be as high as some hundreds of volts. For combination type SPDs more complex formulae may be needed.

When the SPD is installed at the line entrance into the structure, At/ = 1 kV per m length, should be assumed. When the length of the connection conductors is < 0,5 m, U_p/F = 1,2 x U_p can be assumed. When the SPD is carrying induced surges only, At/ can be neglected.

During the operating state of an SPD, the voltage between the SPD terminals is limited to Up/p at the location of the SPD. If the length of the circuit between the SPD and the equipment is too long, propagation of surges can lead to an oscillation phenomenon. In the case of an open-circuit at the equipment’s terminals, this can increase the overvoltage up to

2 x Up/p and failure of equipment may result even if L/_p/F < t/_w

Information on the connecting conductors, connecting configurations and fuse withstand levels for SPDs can be found in IEC 61643-12 and IEC 60364-5-53.

Moreover lightning flashes to the structure or to ground nearby the structure, can induce an overvoltage Ц in the circuit loop between the SPD and the equipment, that adds to t/_p/F and thereby reduces the protection efficiency of the SPD. Induced overvoltages increase with the dimensions of the loop (line routing: length of circuit, distance between PE and active conductors, loop area between power and signal lines) and decrease with attenuation of the magnetic field strength (spatial shielding and/or line shielding).

NOTE 5 For evaluation of induced overvoltages U_{, Clause A.4 applies.

Internal systems are protected if

• they are energy coordinated with the upstream SPD(s), and

• one of the following three conditions is fulfilled:

1. U_p/F< U_w: when the circuit length between the SPD and the equipment is negligible (typical case of an SPD installed at equipment terminals);

2. t/_p/F < 0,8 l/_w: when the circuit length is not greater than ten metres (typical case of SPD installed at a secondary distribution board or at a socket outlet);

NOTE 6 Where failure on internal systems may cause loss of human life or loss of service to the public doubling of voltage due to oscillations should be considered and the criteria U_p/F < U_w /2 is required..

3. U_p/F < (t/_w - t/|) / 2: when the circuit length is more than ten metres (typical case of SPD installed at the line entrance into the structure or in some cases at the secondary distribution board).

NOTE 7 For shielded telecommunication lines, different requirements may apply due to the steepness of the wave front. Information on this effect is provided in Chapter 10 of the ITU-T lightning handbook ^[7].

If spatial shielding of the structure (or of the rooms) and/or line shielding (use of shielded cables or metallic cable ducts) are provided, induced overvoltages 11, are usually negligible and can be disregarded in most cases

.

Кеу

partial lightning current

i

^ul

Up/_F=Up+ b.U

Up

ьи= Ди_и + au_l2

H, dHIdt

nduced overvoltage

surge voltage between live conductor and bonding bar limiting voltage of SPD

inductive voltage drop on the bonding conductors

magnetic field and its time derivative

NOTE The surge voltage Up/p between the live conductor and the bonding bar is higher than the protection level l/p of the SPD, because of the inductive voltage drop Д1/ at the bonding conductors (even if the maximum values of Up and ДО do not necessarily appear simultaneously). That is, the partial lightning current flowing through the SPD induces additional voltage into the loop on the protected side of the circuit following the SPD. Therefore the maximum voltage endangering the connected equipment can be considerably higher than the protection level Up of the SPD.

Figure C.1 - Surge voltage between live conductor and bonding bar

C.2.2 Selection with regard to location and to discharge current

SPDs should withstand the discharge current expected at their installation point in accordance with Annex E of IEC 62305-1:2010. The use of SPDs depends on their withstand capability, classified in IEC 61643-1 for power, and in IEC 61643-21 for telecommunication systems.

The selection of an SPDs discharge current rating is influenced by the type of connection configuration and the type of power distribution network. More information on this may be found in IEC 61643-12 and IEC 60364-5-53.

SPDs should be selected in accordance with their intended installation location, as follows:

1. At the line entrance into the structure (at the boundary of LPZ 1, e.g. at the main distribution board MB):

• SPD tested with /_imp(class I test)

The required impulse current /_im of the SPD should provide for the (partial) lightning current to be expected at this installation point based on the chosen LPL in accordance with Clause E.2 (source of damage S1) and/or E.3.1 (source of damage S3) of IEC 62305-1:2010.

• SPD tested with /_n(class II test)

This type of SPD can be used when the lines entering are entirely within LPZ 0_B or when the probability of failures of the SPD due to sources of damage S1 and S3 can be disregarded. The required nominal discharge current /_n of the SPD should provide for the surge level to be expected at the installation point based on the chosen LPL and related overcurrents, in accordance with E.3.2 of IEC 62305-1:2010.

NOTE 1 The risk of failures of the SPDs due to sources of damage S1 and S3 can be disregarded if the total number of direct flashes to structure (N_D) and to line (N_L) complies with the condition N_o+ N_L< 0,01.

2. Close to the equipment to be protected (at the boundary of LPZ 2 and higher, e.g. at a secondary distribution board SB, or at a socket outlet SA).

• SPD tested with /_n(class II test)

The required nominal discharge current /_n of the SPD should provide for the surge current to be expected at this point of the installation, based the chosen LPL and related overcurrents in accordance with Clause E.4 of IEC 62305-1:2010.

NOTE 2 An SPD having the characteristics of class I and class II tests may be used in this location.

• SPD tested with a combination wave U_oc(class III test)

This type of SPD can be used when the lines entering are entirely within LPZ 0_B or when the risk of failures of the SPD due to sources of damage S1 and S3 can be disregarded. The required open circuit voltage rating U_oc of the SPD (from which the short-circuit current /_sc can be determined, since test class III is carried out using a combination wave generator with a 2 Q impedance) should provide for the surge level to be expected at the installation point, based on the chosen LPL and related overcurrents,in accordance with Clause E.4 of IEC 62305-1:2010.

C.3 Installation of a coordinated SPD system

C.3.1 General

The efficiency of a coordinated SPD system depends not only on the proper selection of the SPDs, but also on their correct installation. Aspects to be considered include:

• location of the SPD;

• connecting conductors.

C.3.2 Installation location of SPDs

The location of the SPDs should comply with C.2.2 and is primarily affected by:

• the specific source of damage e.g. lightning flashes to a structure (S1), to a line (S3), to ground near a structure (S2) or to ground near a line (S4),

• the nearest opportunity to divert the surge current to ground (as close to the entrance point of a line into the structure as possible).

The first criterion to be considered is: the closer the SPD is to the entrance point of the incoming line, the greater the amount of equipment within the structure that will be protected by this SPD (economic advantage). Then the second criterion should be checked: the closer an SPD is to the equipment being protected, the more effective its protection will be (technical advantage).

C.3.3 Connecting conductors

The SPDs connecting conductors should have a minimum cross-sectional area as given in Table 1

.С.3.4 Coordination of SPDs

In a coordinated SPD system, cascaded SPDs need to be energy coordinated in accordance with IEC 61643-12 and/or IEC 61643-22. For this purpose, the SPD manufacturer should provide sufficient information as to how to achieve energy coordination between his different SPDs.

C.3.5 Procedure for installation of a coordinated SPD system

A coordinated SPD system should be installed as follows:

• At the line entrance into the structure (at the boundary of LPZ 1, e.g. at installation point MB) install SPD1 fulfilling the requirements of C.2.2.

• Determine the impulse withstand voltage U_w of internal systems to be protected.

• Select the voltage protection level U_P1 of SPD 1.

• Check the requirements of C.2.1aremet.

If this requirement is met, the equipment is adequately protected by SPD 1. Otherwise, an additional SPD 2(s) is/are needed.

• If so required, closer to the equipment (at the boundary of LPZ 2, e.g. at the installation point SB or SA), install SPD 2 fulfilling the requirements of C.2.2 and energy coordinated with the upstream SPD 1 (see C.3.4).

• Select the protection level U_P2 of SPD 2.

• Check the requirements of C.2.1 are met.

If this requirement is met, the equipment is adequately protected by SPD 1 and SPD 2.

• Otherwise, close to the equipment (e.g. at installation point SA socket), additional SPD 3(s) is/are needed fulfilling the requirements of C.2.2 and energy coordinated with the upstream SPD 1 and SPD 2 (see C.2.3),

Check the condition L/_p/F3 < (J_w is fulfilled (see C.2.1)

.Annex D
(informative)

Factors to be considered in the selection of SPDs

D.1 Introduction

/_imp, /_max and /_n, are test parameters used in the operating duty test for class I and class II tests. They are related to the maximum values of discharge currents, which are expected to occur at the LPL probability level at the location of installation of the SPD in the system. /_max is associated with class II tests and /_imp is associated with class I tests.

The preferred values for /_imp, Q, W/R, in accordance with the future IEC 61643-11 ^[81 are reproduced in Table D.1.

Table D.1 - Preferred values of /_jmp^a

D.2 Factors determining the stress experienced by an SPD

The stress, which an SPD will experience under surge conditions, is a function of many complex and interrelated parameters. These include:

• location of the SPD(s) within the structure - See Figure D.1;

• method of coupling of the lightning strike to the facility (see Figure D.2) - for example, is this via a direct strike to the structure’s LPS (S1), or via induction onto building wiring due to a nearby strike (S2), or services feeding the structure (S3 and S4);

• distribution of lightning currents within the structure - for example, what portion of the lightning current enters the earthing system, and what remaining portion seeks a path to remote earths via services which enter the structure such as the power distribution system, metallic pipes, telecom services, etc. and the equipotential bonding SPDs used on these;

• the resistance and inductance of services entering the structure, as these components effect the current peak value, /, and charge Q distribution ratios;

• additional conductive services connected to the facility - these will carry a portion of the direct lightning current and therefore reduce the portion which flows through the power distribution system via the lightning equipotential bonding SPD(s). Attention should be paid to the permanence of such services due to possible replacement by non-conductive parts;

• type of waveshape being considered - it is not possible to consider simply the peak current which the SPD will have to conduct under surge conditions, one also has to consider the waveshape of this surge (for example, 10/350 ps covering direct and partial lightning current, 8/20 ps covering induced lightning current) and the bulk charge Q;any additional structures which are interconnected to the primary structure via the power service, as these will also effect the current sharing distribution.

Key

1. origin of the installation

2. distribution board

3. distribution outlet

4. main earthing terminal or bar

5. surge protective device, class I or II tested

6. earthing connection (earthing conductor) of the surge protective device

NOTE Refer to IEC 61643-12 for further information.

7. fixed equipment to be protected

8. surge protective device, class II tested

9. surge protective device, class II or class III tested

10. decoupling element or line length

F1, F2, F3 overcurrent protective disconnectors

Figure D.1 - Installation example of test class I, class II and class III SPDs

LI 12

13 PEN

' з Apparatus

Serail

. water)

_ ^/earthing Power line of low voltage ~ energy technical network

Flash near the service connected to the structure

Flash to the service connected to the structure

Flash to the structure

Flash near to the structure

Signal line of information technical network

EC 2873/tO

/power

/data * '

Source of damage:

Figure D.2 - Basic example for different sources of damage to a structure
and lightning current distribution within a system

D.3 Quantifying the statistical threat level to an SPD

D.3.1 General

Many attempts have been made to quantify the electrical environment and “threat level” which an SPD will experience at different locations within a facility. For example, for a service entrance SPD where a structural LPS is fitted, the threat level depends on the required LPL according to risk assessment for the involved structure in order to limit such risk to the tolerable value (see Clause 6 of IEC 62305-1:2010).

This standard postulates that under an LPL I the magnitude of a direct strike (S1) to the structure’s LPS may be as high as 200 kA with a waveshape of 10/350 ps (see 8.1 and Annex A of IEC 62305-1:2010). However, whilst the SPDs should be selected to meet the required LPL identified by the risk assessment, there are further factors that would affect the magnitude of lightning current to which SPD is subjected.

D.3.2 Installation factors effecting current distribution

When no specific calculation of current sharing (see Clause E.2 of IEC 62305-1:2010) is carried out, a general assumption is made that 50 % of this current is conducted to the building’s earthing system, and 50 % returns via the equipotential bonding SPD(s). For LPL I, this implies that the portion of the initial 200 kA discharge experienced by each SPD, /_irnp, is 25 kA for a three phase plus neutral power distribution system - see Figure D.3.

Figure D.3 - Basic example of balanced current distribution

If, however, three metallic services supply the structure, and the model of Clause E.2 of IEC 62305-1:2010 is adopted, the total current, /_imp, to each equipotential bonding SPD in the three-phase system becomes 8,3 kA.

The distribution of lightning current on a power distribution system is strongly influenced by the earthing practice of the services entering the structure. For example, in the TN-C system with its multiple-earthed neutral, a more direct and lower impedance path to earth is provided for lightning currents than in a TT system.

Simplified assumptions of current dispersion are useful in considering the possible threat level, which the SPD(s) may experience, but it is important to keep in context the assumptions being made. In addition, it has been assumed that the waveshape of this current component through the SPD(s) will be the same waveshape as the initial discharge, whereas in reality the waveshape may have been altered by the impedance of building wiring, etc.