Страница 9: ДСТУ EN 62305-4:2012. Protection against lightning — Part 4: Electrical and electronic systems within structures (61692)

Computer simulations can be a helpful tool in order to consider these factors for the correct selection of SPDs. In order to evaluate the lightning current dispersion for a complex system it is necessary to convert the real world system, as shown in the example of Figure D.2 into an equivalent electrical circuit diagram.

Many standards have sought rather to base their considerations of the threat level to which an SPD may be subject, on field experience collected over time. Table E.2 of IEC 62305-1:2010 is based mainly on field experience (see IEEE C62.41 series ¹⁹¹).

D.3.3 Considerations in the selection of SPD ratings: l,_mn, /_m,_v, U_nr** lill p ІІІаЛ fl 4/V

From the above, it is apparent that the selection of the appropriate ratings /_imp, /_max, /_n and U_oc of an SPD depends on many complex and interconnected parameters.

It is important to keep in context that the risk of damage to internal systems within a structure due to surges arising from

• induced effects coupling power, phone and data lines (S4),

• LEMP effects of coupling from nearby strikes to the structure (S2),

may often be greater than those due to the effects of surges arising from direct strikes to the structure itself (S1) or to lines (S3).

Many buildings do not require protection against direct strikes to the structure or to incoming lines, and as such the requirement for test class I SPD(s) is not necessary, while a correctly designed test class II SPD system may be appropriate.

In general, the approach should be to use a test class I SPD where direct or partial lightning currents are involved (S1/S3) and a test class ll/ill SPD for induced effects (S2/S4).

When addressing such complexities, one needs to keep in mind that the most important aspect in selecting an SPD is its voltage limiting performance during the expected surge event, and the energy withstand (/_imp, /_max, /_n, U_oc) which it can handle (see NOTE 4 that follows Table B.7 in IEC 62305-2:2010).

At the expected /_n, an SPD with a limiting voltage lower than the withstand voltage of the equipment will ensure equipment protection, particularly considering external factors that create additive voltages (voltage drop on connecting leads, oscillations and induction phenomena). In contrast, an SPD with a withstand energy higher than that required at the point of installation may result only in a longer SPD operating life. However, an SPD with lower limiting voltage may be more susceptible to possible damage from temporary over voltages (TOV) if installed on poorly regulated power systems

.Bibliography

1. I EC 60364-4-44, Low-voltage electrical installations - Part 4-44: Protection for safety - Protection against voltage disturbances and electromagnetic disturbances

2. I EC 61000 (all parts), Electromagnetic compatibility (EMC)

3. ITU-T Recommendation K.20:2008, Resistibility of telecommunication equipment

installed in a telecommunications centre to overvoltages and overcurrents

4. ITU-T Recommendation K.21:2003, Resistibility of telecommunication equipment

installed in customer premises to overvoltages and overcurrents

5. ITU-T Recommendation K.45:2003, Resistibility of telecommunication equipment

installed in the access and trunk networks to overvoltages and overcurrents

6. I EC 61000-5-2:1997, Electromagnetic compatibility (EMC) - Part 5-2: Installation and mitigation guidelines - Earthing and cabling

7. ITU-T Lightning handbook: 1994, The protection of telecommunication lines and equipment against lightning discharges - Chapter 10

8. I EC 61643-11: Low-voltage surge protective devices - Part 11: Surge protective devices connected to low-voltage power distribution systems - Performance requirements and testing methods

9. IEEE C62.41:1991, Recommended practice on surge voltages in low-voltage ac power circuits

1Figures in square brackets refer to the bibliography.