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IEEE C62.41-1991 IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits. The IEEE C62-41 specification is very comprehensive and has been used to define the risk of a surge in defining the minimum surge current capacity of surge protectors to be installed a certain locations on low voltage AC power at a site. This was classified into three general classifications : Class A, Class B and Class C.

Class C defines the most severe risk of a 10 kA surge, delivered by a Utility on AC power lines at the "service entrance" of a building. As you can understand, the IEEE specification with this statement defined the significant threat to be from a surge on the power lines. During 2002 the IEEE specification was update to IEEE C62-21-2002 which includes, amongst other things, a definition of two "Scenario" for the damage coupling mechanism. The traditional damage mechanism threat from a surge delivered on a power line is referred to as "Scenario 1".

Scenario 2 accepts and agrees with the European IEC specifications that defines a direct lightning strike coupling mechanism and the implications to a surge protector. IEEE C62-21-2002 is an updated version that included "two scenarios" for surges in low voltage AC Power Circuits and discusses the IEC specifications in this extract.

As explained in IEEE C62.41.1-2002 and in 4.6 and 7.4 of the present recommended practice, the possible event-rare as it might be-of a direct strike to the building of interest is the basis of the Scenario 2. In contrast, Scenario I represents the event of surges impinging onto the building by way of the incoming power service connection. The database of IEEE Std C62.41.1-2002 contains convincing and long-standing evidence on the nature and severity of Scenario I events, but Scenario 2 is at this stage less thoroughly documented and the consensus somewhat limited. For that reason, this informative annex offers background information and details on the consensus-building process.

Essentially, the nature of Scenario 2 is based on well-documented and well-accepted data based on the parameters of the lightning flash itself. However, the resulting surge currents carried by the building conductors (intended or opportunistic) and any SPDs installed in the building rest on postulates about the dispersion of the flash current. This dispersion process is controlled by the paths that are available to feed the current into many earthing (grounding) electrodes. These electrodes include both those local to the building (intended made electrodes, ground rods, as well as opportunistic electrodes) and distant electrodes accessible by way of the communications, data or power service connection. Therefore, the difference between Scenario I and Scenario 2 is worth repeating here, and is essential for understanding the resulting stresses on SPDS. In Scenario 1, the SPD stress (threat) is associated with surges that impinge upon the building via the service connection or are generated within the building.

In Scenario 2, the damage threat is associated with the portions of the lightning current that exits the building as "flash-overs" to and via the service connections.

Scenario 2, associated with a direct or nearby lighting strike, is the mechanism, causes up to 90% of all lightning related damage.

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