Lightning Risk Components in IEC 62305-2: RA to RZ

Short answer

IEC 62305-2 divides lightning risk into eight components according to the point of strike and the type of damage. Three come from strikes to the structure (RA, RB, RC), one from strikes near it (RM), three from strikes to a connected line (RU, RV, RW) and one from strikes near a line (RZ). All are calculated with the same equation, RX = NX · PX · LX, and are added differently in R1, R2, R3 and R4.

Short answer

IEC 62305-2 splits lightning risk into eight components, each tied to a point of strike (source S1 to S4) and a type of damage (D1, D2 or D3). All have the same form, RX = NX · PX · LX: the number of dangerous events per year, the probability that each event causes damage and the resulting loss. What changes between components is which values enter each factor and how they are combined in the risks R1 to R4.

The eight components

Component	Source	Damage	What it describes
RA	S1: strike to the structure	D1	Injury to living beings from step and touch voltages, inside the structure and outside up to 3 m from the down-conductors
RB	S1	D2	Physical damage from dangerous sparking inside the structure that starts a fire or explosion
RC	S1	D3	Failure of internal systems from the lightning electromagnetic impulse (LEMP)
RM	S2: strike near the structure	D3	Failure of internal systems from LEMP
RU	S3: strike to a connected line	D1	Injury from touch voltage inside the structure, from the lightning current transmitted by the line
RV	S3	D2	Physical damage from sparking between the external installation and metal parts, generally at the point where the line enters
RW	S3	D3	Failure of internal systems from overvoltages induced in the incoming lines and transmitted to the structure
RZ	S4: strike near a line	D3	Failure of internal systems from overvoltages induced in the incoming lines

The lines assessed are only those entering the structure.

Two notes in the standard give context:

Special structures. Where people may be exposed to direct lightning, such as the roof of a parking garage or a stadium, these cases can also be analyzed with the same principles.
Pipes. Strikes on or near pipes are not considered a source of damage when they are bonded to the equipotential bonding bar.

Equation of each component

For strikes to the structure:

RA = ND × PA × LA
RB = ND × PB × LB
RC = ND × PC × LC

For strikes near the structure:

RM = NM × PM × LM

For strikes to a line:

RU = (NL + NDJ) × PU × LU
RV = (NL + NDJ) × PV × LV
RW = (NL + NDJ) × PW × LW

For strikes near a line:

RZ = NI × PZ × LZ

ND, NM, NL, NI and NDJ are the numbers of dangerous events. NL and NI correspond to strikes that produce overvoltages of 1 kV or more on the line. Each is calculated with the equations in Annex A. The probabilities P come from Annex B and the losses L from Annex C.

If a line has several sections, RU, RV, RW and RZ are the sum of the values for each section between the structure and the first node. If there are several lines with the same route, only the one with the worst characteristics is calculated.

How they add up in R1, R2, R3 and R4

Part 2 combines the components as follows:

R1, human life: RA + RB + RC* + RM* + RU + RV + RW* + RZ*
R2, service to the public: RB + RC + RM + RV + RW + RZ
R3, cultural heritage: RB + RV
R4, economic value: RA† + RB + RC + RM + RU† + RV + RW + RZ

* Only for structures with an explosion risk and for hospitals with life-saving electrical equipment, or other structures where failure of internal systems immediately endangers life.

† Only for properties where animals can be lost.

The sums show what enters each decision. For example, R2, the risk of losing a public service, includes neither RA nor RU: it depends only on physical damage (RB and RV) and on failure of internal systems (RC, RM, RW and RZ). R3 depends only on physical damage.

What reduces each component

Annex B of Part 2 indicates which characteristics and measures affect the probability of each component:

RA and RU (electric shock to living beings): insulation of exposed parts, physical restrictions, warning notices and equipotentialization of the ground. For RU, line shielding, the withstand voltage of the connected systems and equipotential bonding at the entrance also count.
RB (physical damage from a strike to the structure): the lightning protection system, according to its class.
RC (failure of internal systems from a strike to the structure): a coordinated SPD system.
RM (failure from a nearby strike): a meshed LPS, shielding, cable routing, the withstand voltage of the equipment, isolating interfaces and coordinated SPDs.
RV, RW and RZ (strikes to the line and near it): line shielding, the withstand voltage of the equipment, equipotential bonding with SPDs at the entrance, coordinated SPDs and isolating interfaces, depending on the component.

In addition, the collection area affects all components. The measure that reduces the risk most is the one that tackles the component that dominates the sum.

Guides in the same series

The factors that feed each component are covered in number of strikes ND, probability of lightning damage and lightning losses LX. The full procedure that uses them is in lightning risk management, and the types of damage and loss in lightning damage.

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References

IEC 62305-2:2010, Protection against lightning – Part 2: Risk management, clauses 4.1 to 4.3 (tables 1, 2 and 3), 6.1 to 6.6 (equations 1 to 13) and 6.9.1, and Annexes A, B and C.

Frequently asked questions

Why do RC, RM, RW and RZ not always count in R1?

Because they describe the failure of internal systems. They put life at risk only when the structure has an explosion risk or is a hospital, or any other where that failure immediately endangers life. In other structures, these components enter R2 and R4, not R1.

What do NL and NDJ in component RU stand for?

NL is the number of strikes to the line entering the structure and NDJ the number of strikes to the structure at the other end of the line. The standard notes that in many cases NDJ can be neglected. Components RU, RV and RW use the sum NL + NDJ.

Does a strike on a metal pipe count as a source of damage?

No, if the pipe is bonded to the structure's equipotential bonding bar. If there is no bonding bar, the threat must be considered, according to the notes to clauses 4.2.4 and 4.2.5.