The loss LX is the fraction of value lost in a lightning event. It is calculated per zone using the number of people or the value of the zone against the total of the structure, and with factors that reduce it (floor type, fire protection, fire risk) or increase it (panic, difficulty of evacuation). Dividing the structure into zones ZS makes it possible to protect each part according to what it needs and to lower the overall cost.
Short answer
In IEC 62305-2, the loss LX is the average relative amount of a type of damage in a dangerous event, considering its extent and its effects. It is calculated per zone of the structure: in each zone, a typical loss value is the starting point, it is multiplied by the fraction of people, users or value in the zone against the total, and it is corrected with factors that reduce or increase it. Dividing the structure into zones ZS and the lines into sections SL makes it possible to apply to each part the measures it needs.
How the structure is divided into zones ZS
A structure can be treated as a single zone or divided into zones with homogeneous characteristics. Zones are defined mainly by:
- the type of soil or floor (components RA and RU);
- fire compartments (RB and RV);
- spatial shields (RC and RM).
They can also be defined by the layout of the internal systems, the existing or planned protection measures, or the loss values LX. The division must take into account that the protection measures are feasible. Zones ZS may coincide with the protection zones LPZ, but they need not.
With a single zone, each measure must cover the whole structure and can be costly. In a multi-zone structure, the total risk is the sum of the risks of each zone, and each zone risk is the sum of its components. Dividing makes it possible to choose measures tailored to each part and to reduce the total cost.
Within a zone, for components RA, RB, RU, RV, RW and RZ a single value per parameter is set; if there are several, the highest is used. For RC and RM, if there is more than one internal system in the zone, the combined probability is:
PC = 1 − (1 − PC1) × (1 − PC2) × (1 − PC3)…
and similarly for PM.
How lines are divided into sections SL
A line can be a single section or be divided into sections. Sections are defined by:
- the line type, overhead or buried;
- the factors that affect the collection area (CD, CE and CT);
- the line characteristics: shielded or unshielded and shield resistance.
If there is more than one value in a section, the one that gives the higher risk is taken. The sections considered are those between the structure and the first node.
What the loss LX is
The loss varies with the type of loss (L1 to L4) and the type of damage (D1, D2, D3):
- L1, human life: number of people at risk.
- L2, public service: number of users without service.
- L3, cultural heritage: economic value at risk of the structure and its contents.
- L4, economic value: value of animals, structure, contents and internal systems.
The loss is assigned per zone, and the standard warns that the typical values are proposed and that each national committee may define others.
Loss of human life (L1)
The equations in Table C.1 are:
- D1 (electric shock to living beings): LA = LU = rt × LT × (nz/nt) × (tz/8,760)
- D2 (physical damage): LB = LV = rp × rf × hz × LF × (nz/nt) × (tz/8,760)
- D3 (failure of internal systems): LC = LM = LW = LZ = LO × (nz/nt) × (tz/8,760)
Where nz is the number of people in the zone, nt the total in the structure, and tz the hours per year the people are in the zone, over the 8,760 h of the year. LT, LF and LO are the typical values of victims per event.
| Damage | Typical value | Type of structure |
|---|---|---|
| D1, injuries (LT) | 10⁻² | All |
| D2, physical damage (LF) | 10⁻¹ | With explosion risk; hospital, hotel, school, civic building |
| 5 × 10⁻² | Public entertainment, church, museum | |
| 2 × 10⁻² | Industrial, commercial | |
| 10⁻² | Others | |
| D3, failure of systems (LO) | 10⁻¹ | With explosion risk |
| 10⁻² | Intensive care and operating theaters of a hospital | |
| 10⁻³ | Other areas of a hospital |
These values assume continuous presence of people. If the loss can affect the outside, an additional loss LE is added.
Factors that reduce or increase the loss
| Factor | What it represents | Values |
|---|---|---|
| rt | Type of floor or soil (reduces shock) | 10⁻² agricultural or concrete; 10⁻³ marble or ceramic; 10⁻⁴ gravel, carpet or rug; 10⁻⁵ asphalt, linoleum or wood |
| rp | Measures against the consequences of fire (reduces) | 1 with no measures; 0.5 with extinguishers, manual alarm, hydrants, fire compartments or escape routes; 0.2 with automatic extinguishing or alarm |
| rf | Fire or explosion risk (reduces) | Explosion: 1 in zones 0 and 20 or with solid explosives, 10⁻¹ in zones 1 and 21, 10⁻³ in zones 2 and 22; fire: 10⁻¹ high, 10⁻² ordinary, 10⁻³ low; 0 if there is no risk |
| hz | Special hazard (increases) | 1 with no hazard; 2 low panic; 5 medium panic or difficulty of evacuation; 10 high panic |
In structures with an explosion risk, rp is 1. For rp, if there are several measures, the lowest value is taken. An automatic alarm counts only if it is protected against overvoltages and the fire brigade can arrive in less than 10 min.
Other losses
- L2, public service: LB = LV = rp × rf × LF × nz/nt and LC = LM = LW = LZ = LO × nz/nt. LF is 10⁻¹ for gas, water and power, and 10⁻² for television and telecommunications. LO is 10⁻² and 10⁻³ respectively.
- L3, cultural heritage: LB = LV = rp × rf × LF × cz/ct, with LF = 10⁻¹ for museums and galleries. cz is the value of the heritage in the zone and ct the total value.
- L4, economic value: LA = LU = rt × LT × ca/ct; LB = LV = rp × rf × LF × (ca + cb + cc + cs)/ct; LC = LM = LW = LZ = LO × cs/ct. Here ca, cb, cc and cs are the value of animals, building, contents and internal systems of the zone, and ct the total value.
In L4, the value ratios are considered only in the cost-benefit analysis; if the representative value of the tolerable risk is used, they are replaced by 1.
What you need to know about your structure
To set the loss and the factors you need: how many people are in each zone and for how many hours per year, the type of floor, the fire protection measures, the fire or explosion risk, the type of service the structure provides and the value of its parts.
Guides in the same series
LX multiplies the probability in each risk component; the full procedure is in lightning risk management. The cost of those losses is compared with that of the measures in cost-benefit analysis, and the types of loss are defined in lightning damage.
When to contact us
If you need to divide a building or plant into zones and assign the loss of each one for a lightning risk study, write to us with the drawings and a description of use. Learn about our engineering design and studies service and write to us through contact.
Related guides
- IEC 62305 lightning protection: what each part covers
- Lightning risk management: when protection is needed
- Lightning risk components in IEC 62305-2: RA to RZ
- Cost-benefit analysis of lightning protection
- Transient overvoltages in the plant
References
- IEC 62305-2:2010, Protection against lightning – Part 2: Risk management, clauses 6.6 to 6.9 (equations 14 and 15) and Annex C (clauses C.1 to C.6, equations C.1 to C.15 and tables C.1 to C.12).
Frequently asked questions
Why divide a structure into zones ZS instead of calculating it as a whole?
Because with a single zone every protection measure must cover the whole structure, which can be expensive. With several zones, the designer fits the measures to each part and the total risk is the sum of the zone risks, which reduces the overall cost.
Are the LX values in the standard mandatory?
No. The standard gives them as proposed typical values and says that each national committee, or a detailed investigation, may assign others. The assessor sets the LX value, and the standard recommends using the Annex C equations as the main source.
Is a zone ZS the same as a zone LPZ?
Not necessarily. The standard indicates that the zones ZS of Part 2 may coincide with the LPZ of Part 4, but they may also be different: they are defined by, among other things, the type of soil or floor, fire compartments and shields.