A ground flash can be downward (starting in the cloud) or upward (starting from a tall structure). It is made up of short strokes, shorter than 2 ms, and long strokes, between 2 ms and 1 s. For each protection level, IEC 62305-1 sets the peak, charge and specific energy used to size the protection measures; the first positive stroke is 10/350 µs.
Short answer
Lightning current is described by a few parameters: peak value, charge, specific energy, and front and tail times. IEC 62305-1 sets them for each lightning protection level (I to IV), and these values size conductor cross-sections, sheet metal thicknesses, the rating of surge protective devices (SPDs) and the separation distances against dangerous sparking. For level I the first positive stroke is 200 kA, 100 C and 10 MJ/Ω, with a 10/350 µs waveshape.
Downward and upward flashes
There are two types of ground flash:
- Downward: started by a leader that travels from the cloud to the ground. It predominates on flat terrain and on low structures.
- Upward: started by a leader that rises from a grounded structure toward the cloud. It predominates on exposed or tall structures.
The greater the effective height of a structure, the greater the probability of a direct strike, and the physical conditions of the flash change. A downward flash starts with a first stroke, which may be followed by subsequent strokes, and one or more strokes may end in a long stroke. An upward flash starts with a first long stroke, with or without superimposed impulses, and may continue with subsequent strokes.
Short strokes and long strokes
A lightning current is made up of one or more of these components:
- Short stroke: tail duration, to half value, typically shorter than 2 ms.
- Long stroke: duration TLONG, measured between 10 % of the value on the rise and 10 % on the decay, typically longer than 2 ms and shorter than 1 s.
Each component has positive or negative polarity and a position in the flash: first, subsequent or superimposed. The standard assumes a polarity ratio of 10 % positive and 90 % negative flashes, which depends on the territory; if there is no local information, that ratio is used.
A multiple flash has on average three to four strokes, with a typical interval of about 50 ms between them. The standard notes that flashes of up to a few tens of strokes have been recorded, with intervals of 10 ms to 250 ms.
How the shape of an impulse is measured
The standard defines the parameters from the current curve:
- Peak value (I): the maximum of the current.
- Front time (T1): a virtual parameter equal to 1.25 times the interval between 10 % and 90 % of the peak.
- Time to half value (T2): the interval between the virtual origin and the instant the current falls to half of the peak on the tail. The virtual origin is 0.1 times T1 before the current reaches 10 %.
- Impulse charge (QSHORT): the integral of the current over the impulse.
- Specific energy (W/R): the integral of the square of the current. It represents the energy the current dissipates in a 1 Ω resistance; for a long stroke it is negligible.
- Average steepness (di/dt): the change in current over a time interval, divided by that time.
For analysis, Annex B of the standard gives a function of time with the peak, a correction factor and two time constants, with different values for each waveshape. The statistical data come from CIGRE measurements and follow a log-normal distribution.
Design values by protection level
Table 3 of Part 1 gives the maximum values used to size the protection components. Levels III and IV share values.
| Parameter | Level I | Level II | Levels III and IV | Waveshape |
|---|---|---|---|---|
| First positive stroke: peak | 200 kA | 150 kA | 100 kA | 10/350 µs |
| First positive stroke: charge | 100 C | 75 C | 50 C | |
| First positive stroke: specific energy | 10 MJ/Ω | 5.6 MJ/Ω | 2.5 MJ/Ω | |
| First negative stroke: peak (the standard defines it up to level III) | 100 kA | 75 kA | 50 kA | 1/200 µs |
| Subsequent stroke: peak | 50 kA | 37.5 kA | 25 kA | 0.25/100 µs |
| Subsequent stroke: average steepness | 200 kA/µs | 150 kA/µs | 100 kA/µs | |
| Long stroke: charge | 200 C | 150 C | 100 C | TLONG of 0.5 s |
| Whole flash: charge | 300 C | 225 C | 150 C |
The values for levels II, III and IV are obtained by reducing those of level I: 75 % for II and 50 % for III and IV, linearly for the current, charge and steepness, and quadratically for the specific energy. The times do not change. The first negative stroke of 1/200 µs is used for calculations only, not for testing.
There is another group of values, the minimums, which are not used to size conductors but to place the air-termination. They are explained in the guide on protection levels.
Why each parameter matters
Annex D of Part 1 relates each parameter to an effect on the components:
- Specific energy: resistive heating of the conductors that carry the current.
- Charge: thermal damage at the point of strike, where the arc melts the metal.
- Peak value: mechanical stress, from the electrodynamic forces between conductors.
- Steepness: voltage induced in a loop, proportional to the self-inductance times the rate of change of current.
These values are those of the current at the point of strike. Because the current divides among several down-conductors, pipes and lines, the current through a given component is a fraction, and this is calculated case by case. The selection of surge protective devices is covered in the guide on transient overvoltages in the plant.
Guides in the same series
The protection level sets the maximum current value used for design: see protection levels. That current sizes the whole system (LPS, classes I to IV) and is shared among the LPS down-conductors. The part of the current that enters through the lines is what the coordinated SPDs must withstand.
When to contact us
If you need to check that the SPDs, conductors and earthing of your installation are sized for the protection level that applies to it, write to us. Learn about our engineering design and studies service and send your inquiry through contact.
Related guides
- IEC 62305 lightning protection: what each part covers
- Lightning protection levels: LPL I to IV
- Coordinated SPDs: classes I, II and III and Iimp current
- Lightning protection system (LPS): classes I to IV
- Transient overvoltages in the plant
References
- IEC 62305-1:2010, Protection against lightning – Part 1: General principles, clauses 3.1 to 3.21, 4 and 8.2 (tables 3 and 4), Annex A (clauses A.1 and A.2, figures A.1 to A.4 and table A.1), Annex B (equation B.1 and table B.1) and Annex D (clauses D.3 to D.5).
Frequently asked questions
What does 10/350 µs mean in the lightning waveshape?
It is the front time (T1) and the time to half value on the tail (T2) of the first positive stroke, both in microseconds. The front time T1 is defined as 1.25 times the interval between 10 % and 90 % of the peak value, and T2 is measured from the virtual origin of the wave until the current falls to half of the peak.
Why does the standard use three different waveshapes for the impulse?
Because the first positive stroke (10/350 µs), the first negative stroke (1/200 µs) and the negative subsequent strokes (0.25/100 µs) have different fronts and tails. The positive stroke carries more charge and energy; the subsequent stroke has the steepest front. The standard states that the 1/200 µs shape is used for calculations only, not for testing.
Do upward flashes exceed the design values?
No. The standard notes that all the impulse parameters of upward flashes are lower than those of downward flashes. The upward flash parameters are considered covered by the downward maximums, although a higher charge in the long-stroke current is not yet confirmed.