Overvoltage characteristics of atmospheric origin
From Electrical Installation Guide
- Overvoltages of atmospheric origin.
Contents |
Overvoltage characteristics of atmospheric origin
| Lightning strokes in a few figures: Lightning flashes produce an extremely large quantity of pulsed electrical energy (see Fig. J4)
|
Between 2000 and 5000 storms are constantly undergoing formation throughout the world. These storms are accompanied by lightning strokes which represent a serious hazard for persons and equipment. Lightning flashes hit the ground at an average of 30 to 100 strokes per second, i.e. 3 billion lightning strokes each year.
The table in Figure J3 shows the characteristic lightning strike values. As can be seen, 50% of lightning strokes have a current exceeding 33 kA and 5% a current exceeding 65 kA. The energy conveyed by the lightning stroke is therefore very high.
| Cumulative probability (%) | Peak current (kA) | Gradient (kA/µs) |
| 95 | 7 | 9.1 |
| 50 | 33 | 24 |
| 5 | 65 | 65 |
| 1 | 140 | 95 |
| 0 | 270 |
Fig. J3: Lightning discharge values given by the IEC 62305 standard
Fig. J4: Example of lightning current
Lightning also causes a large number of fires, mostly in agricultural areas (destroying houses or making them unfit for use). High-rise buildings are especially prone to lightning strokes.
Effects on electrical installations
Lightning damages electrical and electronic systems in particular: transformers, electricity meters and electrical appliances on both residential and industrial premises.
The cost of repairing the damage caused by lightning is very high. But it is very hard to assess the consequences of:
- disturbances caused to computers and telecommunication networks;
- faults generated in the running of programmable logic controller programs and control systems.
Moreover, the cost of operating losses may be far higher than the value of the equipment destroyed.
Lightning stroke impacts
| Lightning is a high-frequency electrical phenomenon which causes overvoltages on all conductive items, especially on electrical cabling and equipment. |
Lightning strokes can affect the electrical (and/or electronic) systems of a building in two ways:
- by direct impact of the lightning stroke on the building (seeFig. J5 a);
- by indirect impact of the lightning stroke on the building:
- A lightning stroke can fall on an overhead electric power line supplying a building (see Fig. J5 b). The overcurrent and overvoltage can
spread several kilometres from the point of impact.
- A lightning stroke can fall near an electric power line (see Fig. J5 c). It is the electromagnetic radiation of the lightning current that
produces a high current and an overvoltage on the electric power supply network.
In the latter two cases, the hazardous currents and voltages are transmitted by the power supply network.
- A lightning stroke can fall near a building (see Fig. J5 d). The earth potential around the point of impact rises dangerously.
Fig. J5: Various types of lightning impact
In all cases, the consequences for electrical installations and loads can be dramatic.
| Lightning falls on an unprotected building. | Lightning falls near an overhead line. | Lightning falls near a building. |
 |  |  |
The lightning current flows to earth via the more or less conductive structures of the building with very destructive effects:
| The lightning current generates overvoltages through electromagnetic induction in the distribution system. These overvoltages are propagated along the line to the electrical equipment inside the buildings. | The lightning stroke generates the same types of overvoltage as those described opposite. In addition, the lightning current rises back from the earth to the electrical installation, thus causing equipment breakdown. |
| The building and the installations inside the building are generally destroyed | The electrical installations inside the building are generally destroyed. | |
Fig. J6: Consequence of a lightning stoke impact
The various modes of propagation
- Common mode
Common-mode overvoltages appear between live conductors and earth: phase-to-earth or neutral-to-earth (see Fig. J7). They are dangerous especially for appliances whose frame is connected to earth due to risks of dielectric breakdown.
Fig. J7: Common mode
- Differential mode
Differential-mode overvoltages appear between live conductors:
phase-to-phase or phase-to-neutral (see Fig. J8). They are especially dangerous for electronic equipment, sensitive hardware such as computer systems, etc.
Fig. J8: Differential mode
Characterization of the lightning wave
Analysis of the phenomena allows definition of the types of lightning current and voltage waves.
- 2 types of current wave are considered by the IEC standards:
- 10/350 µs wave: to characterize the current waves from a direct lightning stroke (see Fig. J9);
Fig. J9: 10/350 µs current wave
- 8/20 µs wave: to characterize the current waves from an indirect lightning stroke (see Fig. J10).
Fig. J10: 8/20 µs current wave
These two types of lightning current wave are used to define tests on SPDs (IEC standard 61643-11) and equipment immunity to lightning currents.
The peak value of the current wave characterizes the intensity of the lightning stroke.
- The overvoltages created by lightning strokes are characterized by a 1.2/50 µs voltage wave (see Fig. J11).
This type of voltage wave is used to verify equipment's withstand to overvoltages of atmospheric origin (impulse voltage as per IEC 61000-4-5).
Fig. J11: 1.2/50 µs voltage wave
