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: thermal effects: Very violent overheating of materials, causing fire, mechanical effects: Structural deformation, thermal flashover: Extremely dangerous phenomenon in the presence of flammable or explosive materials (hydrocarbons, dust, etc.).	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