Transient overvoltages have four main causes :

  • Lightning
  • Industrial and switching surges
  • Electrostatic discharges (ESD)
  • Nuclear electromagnetic pulses (NEMP) Overvoltages differ in amplitude, duration and frequency. Lightning and industrial overvoltages have been with us for a long time but ESD and NEMP disturbances are new concerns that arise from recent technological developments in the widespread use of semiconductors and pervasive threat of nuclear terrorism.

Indirect Lightning Strikes

There are three types of indirect electrical effects resulting from lightning strikes:

Impact on overhead lines Overhead power and communication lines are very exposed and may be struck directly by lightning, which will first partially or completely destroy the cables, then cause high surge voltages to can naturally travel along the conductors to line-connected equipment. The extent of the damage depends on the distance between the strike and the equipment.

Rise in ground potential The flow of lightning in the ground causes the electrical potential in the earth to increase and will vary depending on the size of the impluse and the local earth impedance. In an installation that may be connected to several grounds (e.g. a link between buildings and/or multiple ground systems), a strike will cause a very large potential difference and equipment connected to the affected networks will be destroyed or severely disrupted.

Electromagnetic radiation The lightning strike may be regarded as an antenna several miles high carrying an impulse current of thousands of amps that radiate an intense electromagnetic field (several kV/m at more than 1 km). These fields induce strong voltages and currents into power and communication lines near or connected to equipment. The values depend on the distance from the ligthing strike and the properties of the connection between the link.

Industrial switching surges

This term covers a phenomena caused by switching electric power sources on or off.

Industrial switching surges are caused by:

  • Starting motors or transformers
  • Neon and sodium light starters
  • Switching power networks
  • Switch «bounce» in an inductive circuit
  • Operation of fuses and circuit-breakers
  • Falling or downing of power lines...

Switching events generate transients of several thousand volts with rise times of only a few microseconds. They can disturb equipment connected in the same networks as the source of disturbance.

Electrostatic overvoltages (ESD) Electrically, a human being has a capacitance ranging from 100 to 300 picofarads. This means that any person, by simply walking on a carpet and touching a piece of equipment, can generate a charge of up to 15kV and about ten amps of current.

All integrated circuits (CMOS, etc.) are especially vulnerable to this kind of disturbance and are protected by properly shielding and grounding of the equipment.

NEMP (Nuclear ElectroMagnetic Pulses) A high-altitude nuclear explosion, above the atmosphere, creates an intense electromagnetic field (up to 50 kV/m in 10ns) that is radiated to the ground in an area that can be as large as 1200 kilometers in radius. In the ground, the field induces a very large transient overvoltage into power and transmission lines, antennas, etc., destroying the terminal equipment (power circuit, computer terminals, telephone equipment, etc.). The field rise may reach several kV/ns. While it is difficult to eliminate all overvoltages induced by an electromagnetic pulse, there are ways to reduce them and strengthen the systems to be protected. In spite of the large amplitude of these events, protection can be provided by shielding, filtering, surge protection adapted specifically to NEMP.

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