EMERGENCE OF OVERVOLTAGES

TRANSIENT OVERVOLTAGES HAVE FOUR MAIN CAUSES:

• Lightning strike
• Voltage peaks due to switching operations, e.g. in industrial plants
• Electrostatic Discharge (ESD)
• Nuclear Electromagnetic Pulses (NEMP)

Surges differ in their amplitude, duration and frequency

Surges caused by lightning and switching operations of industrial plants have been with us for a long time. ESD and NEMP disturbances, on the other hand, are far more specific influences and have resulted from more recent technological developments. For example, the massive use of semiconductors has led to susceptibility to ESD disturbances, while NEMP disturbances are caused by nuclear weapons.

1. INDIRECT COUPLING OF OVERVOLTAGES THROUGH LIGHTNING STRIKES

A distinction is made between three paths of indirect coupling by lightning strikes

Impact in overhead lines

Such lines can be struck directly by lightning due to their very exposed location. This first partially or completely destroys the conductors, and then high surge voltages build up which propagate through the lines and finally reach the electrical installations connected to the overhead line. The extent of the damage depends on the distance between the point of impact and the installations.

Increase of the earth potential

The entry of lightning into the ground causes an increase in the earth potential, which varies depending on the current intensity and the local earth impedance. In a system that may be connected to several earthing points (e.g. a connection between buildings), a lightning strike causes a very large potential difference that results in equipment connected to the affected networks being destroyed or massively impaired in their operation.

Electromagnetic radiation

Lightning can be seen as an antenna reaching several kilometres high, carrying a pulse current of several tens of kiloamperes and emitting correspondingly strong electromagnetic fields (with field strengths of several kV/m at a distance of more than one kilometre). These fields induce high voltages and currents in lines laid in or near electrical installations. The values occurring in practice depend on the distance of the lightning strike and on the physical properties of the connection.

2. INDUSTRIALLY INDUCED SHOCK VOLTAGES

This term covers phenomena that are caused by switching electrical energy sources on or off.

Industrially induced surge voltages are caused by:

• Switching operations of inductive loads such as motors or transformers
• Coupling of ignition voltages from conventional gas discharge luminaires
• Switching circuits with inductive loads or capacitive loads
• Tripping of fuses and circuit breakers
• Unintentional fault conditions in the supply network

These phenomena cause transients of several kV with rise times in the order of a few microseconds and interfere with the operation of equipment in networks to which the interference source is connected.

3. SURGES DUE TO ELECTROSTATIC DISCHARGES

Electrically, the human body has a capacitance in the range of 100 to 300 picofarads. This capacitance can charge up to voltages of 20 kV when walking on a carpet. If one subsequently touches a conductive object, this charge flows off as a current flow in a few nanoseconds. All integrated circuits, but especially those in CMOS technology, are quite susceptible to this kind of interference, which can generally be eliminated by shielding and earthing measures.

4. THE NEMP PHENOMENON

(Nuclear ElectroMagnetic Pulse) A nuclear electromagnetic pulse at high altitude above the atmosphere produces a strong electromagnetic field (up to 50 kV/m in 10 ns) covering an area with a radius of 1200 kilometres on the earth's surface. On the ground, the field induces very high transient overvoltages in power supply and data transmission lines, antennas and other electrical equipment, destroying the connected terminal equipment (circuits, computer terminals, telephones, etc.). The increase in field strength can be several kV/ns. Although it is difficult to eliminate all overvoltages induced by an electromagnetic pulse, there are ways to attenuate them and at the same time make the systems to be protected more resistant. Despite the enormous amplitude of this phenomenon, effective protection can be achieved through shielding, filtering and surge protection measures designed for NEMP effects.