TRANSIENT SURGES HAVE FOUR MAJOR CAUSES:
- Lightning strike
- Voltage peaks due to switching operations, e.g. in industrial plants
- Electrostatic Discharge (ESD)
- Nuclear electromagnetic pulses (NEMP)
Overvoltages differ in amplitude, duration and frequency.
Overvoltages caused by lightning strikes 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 new technological developments. For example, the massive use of semiconductors has led to susceptibility to ESD interference, while NEMP interference is caused by nuclear weapons.
1. INDIRECT COUPLING OF OVERVOLTAGES BY LIGHTNING STRIKES
One distinguishes three ways from indirect coupling by lightning strikes
Impact in overhead lines
Due to their very exposed position, such cables can be struck directly by lightning. First, the conductors are partially or completely destroyed, and then high surge voltages build up, which propagate via the cables and finally the cables connected to the overhead line are destroyed.
electrical systems connected to it. The extent of the damage depends on the distance between the point of impact and the installations.
Rise of the earth potential
The entrance of the lightning into the ground causes an increase of the earth potential, which varies depending on the current strength and the local earth impedance. In a system that may be connected to several grounding points (e.g. a connection between buildings), a lightning strike will cause a very large potential difference which will destroy or massively affect the operation of equipment connected to the affected networks.
The lightning can be regarded as an antenna reaching several kilometres in height, which carries a pulse current of several tens of kiloamperes and emits 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 that occur in practice depend on the distance of the lightning strike and the physical properties of the connection.
2. INDUSTRIALLY CAUSED SURGE VOLTAGES
This term covers phenomena that are caused by switching electrical energy sources on or off.
Industrially caused surge voltages are caused by
- Switching operations of inductive loads such as motors or transformers
- Coupling of ignition voltages from conventional gas discharge lamps
- Switching circuits with inductive loads or capacitive loads
- Tripping of fuses and circuit breakers
- Unwanted error conditions in the supply network
These phenomena cause transients of several kV with rise times of the order of a few microseconds and interfere with the operation of equipment in networks to which the source of the interference is connected.
3. OVERVOLTAGES DUE TO ELECTROSTATIC DISCHARGES
Electrically speaking, the human body has a capacity in the range of 100 to 300 picofarads. This capacity can charge up to 20 kV when walking on a carpet. If a conductive object is subsequently touched, this charge flows off as a current flow in a few nanoseconds. All integrated circuits, especially those in CMOS technology, are quite susceptible to this type of interference, which can generally be eliminated by shielding and grounding.
4. THE PHENOMENON NEMP
(Nuclear ElectroMagnetic Pulse) A nuclear electromagnetic pulse at a high altitude above the atmosphere produces a strong electromagnetic field (up to 50 kV/m in 10 ns) that covers an area of 1200 km radius on the Earth's surface. At ground level, the field induces very high transient overvoltages in power and data transmission lines, antennas and other electrical equipment, destroying the connected terminals (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 of attenuating them and making the systems to be protected more resistant. Despite the enormous amplitude of this phenomenon, effective protection can be achieved by shielding, filtering and overvoltage protection measures designed for NEMP effects.