THE ORIGIN OF OVERVOLTAGES
Transient overvoltages have four main causes :
- 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 much more specific and arise from recent technological developments (massive use of semiconductors for the former, thermonuclear weapons for the latter).
There are three types of indirect electrical effects :
Impact on overhead lines Such 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 that travel naturally 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 earth potential increases that vary according to the current intensity and the local earth impedance. In an installation that may be connected to several grounds (e.g. a link between buildings), 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 flash may be regarded as an antenna several miles high carrying an impulse current of several tenth of kilo-amperes, radiating intense electromagnetic fields (several kV/m at more than 1 km). These fields induce strong voltages and currents in lines near or on equipment. The values depend on the distance from the flash and the properties of the link.
This term covers phenomena caused by switching electric power sources on or off.
Industrial 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 power lines...
These phenomena generate transients of several kV with rise times in the order of a few microseconds, disturbing equipment in networks to which the source of disturbance is connected.
Electrostatic overvoltages (ESD) Electrically, a human being has a capacitance ranging from 100 to 300 picofarads, and can pick up a charge of as much as 15kV by walking on a carpet, then touch some conducting object and be discharged in a few nanoseconds, with a current of about ten Amperes. All integrated circuits (CMOS, etc.) are quite vulnerable to this kind of disturbance, which is generally eliminated by shielding and grounding.
NEMP phenomena (Nuclear ElectroMagnetic Pulses) A high-altitude nuclear explosion, above the atmosphere, creates an intense electromagnetic field (up to 50 kV/m in 10ns), radiated to a ground area up to 1200 kilometers in radius. In the ground, the field induces very large transient overvoltages in 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 amplitude of the phenomenon, protection can be provided by shielding and filtering/surge protection adapted to NEMP.