Lightning
Lightning, investigated since Benjamin Franklin’s first research in
1749, has paradoxically become a growing threat to our highly electronic
society.
Lightning
formation
A lightning flash is generated between two zones of opposite charge,
typically between two storm clouds or between one cloud and the ground.
The flash may travel
several miles, advancing toward the ground in successive leaps: the
leader creates a highly ionized channel. When it reaches the ground, the
real flash or return stroke takes place.
A current in the tens
of thousands of Amperes will then travel from ground to cloud or vice
versa via the ionized channel.
Direct effects
At the moment of discharge there is an
impulse current flow that ranges from 1,000 to 200,000 Amperes peak,
with a rise time of about a few microseconds. This direct effect is a
small factor in damage to electric and electronic systems, because it is
highly localized.
The best protection
is still the classic lightning rod or Lightning Protection System (LPS),
designed to capture the discharge current and conduct it to a particular
point.
Indirect effects
There are three types of indirect
lightning effects:
Impact on overhead lines
Such lines are very
exposed and ma be struck directly by lightning, which will first
partially or completely destroy the cables, and 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. 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 tenths of kilo-amperes, radiating intense electromagnetic fields
(several kV/m at more than 1km). 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.
Industrial surges
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
- Poor or
intermittent contacts
These phenomena
generate transients of several kV with rise times of the order of the
microsecond, disturbing equipment in networks to which the source of
disturbance is connected.
Electrostatic overvoltages
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 carpet, then touch some
conducting object and be discharged in a few microseconds, 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.
Effects of overvoltages
Overvoltages have many types of effects on electronic equipment in order
of decreasing importance:
Destruction:
-
Voltage breakdown
of semiconductor junctions
-
Destruction of
bonding of components
-
Destruction of
tracks of PCBs or contacts
-
Destruction of
triacs/thyristors by dV/dt.
Interference with
operations:
-
Random operation of
latches, thyristors, and triacs
-
Erasure of memory
-
Program errors or
crashes
-
Data and
transmission errors
Premature ageing:
Components exposed to overvoltages have a shorter life.
Surge Protection Devices
The Surge Protection Device (or SPD:
this is a generic name for any device to protect from voltage surges) is
a recognized and effective solution to the overvoltage problem. For
greatest effectiveness, however, it must be chosen according to the risk
and installed in accordance with the rule of the art.
Standards
Because of the diversity and importance
of transients, standards organizations have created specifications for
testing the reactions of equipment to overvoltages.
The phenomena were
first characterized and a series of standardized waves created (1.2/50µs
voltage wave and 8/20µs and 10/350µs current waves), then a number of
standards defining surge arrestor performance were issued, among them:
Surge Protectors for low-voltage
installations
-
NFC 61740 (France)
- VDE
0675 (Germany)
- UL 1449 (USA)
- IEC
61643-1-1 (International)
Surge Protectors
and recommendations for telecom equipment
-
IEC
61644-1 (International)
-
ITU-T
recommendations K11, K12, K17, K20, K22, K36 (International)
-
UL 497A/B (USA)
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