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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