LIGHTNING AND SURGE PROTECTION -BASIC PRINCIPLES

INTRODUCTION
Rarely does the power of nature strike an observer more forcibly than the
sight for the first time of a tropical thunderstorm in full flow. Most people,
even those not frightened by thunderstorms as children, can appreciate
that forces of great magnitude are unleashed and that some means of
protection against the effects of lightning must be highly desirable. It is the
intention of this application note to discuss suitable techniques to protect
electronic circuits and equipment from high voltages and surge currents
induced by lightning and other forms of transients.

The need for surge protection
Most process control or telemetry installations are interconnected by power
and signal cables which run on trays, in ducting or via overhead poles.
Lightning strikes, static discharges and induction from power cabling are
typical sources of transient voltages which can be coupled into signal cables
and hence transmitted to electronic equipment. Field transmitters, computer
terminals, etc. containing low-power semiconductor devices can be damaged
by overvoltages of only tens of volts. The longer the cables, the more frequent
the occurrence of high voltage transients through shifts in ground potential, so
devices controlling or monitoring events in remote locations are more likely to
suffer from overvoltages and consequent component failures. Significant
damage can also be found in equipment connected by relatively short cables if
the circuits or components are particularly sensitive- as is the case for
computer data communication ports.

As an illustration, consider the effects of a lightning strike to a building,
housing control and telemetry equipment, of which the fabric is protected
from a direct strike by lightning conductors and ground rods. The conductor carries the very large strike current into the earth termination and dissipates the charge transfer into the mass of the earth.

The effect of this current is to elevate the reference potential at the building.
For example, if the strike current is 100kA and the conductor/ground
impedance, Re, is 10½, then the potential above ground is 1 million volts.
Exposed metalwork within the building is bonded to the same reference
potential and so only small voltage differences exist to pose little risk to
personnel.

The field transmitter is pole-mounted away from the control building but
connected to the telemetry electronics by signal cabling. Most transmitters
incorporate some level of isolation from structural earth, typically 500V.
This level of isolation now has to withstand the transient voltage between the
new building reference potential and its local earth potential. Many
transmitters can be destroyed in this way, even though the actual lightning
strike was to a protected building.

Surge protection devices (SPDs)
Electronic equipment can be protected from the potentially destructive
effects of high-voltage transients. Protective devices, known by a variety of
names (including "lightning barriers", "surge arrestors", "lightning protection
units", etc.) are available. The "correct" name (accepted internationally) is
"surge protection devices" or "SPDs" and this nomenclature is used
throughout this publication.

Surge protection devices should ideally operate instantaneously to divert
a surge current to ground with no residual common-mode voltage
presented at the equipment terminals. Once the surge current has
subsided, the SPD should automatically restore normal operation and
reset to a state ready to receive the next surge.

Atlantic Scientific specialise in the design and manufacture of SPDs. The range of products available includes models for virtually all applications. They are based on gas discharge tubes (GDTs), voltageclamping diodes, and metal-oxide varistors (MOVs) which feature rapid operation, accurate voltage control and automatic resetting once the overvoltage has ceased possible insulation breakdown between inside and outside Local 0V reference.

LIGHTNING
Introduction
This section describes the mechanism by which lightning is generated and
the ways by which high voltages produced by lightning discharges find their
way into instrumentation and communications systems. Other sources of
high-voltage transients are also described, such as static electricity and
induction or direct contact with power cables.

Generation of atmospheric discharges
Updraughts and downdraughts of air are fairly common events experienced
by most of us and, indeed, used by glider pilots and balloonists to further
their flights or bring them to a premature end. Such movements of air may
be generated by heat coming from hillsides in full sun or by cold air masses
pushing underneath warmer air in a frontal weather system. As the warm
air rises, it progressively cools and forms a cloud consisting of water
droplets and, at greater heights, ice crystals. A "thunder cloud" is a system
of this type in which the air velocities are much greater than normal.

In undisturbed fine weather, the earth carries a negative charge with the
corresponding positive charge in the upper atmosphere. By convention,
this results in a positive (downwards) field V of typically 100V/m.
Immediately below the thundercloud charge centre, the electric field may
exceed 20kV/m. Fields of such magnitude can lead to point discharges
taking place from sharp objects such as the tips of radio masts and
flagpoles. These objects are essentially conductors short-circuiting part
of the vertical field and hence producing an intense field concentration at
the tip. In maritime terminology these discharges are called St. Elmo's
Fire when they are seen on ships masts, etc. Natural objects can also
promote point discharges, particularly in mountainous areas where
physical elevation further intensifies the field. Climbers often experience
the phenomenon of hair standing on end in thunderstorm conditions and
point discharges from the tips of outstretched hands have been
reported. The discharges themselves are of no great magnitude and are
thus relatively harmless, but they serve as a timely reminder that true
lightning discharges may be imminent.

No means are known for directly measuring the potential of the cloud
charge centres with respect to ground. Indeed, people have been killed in
the attempt (emulating Benjamin Franklin by flying a kite in a storm with a
multimeter attached is not recommended!), but it is estimated to be of the
order of 107 to 108 volts, i.e. 10 million to 100 million volts. The intense
field which is generated between the charge centres causes ionisation of
air molecules to take place and a conducting channel is opened which
permits charge neutralisation to occur, i.e. a lightning stroke.
Most lightning is within the cloud or cloud system. Something like 15% are
cloud-to-ground discharges, these being responsible for the bulk of
damaging effects. Cloud-to-cloud discharges can generate radio
interference often heard as clicks and bangs from nearby storms, or
whistles and howls from storms on the other side of the planet.
This publication is solely concerned with cloud-to-ground discharges
and the effects on cable-connected equipment. The importance of
this is emphasised by evidence which suggests that the frequency of
thunderstorms and related lightning strikes is currently on the
increase globally.

The magnitude of lightning discharges around the world have been
measured from 2000A to more than 200kA, with rise times to peak
current of less than 10µs. The variation in magnitude and rise times follows
the "log-normal" distribution typical of many natural phenomena. BS6651
gives the following data:
1% of strokes exceed 200kA
10% of strokes exceed 80kA
50% of strokes exceed 28kA
90% of strokes exceed 8kA
99% of strokes exceed 3kA

Lightning discharges rarely consist of one stroke only, although the human
eye "runs together" multiple strokes into one persistent image. The process
begins with a stepped leader discharge making its way to ground via
pockets of charge in the atmosphere, giving rise to the typical strongly
branched appearance. As it nears the ground, local charge concentrations
tend to be greatest at high or sharp points so the initial stroke is most likely
to hit tall objects such as masts, towers, trees, etc. Often a point discharge
from the tall object reaches up to make contact with the downward
travelling leader stroke. Once the ionised channel between cloud and
ground is complete, a conducting path is formed, short-circuiting the
charge centres. The main current or return stroke can flow so as to
neutralise the charge imbalance. Thunderclouds are normally positive at
the top and negative at the bottom with a positive charge "shadow" induced
on the ground. Thus, the negative charge close to the ground moves down
the channel to be neutralised by the positive charge in the earth. By
convention, the current therefore moves upward and this initial return
stroke has usually the highest magnitude of the multiple stroke series. The
heating effect of this current on the atmosphere produces the violent air
expansion which we recognize as a thunder-clap.

The initial leader stroke and main return stroke are generally followed
by subsequent leaders and return strokes in rapid succession. Up to
42 separate strokes have been recorded as forming one discharge.
Stroke spacing is in tens of milliseconds and, physically, each follows
the initial leader track unless heavy winds or other disturbances can
move the channel.

Some 95% of ground strokes are negative strokes with respect to ground.
When positive strokes do occur, they are usually at the end of the active life
of a particular thundercloud, and a single stroke may discharge the whole
of the upper positive cloud charge centre in a stroke of exceptional severity.