Due to the random and statistical manner in
which lightning related events occur and cause equipment damage, lightning surge
protection equipment is often only tested in the field for a brief moment,
sometimes years after its purchase and installation. If the surge protection was
correctly rated for the application, manufactured correctly and installed
correctly there is no reason why it should not operate and adequately protect
the equipment installed ‘downstream’ of the surge. If the rating, manufacture or
installation of the product is not adequate, however, the surge protector may
remain installed without incident until required to handle a large surge. When
the product then fails, people search for reasons and due to the uncertainty
associated with the events surrounding a given a lightning strike, reasons are
sometimes difficult to substantiate and mistrust of the product and the industry
which supplied it are often the end result.
Over the last few years, there have been a
proliferation of articles claiming to ‘explain away the myths’, surrounding the
field of lightning surge protection. Many of these articles have been written
either in order to bring the layman into the picture or to promote a particular
product. This article will highlight some of the contentious issues and trends
which have arisen in the field of lightning surge protection recently and give
an impartial analysis of what the implications are.
Surge component criticism
Very often, specific surge protection
components are criticised for their general performance in favour of another
product or component. What is seldom highlighted in the argument is the fact
that there are many different situations in which the three main groups of surge
protective components, Metal Oxide Varistors (MOVs), Gas Arresters and Silicon
Clamping Diodes (SCD’s) may be applied. Each has characteristics making it the
best choice for a given application. The sensitivity and cost of the equipment
to be protected coupled with the risk of sustaining surge related damage
determines the component or combination of components that is most suitable. The
following discussion reviews the three main component types and what application
they are suited for, their advantages and disadvantages:
MOVs: Metal
Oxide Varistors are commonly used for mains protection applications and are
available in a wide range of clamping voltage and peak impulse current
variations. MOVs are extremely cost effective components and have been proven by
time to work efficiently in many applications. There are three main drawbacks in
the use of MOVs namely their tendency to degrade with use, a relatively high
terminal voltage when clamping high current impulses and a response time which
could be considered slow when compared with silicon clamping technology. Despite
these weaknesses, the MOV is perfectly suited to a number of applications.
- The ‘aging’ characteristic describes the degradation of
the micro-granular structure of the material comprising the MOV. The result is
an increase in leakage current drawn by the MOV at normal system voltages. If
this current increase is suitably high, it is liable to drive the MOV into
thermal runaway – a condition where the device is absorbing energy more
quickly than it can be dissipated and it heats up to dangerous temperature
levels. This hazard is effectively countered with the use of an effective
thermal disconnect device which disconnects the MOV from the supply if thermal
runaway occurs and should visually indicate the failure on the device on the
outside of the casing. MOVs should, therefore, be chosen so that they will be
exposed to surges which are well within the limits of their specified energy
handling capability and should, like any other piece of industrial equipment,
be checked and replaced if found to be faulty. I this is done correctly, the
service lifetime of MOVs is increased dramatically and they provide a very
cost effective surge protective solution.
- There is often confusion surrounding the quoted clamping
voltage of a MOV and the fact that during a surge, the voltage across its
terminals may reach three or four times this value. The clamping voltage is
the voltage at which the MOV will begin to lower its impedance and begin
drawing surge current. As this current flowing through the MOV increases, it
raises the voltage across its terminals (the MOV is after all just a
non-linear resistor with a very small but finite resistance and as the current
through it increases, so must the voltage across it!). The fact is that in
most mains applications, the equipment input is quite robust and will more
than adequately be protected by a correctly chosen MOV product even if the
terminal voltage does exceed the clamping voltage by 3 or 4 times for a short
duration.
- The manufacturer quoted response time of the MOV is
usually of the order of tens of nanoseconds which is once again more than
adequate for most mains applications. Response time becomes critical when the
surge is applied across sensitive silicon-based inputs which react quickly to
a change in terminal voltage. The speed of response of the MOV material is
almost instantaneous, however, and it is largely the inductance in the leads
connecting the surge protector to the supply which causes a delay in response.
Gas arresters
operate on the principle of electrical breakdown across a gap between two
conductive plates. They are capable of repeatedly diverting huge surge currents
without the degree of degradation experienced by MOVs (although they may be
damaged by repeated operation due to large surges) and are used extensively as
the front end to hybrid communications and data protection circuits. Gas
arresters have two main drawbacks: follow on currents at large system voltages
and they often experience a lengthy statistical breakdown delay before operation
leading to a large transmitted voltage spike before clamping.
- Follow on currents make conventional gas arresters
impractical for use on mains systems. Once the device operates in response to
a surge, the impedance across the arrester is extremely low and normal mains
voltage can sustain the arc leading to the rapid heating and destruction of
the device. Modern developments in the field have yielded special gaps which
use circuit breaker principles where the heat of the arc is used to stretch it
between two diverging electrodes thus raising its impedance and eventually
quenching it. These gaps may be used on mains systems and are normally used in
conjunction with MOVs and a series impedance providing good protection against
large surge currents.
- The statistical delay causing a voltage spike in the
clamping characteristic of the gas arrester is normally removed by placing a
series impedance behind the gap (to prevent the spike from driving a large
current into the device being protected) followed by a MOV or silicon clamping
device which operates quickly enough to clamp the spike until the voltage
across the gas tube drops. Modern hybrid spark gaps include the tiny hybrid
components between the terminals and legs of the gas arrester component and
function without the series impedance.
Silicon Clamping
Diodes (SCDs) are well know for their rapid response times, excellent
clamping characteristic and extended life-time. Unfortunately, their inability
to deal with high surge currents and the relatively high cost are the drawbacks
associated with silicon surge technology. SCDs are used extensively as the
secondary stage of the hybrid protection described previously. The rapid
response times (a few nanoseconds once again dependant on component lead
length!) make them ideal for circuits used to protect sensitive silicon inputs
such as those found in computer networking and most digital telecommunications
systems. There are a few silicon based products which have been designed to cope
with larger currents and are sometimes used for specialised mains applications.
These products are usually enormously expensive and cannot handle surge currents
which MOV or the new gas arrestor technology can. As mentioned previously, the
majority of mains protection is placed in front of reasonably robust inputs
which can handle high voltages for a short duration and there is subsequently no
need for nanosecond response times and rigid clamping in this application.
From the above synopsis, it should be clear
that no one surge protective component provides the ‘ultimate solution’. The
answer lies in choosing or combining components to achieve the desired
performance for a particular application at an acceptable cost.
The ‘numbers game’
A common claim in the lightning protection
industry is that the device able to withstand the highest current magnitude is
the superior product and is worth the extra expense. This has led to a race in
which competitors are producing devices capable of withstanding extremely large
lightning strike currents when this is almost never required in any practical
situation. It is well known that, during a lightning strike to a structure, a
conservative estimate of 50% of the strike current is dissipated by the
structure itself and the rest divides itself up more or less equally in the
cables entering the building. Assuming a lightning stroke with a magnitude of
100kA which, according to the IEC-SABS 1024-1-1, is large, has a 5% chance of
occurring and is thus quite rare, we can make some simple deductions about the
common mode currents entering a building via power and signal cables. If we
assume that we have a conservative number of five cables entering the structure
(3 phases of the supply and telephone/fax for example), a 10kA impulse could be
expected on each incoming line. The decision to place a 100kA surge protection
device on each phase of the incoming power lines would then, for example, be
unnecessary and a poor economic decision.
Lightning Protection Zones
The concepts of defined lightning zones and
boundaries are sometimes misunderstood and poorly defined. A lightning
protection zone is an area in which a similar electromagnetic environment can be
described. A significant change in this environment occurs at the boundary of a
lightning protection zone. A zone boundary may be established by a structure
exhibiting a certain degree of electromagnetic shielding or the placement of a
surge protective device. Zones may be classified as follows:
LPZ0A : Area exposed to direct lightning
strike and unattenuated electromagnetic fields.
LPZ0B: Area shielded from direct lightning
strikes but still exposed to unattenuated electromagnetic fields.
LPZ1: Area shielded from direct lightning.
Fields and currents on conductors are significantly attenuated compared with
LPZ0A and LPZ0B.
LPZ2: Area shielded from direct lightning.
Fields and currents on conductors are significantly attenuated compared with
LPZ1.
etc.
How this concept affects the ratings and
placement of surge arresters is probably best described by the following
diagram:
The diagram shows that by slightly increasing
the clamping voltage of the surge protective devices at consecutive zone
boundaries, the surge current may be diverted in a controlled manner with most
of it being dealt with at the Zone 0B/1 boundary. As the surge currents then
entering the building are smaller, lower rated and cheaper surge arresters may
be used inside the building. This approach is often called cascading. Note that
one of the main reasons why cascading surge arresters is a valid practice is not
that the initial device at the Zone 0B/1 boundary is inferior but that in
addition to any transmitted surges, the cabling lying inside Zone 1 & 2 etc.
may be exposed to further induced surges, making it important to place further
surge protective devices closer to the equipment to be protected. If these surge
protectors can be smaller and cheaper yet provide completely adequate
protection, it is economically beneficial. Cascading surge arresters makes this
possible.
Conclusions
An attempt has been made to highlight some of
the issues which pertain to the field of lightning surge protection at the
present moment. The components commonly used in the manufacture of surge
protective devices have been reviewed and critically analysed. It is clear that
each has its benefits and short comings and that effective surge protection
often requires the use of more than one component to achieve the required
performance. The need for surge arresters which protect against huge impulse
currents was critically analysed for realistic scenario and the concepts of
zones, boundaries and the cascading of surge arresters were
explained.
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