Protection From Power Surge
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| Capture the
lightning Strike. The first point
of the six point plan involves directing the
lightning strike to a preferred point on purpose-designed
air terminals without protection the highest
point on a structure is the most likely to
attract lightning, such as antennas, microwave
dishes and their associated equipment. By
correctly installing a purpose - designed
air terminal, lightning can be attracted to
preferred point. Located away from antennas
and cabling. |
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Conduct the lightning current to
ground safely
Once lightning has been captured at a preferred point,
it is necessary the discharge current safely to ground,
and to minimise the induction to ancillary conductors
such as nearby internal and external cabling. This can
be implemented by the use of a triax down conductor. The
multiple screens serve to reduce induction and the risk
of side flashing.
Dissipate the energy into the earth
Next step is to dissipate the transient energy to earth
with minimal rise in ground potential.
Eliminate earth loop differentials
Once the energy is conducted to ground, a low impedance
earth is essential to dissipate the lightning energy into
the earth mass. The use of earth rings, crow's foot earthing
techniques should be employed to lower impedance, as they
the lightning energy to diverge as each conductor take
a share of the current special earthing compounds can
used to lower grounding impedances. Earth potential bonding
should also be ensured between lightning protection, telecom
power grounding systems.
Protect equipment from surges and
transients on power lines
Even if a structure is provided with an integrated direct
strike system, there still remains the largest risk of
overvoltage transients entering an installation via external
cables. Efficient clamping and filtering of power transients
at the point of entry of power lines to building is essential
if the risk of physical equipment damage, loss of operation
and economic loss is to be prevented. Standard surge diverters
installed at the main switchboard may not provide adequate
protection. In order to protect sensitive equipment, it
is necessary to limit residual voltage within the maximum
operation tolerance of the internal equipment. For equipment
operation at 240 volts rms, component damage result from
transients with peak values as low as 700 volts. surge
diverters can clamp dangerous overvoltages to safe levels,
but they do not limit the fast rate of rise of current,
which can be as high as 10,000 amps every microsecond.
These high rates of rise of current can induce destructive
high voltage downstream from the surge diverter and can
be generated within the equipment itself leading to failure.
low pass filter techniques following the primary surge
diverters will reduce the peak residual voltage, and voltage
rise reaching the equipment. Surge reduction filters provide
multistage attenuation bt clamping and then filtering
transients on power lines.
Protect equipment from surges and
transients on communication, data and signal lines
Nprotection devices for these circuits fall into several
levels according to performance. Level one protection
is generally a simple gas arrester with high surge div
ersion capability. However, the gas arrester is a relatively
slow device, and fast transients will overshoot the rated
voltage (for example, 700-800 volts for a 230 volt rated
device). Level two protection incorporates high surge
diverters with semiconductor transient suppressers which
operate fast to clamp the voltage at lower voltages until
the gas surge arrester operates. Level three protection
incorporates the use of a low pass filter allows the protector
to clamp the transient at an extremely low level (below
the normal operating voltage of the equipment during the
transient) but is transparent under operating condition.
For example, a typical PABX installation will have a peak
operating voltage of 150 volts and the level three protector
will clamp surge the transients to 50 volts, well below
the maximum operating voltage to provide protection.
Summary
Overvoltage transients and surges are a major cause of
damage and loss of operating to communications and IT
equipment. An integrated approach to protection is required
at the facilties, systems and equipment levels. Twenty
years of analysis of damage to installations has shown
that no single protection device can provide lightning
immunity. Comprehensive protection can be provided by
employing an integrated approach using the six point protection
plan. This practical approach as described has proven
to be very successful on major facilities throughout the
world.
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About
the author:
Alistair Hunt is the Technical Director for Telecom
protection Technologies. A physics graduate, he
has been involved in the design and development
of transient overvoltage protection components and
protection system for mains, power data, signalling
and telecom equipment for over 16 years. He is also
an active member of the BSI and ICE working group
committees for the setting of specifications and
standards in the protection of electrical and electronic
systems against lightning and transient over voltages.
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