Sensitive critical process equipment is deemed to be
at greatest risk and should be assessed for surge protection
based on Points 4, 5 and 6 of the 6 Point Plan, as detailed
in the Site Audit Methodology within the
of this site.
The greatest risk to any critical operational and electronic
process management systems is through the service entry
of unshielded, buried or exposed conductors that may
enter (or exit) a building or structure, or which may
travel and interconnect between different buildings/structures.
Such cablings will include:
and outgoing mains power cabling
Incoming and outgoing
telecoms and extension cabling
LAN and serial data
RTU telemetry cabling
Various instrumentation cabling.
Lightning does not discriminate in its treatment of
any long metallic conductor that is exposed to its direct
and indirect coupled energy and EMF, and it is the exposure
to these indirect effects of magnetic induction, capacitive
coupling, and ground potential rise, (not withstanding
the lesser prevalent direct strike risk) which causes
the greatest risk for the incidence of lightning related
damage and disruption.
Point of Entry/Point of Exit
To deal with this risk, a sites engineering and operations
management team need to look at a holistic regime so
as to isolate their most critical process equipment
from these directly connected and exposed cablings,
where the most effective way to manage this, is through
a basic and simple protection philosophy, that we refer
to as “Point of Entry/Point of Exit”.
The Point of Entry, and Point of Exit, involves individually
auditing and considering the risk that all incoming
and outgoing copper conductors place on the connected
equipment when these cables enter, or leave a structure,
and where these cablings are shown to present a direct
connection to internal critical systems, these cablings
should then be fitted with suitably rated SPD’s (Surge
Protection Devices) installed at the cablings point
of entry, and/or point of exit. The presence of any
installed SPD at Point of Entry, therefore delineates
that a protected zone has been created, where any incoming
surge current will be shunted to earth, prior to it
entering the facilities.
Strategically, the Main Switchboard, the various levels
of Distribution, and possibly even Sub Distribution
boards, offer the most effective and convenient placement
for SPD’s to be installed on incoming mains cabling,
which when considered methodically, provides a sequenced
protection regime which provides transient control to
all downstream connected critical systems.
At large decentralized sites where there are larger
numbers of remote outbuildings, there will most likely
also be larger numbers of downstream Distribution and
Sub Distribution Boards involved, which will all interconnected
by long and often buried lengths of sub mains cabling.
Where these cablings are run remotely from a remote
main switchboard, this can additionally present an elevated
risk from the induced effects of localized lightning
activity, to the connected downstream facilities, irrespective
that the upstream incoming mains may be surge protected
at the Main Switchboard. The reasons for this is that
magnetic induction, capacitive coupling and ground potential
rise, can occur anywhere on a site, therefore transients
may be induced or coupled downstream from Main Switchboard
installed surge diverters.
Where these sub mains cables are buried, or are run
in PVC underground conduits, this plays no role in any
shielding effect to these cablings, hence there will
still a requirement to protect these cables at their
imminent downstream “POE” (Point of Entry) at the relevant
outbuilding or equipment cubicle. The installation of
additional set of SPD’s at the outbuildings main Distribution
Board, or remote equipment control cubicle , then delineates
a further protected and clean zone to which all connected
equipment within and connected from those outbuildings
if then protected.
What is trying to achieve effectively involves the creation
of clean zones where sensitive critical systems are
located. POE is all about the creation of protective
zones, created by the appropriate placement of SPD’s.
POE also has similar application on those other incoming
cabling services, the most common of which will be incoming
telecommunications cablings and any outgoing extension
lines. From a POE perspective, the Telecoms MDF and
IDF termination frames, provides the most strategic
location for the installation of these telecoms line
Where some organizations come unstuck is that these
MDF and IDF Frames must be earthed, and to simply install
SPD’s into these frames which are not earthed, the protection
devices require this earth reference, and any installed
protection device will not operate correctly without
the earth connected.
Diverters versus Filters
Whilst Primary Surge Diverters offer good point of entry
protection by removing much of the brunt of lightning
related transients, there will still be high residual
voltages which remain, which may still be of sufficient
magnitude to still damage the more sensitive semi conductor
The installation of dedicated Surge Filters at those
more critical sensitive loads further reduces the voltages
to within the tolerance of sensitive equipment, whilst
also reducing the high rates of change associated with
voltage and current rise (the dV/dT and dI/dT). These
high rates of change can be just as damaging as the
over-voltage, yet simple surge diverters do not address
this particular aspect.
Such sensitive loads will generally include:
Mainframe Computing and Server equipment
PC and workstations
Programmable Logic Controllers
Radio Telemetry Equipment
The following illustration shows the typical let through
voltage at the output terminal of a surge diverter,
and surge filter when subjected to an identical Cat
B ( 6 kV, 3kA @8/20us) test impulse.
This illustration shows that 6000 volts is impressed
on the Surge Protective Devices with an associated current
of 3000 amps, which reaches 90% of its peak within 8
microseconds, and which decrease to 10 % of its peak
within 20 microseconds.
The residual voltage left over from a surge diverter
( shown in green) has been reduced from 6000 volts down
to 600 volts, whilst the Surge Reduction Filter (Shown
in Red) has slowed down the rate of voltage rise and
outputs a much reduced let through voltage with a peak
of only 240 volts.
Shunt vs. Series voltage levels
As the surge diverter cannot address this rate of change
issue, it therefore only offers a coarse level of protection.
The surge filter (which includes an additional LC wired
circuit in its configuration) slows down these high
rates of dI/dT and dV/dT, ensuring a much lower residual
voltage is presented, and we refer to these Surge Filters
as offering a Fine level of protection and voltage control.
When we discuss “a staged and sequenced protection regime”,
we are effectively talking about taking the main brunt
of lightning related over-voltage away with Point of
Entry Surge Diverters, and using downstream Surge filters
which take the pre-clamped voltage waveform, and then
reduce this let through down to even safer levels, which
are within the over-voltage tolerance of the connected
POE compliance can effectively be automatically included
on any future upgrade projects, by tightening internal
engineering and tender specifications, drawings and
documents , that may be provided to external equipment
suppliers such as switchboard manufacturers, as well
as UPS, PLC, Telemetry, IT and Communications equipment
It is recommended that ANY proposed Surge protection
equipment be UL listed, that is the equipment has been
independently tested and put through rigorous safety
and other testing regimes to ensure the products actually
meet their stated specification and performance criteria.
Surge Protection devices in particular should be UL
listed and hold compliance to UL 1449 Edition 2 and
Australian Standard AS1768-2007 recognizes the requirement
for UL testing in ensuring that any Surge Protective
device which may fail due to certain fault conditions,(
such as Temporary Over Voltage( TOV)) fails in such
a manner so as to not cause a fire.