|
by Ernest M.
Duckworth Jr., P.E. President-LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303
303-688-5800
How does one make a wise engineering choice in choosing the best
method of isolation from a Ground Potential Rise (GPR) due to an
earth return fault current or lightning strike energy?
Choosing a fiber facility extension may offer unlimited
isolation from remote ground, provided that the extension is
properly designed. Choosing to isolate a wire-line communication
facility with currently available products will offer
70,000V-Peak or more of isolation from remote ground.
Is 70,000V-Peak a limitation that I need to worry about? No,
absolutely not. 70kV-Peak of isolation is much more isolation
than any grounding facility on earth will ever experience in
Ground Potential Rise (GPR). Why is this? Because the earth is
unable to support a large field strength (greater than 30kV) and
will ionize, thus reducing the ground resistance as the GPR
attempts to rise above approximately 30kV.
Thus, as far as
isolation from a remote ground, there is no advantage of using a
fiber facility extension over wire-line communications
isolation.
Choosing a fiber optic facility extension will require equipment
at two locations: 1) remote ground (300V-Point) and 2) GPR
ground grid. Both locations require powering of sensitive
electronic boards and equipment. The maximum reliability
achievable because of the required powering at each locating is
20 years MTBF. Since there are two locations, this reliability
is halved, or 10 years MTBF for a fiber facility extension
Choosing to isolate a wire-line communications facility will
require equipment at one location: 1) GPR ground grid. This
single location will require no power if passive equipment is
chosen to support the wire-line communications facility, i.e.;
T1 Carrier, HDSL, DATA, SCATA, etc. The maximum reliability
achievable because of no powering requirement and only the need
for one location is approximately 288 years MTBF, the failure
rate for an isolation transformer.
Thus, as far as system reliability, there is an advantage of
28.8 to 1 in the selection of wire-line communications over
fiber facilities. And don't forget costs! Wire-line isolation is
approximately one-half the expense of a fiber optic facility
extension, because the fiber optic facility extension requires
twice the equipment for twice the locations (two).
Per IEEE Std. 487-2000, Class A Service is a reliability level
in which the communications service must work ‘before’,
‘during’ and ‘after’ a power fault event. To provide
Class A Service the engineering choice is a method of isolation
that requires no powering to support. Any powering requirement
of the isolation eliminates Class A Service as a capability.
Provision of Class A Service automatically rules out Fiber Optic
Facility Extensions, because this isolation method requires
powering of the equipment at two locations. Wire Line Isolation
can provide isolation without any powering and thus, can offer
Class A Service.
Choosing a fiber facility extension will require equipment to be
installed and maintained at two locations. This fiber terminal
equipment and fiber cable installation, between these two
locations, will require fiber installation methods, trained
personnel in fiber, special fiber cable locate equipment, and
fiber installation tools. Because of two locations, installation
and maintenance costs are approximately double that of wire-line
communications isolation.
Choosing to isolate a wire-line communications facility will
require equipment to be installed and maintained at only one
location. This location, known as the High Voltage Interface (HVI)
will require your normal Installation & Repair personnel
with no requirement for special tools, etc. Because of the need
for only one location, installation and maintenance costs are
approximately half that of a fiber facility extension.
Thus, as far as system installation & maintenance costs,
choosing wire-line communications isolation over a fiber
facility extension will cost approximately one-half as much
initially. Over the life of the two different systems, because
of the reliability advantage of wire-line communications
isolation over a fiber facility extension, the maintenance costs
will be much more (28 times more) for maintaining the fiber
facility extension.
To compare the safety of a fiber facility extension to wire-line
communications isolation, one must consider the reliability of
the two methodologies, as well as the local safety procedures
for each. Since wire-line communications isolation is 28 times
more reliable than a fiber facility extension, it could be
considered many times safer, due to reoccurring maintenance
exposure.
Local safety procedures to protect against step and touch
potential at these locations are thoughtfully practiced when
installing wire-line communications isolation. This includes the
use of isolating rubber gloves and a rubber blanket. However,
safety procedures are generally nonexistent when installing a
fiber facility extension, due to the misconception that there is
no danger to personnel.
Ironically, this is a very dangerous misconception. There will
be the same danger to step and touch potential, as well as many
more times of reoccurring maintenance exposure using a fiber
facility extension instead of wire-line communications
isolation. Thus, it is far safer, for installation and
maintenance personnel, to work on a wire-line communication
isolation facility than a fiber facility extension.
Choosing a fiber facility extension will require that the NCTE
equipment (loop backs, terminations, amplifiers, etc.) be placed
at the 300V-Point. This is a very inconvenient location, out in
the middle of a cable facility and not within a building. It is
also not at the end of the communications network where this
NCTE equipment is suppose to be located for testing and
monitoring purposes.
Choosing wire-line communications isolation places the NCTE
equipment in a building on the station side of the HVI
(isolation equipment), and just where it belongs at the end of
the communications network. This is a very convenient location,
protected from the elements, environmentally controlled, and
located exactly where it was designed and meant to be placed.
Thus, choosing a fiber facility extension will require the
misplacement of the Network Channel Terminating Equipment (NCTE).
This placement in the middle of a cable facility will require a
special weather proof housing, and dispatch of maintenance
personnel even if the network tests as ‘no trouble found’,
because this NCTE is not located at the end of the
communications network.
Choosing wire-line communications isolation, places the Network
Channel Terminating Equipment (NCTE) exactly where it was
designed to be located per all communications standards and
practices.
Choosing a fiber facility extension will incorporate
non-standard protocols over the last section of the
communications network. Eventually, this system will have to be
replaced, because it cannot be integrated into a total network
fiber optic infrastructure.
Choosing wire-line communications isolation has not incorporated
any non-standard protocols in the communications network
whatsoever. This type of isolation is transparent to the network
transmission, and any network system upgrade will be equivalent
to upgrading any wire-line communications into the total fiber
infrastructure.
Thus, choosing wire-line communications isolation will prevent
the future need for having to terminate service in order to
junk-out a non-standard protocol fiber facility extension and
replace it with a standard protocol total fiber infrastructure.
|
