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An Electrical Engineering Protection Decision ..... Should I use wire-line isolation or a fiber optic facility extension?

Please keep in mind that a 'Fiber Optic Facility Extension' is not a fiber optic network solution!

 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.

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