Your OSP system is an investment you can't afford to lose, so keeping it safe should be a high priority.
Life. Property. Money. Performance. Each one of these important design elements is vulnerable to the effects of stray currents, so it's important to properly design and install an electrical grounding system on every outside plant (OSP) project. A good protection system will mitigate hazardous currents and voltage potentials and provide proper protection for all of those elements.
In this sixth installment of EC&M's series on the Second Edition of BICSI's Customer-Owned Outside Plant Design Manual, we look at the key design elements of an OSP bonding and grounding system, which must meet the rules of the NEC and the National Electric Safety Code (NESC).
Is your OSP exposed?
One of the first items to address in designing a protective system is to determine if your plant is considered exposed or non-exposed. An exposed system is vulnerable to unwanted sources of current and voltage. You must provide protective measures for aerial, direct-buried, and underground cable when there is exposure to:
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Disturbances from lightning.
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Accidental contact with power conductors operating at more than 300V to ground.
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Ground potential rise (GPR) exceeding 300V.
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Voltage induction, such as AC power, exceeding 300V.
An OSP system subject to electrical disturbances from any of these sources is considered exposed. A system not subjected to these effects is considered nonexposed, in which case you needn't do anything.
Even though a cable is buried and not located adjacent to other electrical cables, it's still considered exposed to lightning transients. In rare cases, the cable may even come in contact with a foreign conductive material while still in the ground. A good rule of thumb is to first assume all system components are exposed unless you can absolutely rule out any reasonable possibility of foreign potentials or can qualify it by way of policy directed by authorities. Some AHJs may consider all your cable exposed and force you to treat it as such, so it's important to consult with your local authority having jurisdiction (AHJ) prior to moving too far along on the project to avoid conflict down the road.
Bonding and grounding.
A good reliable protection system requires effective bonding and grounding. These two separate requirements, make up a single critical system.
As defined in BICSI's manual, bonding refers to the electrical interconnection of conductive parts designed to maintain a common electrical potential. You must size bonding conductors to be of sufficient gauge to carry anticipated fault current. Grounding refers to the electrical connection of telecommunications hardware to an effective electrical ground, which can be the vertical down lead of a power system multiground neutral (MGN), a grounded neutral of a secondary power system, or a specially constructed grounding system.
All exposed telecommunications cables that contain metallic components such as a metallic shield, metallic strength member, or metallic pair require some form of electrical protection at the building entrance. It's critical that you connect the metallic sheath components and metallic strength members of all cables entering the building to the telecommunications main grounding busbar (TMGB), which is the location where all grounding conductors must be connected to the earth electrode.
You should also bond all cable shields at all splice locations as well as to the strand of aerial cables. And you should bond all buried cables to ground rods or an MGN. Remember, no system is properly grounded if you fail to properly tie it into an effective grounding system.
Continuity is a must.
When it comes to qualifying your system, check your continuity. Bad connections, broken ground wires, and ground rods improperly placed or installed in a high resistivity soil will create fallible systems.
Visual inspections may not always reveal these problems. That's why you should use test equipment to verify your ground system is intact. Every component of the system must be intact and well bonded to provide proper protection. Unfortunately, continuity weaknesses are found to be the source of equipment damage, electrical noise, or injuries only after the fault is introduced. Most of the time these situations can be prevented with proper design, inspection, or test methods.
Protectors.
The foreign currents and unwanted voltage levels that may find their way into your telecommunications system can be mitigated with primary and/or secondary protectors. Protectors limit the potential difference between conductors and ground by providing a low impedance path to ground for current, once the voltage of the protector unit is reached. Protector units are required on all exposed circuits entering a building, which include tip and ring conductors contained in conventional paired conductor cables and those in hybrid cables. These protectors are typically attached to the system via termination point hardware.
Most protectors can be classified as either fused or fuseless. The fused protection system may use an air gap discharge, gas tube, solid state, current-interrupting device, or isolating transformer as methods of application. On the other hand, fuseless protectors don't offer protection for sustained fault current like fused protectors. In conjunction with the use of fuseless protectors where power exposure exists, a fuse link is required between exposed system components and the protector in order to minimize any fire or shock hazard in the event of a sustained power contact.
Fuse links are sections of finer gauge cable shorter than normally required for transmission purposes. In the event of prolonged current flows caused by foreign potentials, fuse links burn open, protecting terminating equipment or cabling. For transmission reasons, make fuse links as short as possible.
Consider providing primary overvoltage protection and secondary overcurrent protection for OSP exposed twisted-pair copper cables. Secondary protection helps protect equipment from continuous stray currents exceeding 0.35A. These currents typically aren't large enough to engage primary protectors, but can damage equipment and present a fire hazard. You should install fast response secondary protectors in series between the primary protectors and the switching equipment at the main building, and between the primary protectors and the station equipment at the remote buildings.
It's also permissible to use a single assembly that integrates both primary and secondary overvoltages and overcurrent protection in a single device. If you do, be sure the protector module is equipped with in-service test points so you can pinpoint faulty or blown modules without accidentally disengaging a working circuit during testing.
Grounding it all together.
With a good understanding of grounding system design and awareness of standard components required to build a system, you can create the necessary protection for your next OSP network. All it takes is the proper application of solid design principles, good work practices, and regular testing of your system.
Next month, we'll look at air pressures systems for OSP systems.
Hite is special projects engineer-OSP for CT Communications, Inc., Concord, N.C.
The material for this article was excerpted with permission from BICSI's Customer-Owned Outside Plant Design manual, Second Edition.
