Fiber Optic Cabinets, Cables, Pedestals and Terminals

Working with customers across the country to install our duct and buried fiber delivery solutions, we’re often asked how grounding applies to the installation. As I began to research the topic more fully, I discovered this was a bit of a hot topic with basically two camps of thought: one camp still treating fiber like old school copper cables and another enlightened camp that treats fiber like…fiber.

Bonding, earthing, grounding and drain wires…throughout my previous career as a transport network installer and engineer, I dealt with grounding on almost every project, whether it was in the form of a building, grid, frame, chassis, cable rack, aisle or shielded copper cabling. As an RCDD, I’m very aware of the great number of pages in the BICSI TDMM 13th addition, which detail how to calculate and mitigate electromagnetic interference (EMI) and radio frequency interference (RFI) when designing a network. The drive behind this emphasis on grounding has been for the safety and network performance based on old copper cabling networks. But things have changed and I hope to bring everyone up to speed!

Fiber cable figure

The simplest way to design a network that avoids traditional copper cabling problems and the additional associated costs is to choose an all-dielectric cable. Guess what? It just so happens that optical fiber cable is dielectric, whether singlemode or multimode. Optical fiber cable in general is composed of all-dielectric materials. Dielectric means it has non-conducting properties of a non-metallic, insulating material that resists the passage of electric current. In addition, the signal traversing the fiber’s glass conductor is light, not electrical. Since an optical fiber cable is non-conductive and there is no electric flowing, there are several advantages over a twisted copper cable in deploying:

  • Lightning immunity
  • Electromagnetic interference (EMI) immunity
  • Radio frequency interference (RFI) immunity
  • No crosstalk
  • No grounding requirements
  • Reduced labor and material costs

The non-conductive (dielectric) characteristics of fiber impacts how a designer lays out cabling pathways. When designing with fiber, you can utilize the shortest pathway versus avoiding EMI sources. Traditionally with a twisted copper cable network, you would take into consideration the minimum separation distances from these sources (not an all-inclusive list):

  • Unshielded power lines – 24 inches
  • Power lines enclosed in a grounded metal conduit – 6 inches
  • Electrical motors and transformers – 48 inches

On occasion, you may find a metallic strength member, metallic tone wire or metallic armor in optical fiber cables depending on the application. Since there is some confusion on grounding optical fiber cable, I always say, “When in doubt, code it out!” One code sits on the iron throne and rules them all: the National Electric Code or NEC. The current language regarding optical fiber cabling grounding found in the NFPA 70 NEC 2014 is as follows:

770.93 Grounding or Interruption of Non–Current-Carrying Metallic Members of Optical Fiber Cables. Optical fiber cables entering the building or terminating on the outside of the building shall comply with 770.93(A) or (B).

(A) Entering Buildings. In installations where an optical fiber cable is exposed to contact with electric light or power conductors and the cable enters the building, the non–current-carrying metallic members shall be either grounded as specified in 770.100, or interrupted by an insulating joint or equivalent device. The grounding or interruption shall be as close as practicable to the point of entrance.

(B) Terminating On the Outside of Buildings. In installations where an optical fiber cable is exposed to contact with electric light or power conductors and the cable is terminated on the outside of the building, the non–current carrying metallic members shall be either grounded as specified in 770.100, or interrupted by an insulating joint or equivalent device. The grounding or interruption shall be as close as practicable to the point of termination of the cable.

As you can see in the language of 770.93(A) & (B), the only application that requires the grounding of metallic members in an optical fiber cable is when it is exposed to contact with residential power or other power conductors. However, if that metallic member is interrupted by an insulating joint or equivalent device, grounding is not necessary even when exposed to contact with power.

Direct Bury - ToneableOne of the many benefits of Clearfield’s aerial) which will be subject to NEC 2014 770.93(A) & (B). Always keep in mind that optical fiber is not electrical and is not subject to the same rules as an electrical current carrying cable. All standards and codes are subject to the opinion of the local authority having jurisdiction.