National Electrical Code Tips: Article 770, Optical Fiber Cables and Raceways, Part 1
Understanding the listing requirements of fire alarm circuit cables can help you make sense of the cable alphabet soup. Here are some highlights from Part IV of Article 770.
- The Code Making Panels (CMPs), composed of volunteers with full-time jobs, struggle to standardize and clarify terminology with each Code cycle. One of the terms most commonly rife with confusion is "grounding." The general movement is to be more precise. The last three revisions have brought Article 770 in line with that. The 2014 revision started with an informational about bonding and grounding, as did the 2017 revision. That note is gone with the 2020 revision, and the relevant language in this Article is almost clear. Instead of confusing the issue by using grounding where bonding is meant, it now says "either grounded or bonded as specified in...." Maybe in 2023, this Article will simply say "bond" where bonding is meant.
- Previous editions of Article 770 referred to a "grounding conductor." But in Article 100 we see that "ground" is the earth and to ground something means to connect it to the earth. That is, drive a rod into the dirt. Grounding on the load side doesn't serve any electrical purpose, but it does waste money and cause confusion. So in Article 770, and elsewhere in the Code, we are seeing this supplanted by "bonding conductor" and "grounding electrode conductor" as applicable.
- This Article is about installing, not manufacturing, optical fiber cables and raceways [770.1].
- There's some confusion over what is meant by "abandoned cable." Here in Article 770 and elsewhere in the Code, that confusion is resolved by defining such cables as not being terminated at equipment (the common understanding, or misunderstanding as the case may be) and also not identified for future use with a tag [770.2]. There are specific requirements for such cables.
- There's also considerable confusion, in communication systems, over the meaning of "point of entrance." The definition is pretty simple, and for fiber optic cables it's in 770.2. This isn't where the cable enters a central distribution box, and not necessarily where it enters the building. The key is where it emerges into the usable space. For example, the point of entrance isn't where the cable enters the slab from the outside, but where it emerges from the slab to the inside.
- Other Articles apply. For most applications/installations, you follow the simple formula that the Article you're dealing with (e.g., Article 503 or 626) is something that amends the requirements of Chapters 1 through 4. With communications systems, things are a bit different. With optical fiber, only those sections in Chapter 2 and Article 300 referenced by Article 770 apply [770.3].
- Fiber optic cables don't carry current (unless they are composite types), so you don't need to seal them when installed in hazardous locations, right? Wrong! Here's an example to illustrate the concept. Explosion-proof fittings are not designed to protect conductors from explosions, but to contain an explosion in the raceway and/or prevent it from propagating through the raceway. Using this same logic of containment, you seal the cables. Apply 501.15, 502.15, 505.16, or 506.16, as applicable [770.3(A)]. You're not worried about flame propagating out of the fiber optic cable. You're worried about flame propagating past the cable, so you seal around it.
Just as you don't let cables pile up on ceiling tiles to block access, you don't let optical fiber cables pile up to prevent access behind electrical equipment panels [770.21]. Note that the ceiling tile rule also applies to these cables and is explicitly stated in 770.21.
- Remember we mentioned abandoned cables earlier? What are you supposed to do with them? The answer is you must remove the accessible portions of them [770.25]. A good set of end-cutters works well for this purpose.
- Remember what we said about hazardous locations earlier? A similar concept applies to any optical cables or raceways installed in hollow spaces, vertical shafts, and ventilation/air-handling ducts. You must make provisions to ensure the possible spread of fire or products of combustion will not be substantially increased [770.26]. Make sure you firestop penetrations in walls, using approved methods.