C7 Optical Fiber Cable Installation (NEC Article 770) — Questions and Answers
Question 1: What is the key difference between single-mode (OS2) and multimode (OM3/OM4) optical fiber that determines which type is used for a given installation?
- Single-mode uses plastic fiber; multimode uses glass fiber
- Single-mode supports longer distances with a smaller core; multimode supports shorter distances with a larger core (Correct answer)
- Single-mode is used for audio signals; multimode is used for data
- Single-mode requires no connectors; multimode requires fusion splicing
Correct answer: Single-mode supports longer distances with a smaller core; multimode supports shorter distances with a larger core
Single-mode fiber has a very small core (~9 microns) that carries a single light ray, enabling transmission over many kilometers. Multimode fiber has a larger core (50 or 62.5 microns) that carries multiple light modes, which limits distance due to modal dispersion but works well for shorter runs (up to ~550m for OM3/OM4 at 10Gbps). This determines which to specify for campus backbone vs. in-building links.
Question 2: Per NEC Article 770, optical fiber cables installed in a riser (vertical shaft between floors) must be listed as which fiber cable type?
- OFN (optical fiber nonconductive)
- OFNG (optical fiber nonconductive general purpose)
- OFNR (optical fiber nonconductive riser) (Correct answer)
- OFNP (optical fiber nonconductive plenum)
Correct answer: OFNR (optical fiber nonconductive riser)
NEC 770.154 requires riser-rated cable (OFNR) in vertical runs between floors in a riser shaft. OFNR cables are tested to limit flame propagation in vertical installations. OFNP (plenum) can substitute for OFNR, but OFNG (general purpose) cannot be used in risers or plenums.
Question 3: When installing optical fiber, the minimum bend radius during and after installation must be respected. What is the general rule for the minimum post-installation bend radius of a typical indoor fiber optic cable?
- No smaller than 1× the cable's outer diameter
- No smaller than 10× the cable's outer diameter (Correct answer)
- No smaller than 30× the cable's outer diameter
- Bend radius is not restricted for fiber since glass is flexible
Correct answer: No smaller than 10× the cable's outer diameter
Most indoor optical fiber cables must not be bent tighter than 10 times the cable's outer diameter under static (installed) conditions, and 20× during pulling. Exceeding the minimum bend radius causes microbending losses and can permanently damage or fracture the glass fiber. Manufacturers publish specific values that must be followed.
Question 4: An OTDR (Optical Time Domain Reflectometer) is used after a fiber optic installation to:
- Verify copper continuity end-to-end
- Locate faults, measure splice loss, and verify end-to-end fiber attenuation (Correct answer)
- Test the PoE voltage on fiber-connected switches
- Certify the fiber meets Category 6A standards
Correct answer: Locate faults, measure splice loss, and verify end-to-end fiber attenuation
An OTDR sends pulses of light into the fiber and analyzes the reflections to create a 'picture' of the link. It can locate breaks, measure connector and splice losses, measure fiber length, and verify total attenuation meets the link budget. It is the primary fiber certification tool for installed links.
Question 5: What safety precaution is most critical when working with optical fiber, particularly during cleaving and splicing operations?
- Wearing rubber gloves to prevent electrical shock
- Ensuring all fiber shards are collected and properly disposed of — never left on surfaces or floors (Correct answer)
- Wearing arc flash PPE rated at 40 cal/cm²
- Using only plastic-jacketed tools to avoid conductor damage
Correct answer: Ensuring all fiber shards are collected and properly disposed of — never left on surfaces or floors
Cleaved fiber ends and fragments are extremely sharp glass slivers that are nearly invisible. They can penetrate skin deeply and cause serious injury if stepped on or handled. Proper practice requires collecting all fiber scraps on adhesive pads or tape and disposing of them in a sealable container — never leaving them loose on a work surface.
Question 6: A fiber optic link budget calculation shows a maximum allowable loss of 3.0 dB for a horizontal run. The installed link has two connectors (0.5 dB each) and one fusion splice (0.3 dB). What is the maximum allowable fiber attenuation for the cable segment itself?
- 1.2 dB
- 1.7 dB (Correct answer)
- 2.3 dB
- 3.0 dB
Correct answer: 1.7 dB
Total connector loss = 2 × 0.5 dB = 1.0 dB. Splice loss = 0.3 dB. Total passive component loss = 1.3 dB. Maximum allowable cable loss = 3.0 dB − 1.3 dB = 1.7 dB. Link budget calculations ensure each component's loss is accounted for so the total does not exceed the system's loss budget.
C7 Optical Fiber Cable Installation (NEC Article 770)
What is the key difference between single-mode (OS2) and multimode (OM3/OM4) optical fiber that determines which type is used for a given installation?