C7 Optical Fiber Cable Installation (NEC Article 770)

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?

Accepted Answer

Single-mode supports longer distances with a smaller core; multimode supports shorter distances with a larger core

Answer

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?

Accepted Answer

OFNR (optical fiber nonconductive riser)

Answer

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?

Accepted Answer

No smaller than 10× the cable's outer diameter

Answer

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:

Accepted Answer

Locate faults, measure splice loss, and verify end-to-end fiber attenuation

Answer

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?

Accepted Answer

Ensuring all fiber shards are collected and properly disposed of — never left on surfaces or floors

Answer

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.

(C7) California C7 Low Voltage Systems Contractor Certification Practice Test

(C7) California C7 Low Voltage Systems Contractor Certification Practice Test

C7 Optical Fiber Cable Installation (NEC Article 770)