Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 439

Article 770—Fiber Optic Systems. Article 770 cov-

ers three general types of fiber optic cable: nonconduc-

tive, conductive, and composite. Nonconductive type

refers to cable containing no metallic members and no

other electrically conductive materials. Conductive type

refers to cable containing noncurrent carrying conduc-

tive members such as metallic strength members, etc.

Composite type refers to cable containing optical fibers

and current carrying electrical conductors. Composite

types are classified according to the type of electrical

circuit that the metallic conductor is designed for.

Article 800—Communication Circuits. Article 800
covers multipurpose and communication cable. Multi-
purpose cable is the highest listing for a cable and can be
used for communication, Class 2, Class 3, and
power-limited fire-protective cable. Communication

cable can be used for Class 2 and Class 3 cable and also

as a power-limited fire protective cable with restrictions.

Article 820—Community Antenna Television. Article

820 covers community antenna television and RF cable.

CATV cable may be substituted with multipurpose or

communication listed coaxial cable.

14.28.1 Designation and Environmental Areas

The NEC has designated four categories of cable for var-

ious environments and they are listed from the highest to

the lowest listing. A higher listing can be used as a sub-

stitute for a lower listing.

Plenum—Suitable for use in air ducts, plenums, and

other spaces used for environmental air without conduit

and has adequate fire-resistant and low-smoke produc-

Figure 14-22. Effects of grounding on crosstalk. Courtesy Altec Lansing Corp.

70 V

600 7

Conduit

Amplifier

Two twisted pairs in the same conduit

EN

125 7

40 W

70 V

Conduit

Amplifier

Two twisted pairs in the same conduit

70 V

Conduit

Amplifier

Two twisted pairs in the same conduit

1. Amplifier common grounded 0.1 V 0.4 V 2.0 V


2. Ground removed. 70 V line floating 0.014 V 0.06 V 0.3 V


3. 70 V circuit grounded using a pair of resistors matched to 10% 0.005 V 0.02 V 0.1 V


4. Same with resistors matched to 1% 0.0006 V 0.0025 V 0.012 V


5. 70 V circuit as in 4 0.000 V 0.0016 V 0.008 V


6. Same as 5, except disturbed line is 2 conductor twisted cable 0.0002 V 0.0008 V 0.004 V

250 ft 1000 ft 2000 ft

15,000 7

600 7

600 7

600 7

600 7

600 7

125 7

40 W

125 7

40 W