Cable Width Selection

Cable Tray Width Selection for Installations with 600 Volt Single Conductor Cables
National Electrical Code (NEC) Section 318-11 Ampacities of Cables, Rated 2000 Volts or Less, in Cable Trays. (b) Single Conductor Cables allows cables of identical construction and conductor material to be operated at different maximum ampacities depending on the physical placement of the cables in ladder or ventilated trough cable trays.
NEC Section 318-10 Number of Single Conductor Cables, Rated 2000 Volt or Less, in Cable Trays. (a) Ladder or Ventilated Trough Cable Trays. Doesn't cover the width requirements of ladder or ventilated trough cable tray for all the types of installations that contain single conductor cables.

This is best exhibited by cable tray width calculations for three different examples of single conductor cables in ladder or ventilated trough cable tray that are permitted by NEC Article 318. The examples are based on installations that contain 12 - 500 kcmil cables (Four - three phase - 480 volt circuits or a circuit of four paralleled conductors per phase) The 500 kcmil copper single conductor cables have 600 volt 75 degree centigrade insulation. The cable's diameter equals 1.07 inches and the cable's area equals 0.90 square inches.
Example #1 is based on the requirements in Sections 318-10(a)(2) and Section 318-11(b)(2).
Section 318-10(a)(2) states that the sum of the cross-sectional areas of the single conductor cables shall not exceed the allowable fill area in Column 1 of Table 318-10 for the appropriate ladder or ventilated trough cable tray width.
12 cables x 0.90 square inches /cable = 10.8 Square Inches
Table 318-10 - Column 1 shows that the minimum cable tray width that has adequate fill area is a 12 inch wide cable tray. The 12 inch wide cable tray has an allowable fill capacity of 13.0 square inches which slightly exceeds the installation's 10.8 square inch requirement.
Section 318-10 states that the single conductors or conductor assemblies shall be evenly distributed across the cable tray. This statement leaves the exact cable arrangement in the cable tray up to the designer. Following are two examples of installations that meet the intent of Section 318-10. The cable installation shown in Figure 1A is technically superior to that shown in Figure 1B.

The installation of the cables in the cable tray as per Figure 1A is very desirable as the cables are in an arrangement where they are equilaterally spaced. This will result in equal reactances for the circuit's phase conductors. If each of the phase conductors has the same resistance and reactance, the currents to and the phase voltages at the utilization equipment will be balanced assuming that all the loads are balanced three phase loads. Motors that are supplied with unbalanced three phase voltages experience additional heating due to the voltage unbalance. A few percent voltage unbalance can be very detrimental to the length of the motor's operating life.
For the Figure 1A and Figure 1B cable installations, Section 318-11(b)(2) states that the maximum ampacities of the cables in ladder or ventilated trough cable trays without covers is 65 percent of the values in Table 310-17
For the Figure 1A and Figure 1B installations, the allowable maximum operating ampacities (Table 310-17) for the 500kcmil conductors is 620 Amperes x 0.65 = 403 amperes per conductor (without the use of a maximum ambient operating temperature correction factor).
Example #2 is based on the requirements in Section 318-11(b)(3).
Section 318-11(b)(3)states that where single conductors are installed in a single layer in uncovered cable trays, with a maintained space of not less than one cable diameter between individual conductors, the ampacities of Nos. 1/0 and larger cables shall not exceed the allowable ampacities in Table 310-17.
Section 318-11(b)(3) defines the arrangement of the cables in the cable tray to obtain the conditions that allow the cables to carry the higher ampacities. Section 318-11(b)(3) contains permissible ampacity information and it also contains information that impacts on the cable tray width selection.
If the width of the ladder or ventilated trough cable tray is selected based on the requirements of Section 318-10 for a installation being made as per Section 318-11(b)(3), the cable tray will be of insufficient width for the intended installation.
To determine the required width of a ladder or a ventilated trough cable tray as per Section 318-11(b)(3).
Total width of the Cables -- 12 x 1.07 inches = 12.84 inches
Space between cables must be equal to one cable diameter -- 11 x 1.07 inches = 11.77 inches. Total cable tray width required is 12.84 inches + 11.77 inches = 24.61 inches.
A 30 inch wide cable tray must be used.
For Figure 2 installations, the allowable maximum operating ampacities (Table 310-17) for the 500kcmil conductors: is 620 Amperes per conductor (without the use of an maximum ambient operating temperature correction factor).
This cable arrangement will result in some unbalance in the phase currents and voltages due to the fact that the cables are not equilaterally spaced. The distances from the conductor centerlines of the Phase A to Phase B and from the Phase B to the Phase C are equal but the distance between centerlines of the Phase C conductor to the Phase A conductor is larger. The reactances for the three phases will not be equal which will result in the currents to and the phase voltages at the utilization equipment being unbalanced. If the circuits are of a length where it is possible to transpose the phase conductors, the reactances of the phase conductors can be equalized. Two transposition would allow each phase conductor to occupy each of the three conductor positions for 1/3 of the length of the run. For very long runs, it may be desirable to have many transpositions but regardless of the number of transpositions each phase conductor must occupy each of the three conductor positions for 1/3 of the length of the run.
This type of installation can only be made where the cables can be terminated without entering raceways (The ampacities in Table 310-16 must be used if the cables enter a raceway). Examples would be at a transformer secondary or at a bus extension from switchgear.
It is best to use the 75 degree C ampacity values even if a 90 degree C insulated cable is installed unless it is known that the equipment can accommodate the termination of the higher temperature conductors. For the Figure 2 installation, the 90 C insulated conductor operating at its maximum ampacity will produce 27 percent more heat than will the 75 C insulated conductor. See NEC Section 110-14(c).Temperature Limitation.
Example #3 is based on the requirements in Section 318-11(b)(4).
Section 318-11(b)(4) states that where single conductors are installed in a triangular or square configuration in uncovered cable trays, with a maintained space of not less than 2.15 times one cable diameter between the cable groups, the ampacities of Nos. 1/0 and larger cables shall not exceed the allowable ampacities in Table B-310-2 in Appendix B of the NEC.
Section 318-11(b)(4) defines the arrangement of the cables in the cable tray to obtain the conditions that allow the cables to carry the higher ampacities. So Section 318-11(b)(4) contains permissible ampacity information and it also contains information that impacts on the cable tray width selection.
If the width of the ladder or ventilated trough cable tray is selected based on the requirements of Section 318-10 for a installation being made as per Section 318-11(b)(4), the cable tray will be of insufficient width for the intended installation.
To determine the required width of a ladder or ventilated trough cable tray as per Section 318-11(b)(4).
Total width of the Cables -- 8 x 1.07 inches = 8.56 inches
Space between cables must be equal to 2.15 times one cable diameter -- 3 x 2.15 x 1.07 inches = 6.90 inches. Total cable tray width required is 8.56 inches + 6.90 inches = 15.46 inches.
A 18 inch wide cable tray must be used.
For Figure 3 installations, the allowable maximum operating ampacities (Table B-310-2) for the 500kcmil conductors is 496 amperes per conductor (without the use of an maximum ambient operating temperature correction factor).
The installation of the cables in the cable tray as per Figure 3 is very desirable for the reasons stated concerning Figure 1A.
This type of installation can only be made where the cables can be terminated without entering raceways The ampacities in Table 310-16 must be used if the cables enter a raceway.
It is best to use the 75 degree C ampacity values even if a 90 degree C insulated cable is installed unless it is known that the equipment can accommodate the termination of the higher temperature conductors. For the Figure 3 installation, the 90 C insulated conductor operating at its maximum ampacity will produce 37 percent more heat than will the 75 C insulated conductor. See NEC Section 110-14(c). Temperature Limitation.
When utilizing cable tray to support cables, the designer has cable installation arrangement options available which allow the same size cables to operate at different ampacities if the appropriate cable tray width is selected.
The maximum allowable ampacity for the 500 kcmil cables installed as per Figures 1A and 1B is 403 amperes - (12 inch wide cable tray).
The maximum allowable ampacity for the 500 kcmil cables installed as per Figure 2 is 620 amperes - (30 inch wide cable tray).
The maximum allowable ampacity for the 500 kcmil cables installed as per Figure 3 is 496 amperes - (18 inch wide cable tray).

www.cabletrays.com/techbl14.htm

Cable Tray/Ladder

http://www.cabletrays.com/faqs.html#nec
What is a Cable Tray System?
Per the National Electrical Code, a cable tray system is "a unit or assembly of units or sections and associated fittings forming a rigid structural system used to securely fasten or support cables and raceways."

What does this mean?
Cable trays support cable the way that roadway bridges support traffic.
A bridge is a structure that provides safe passage for traffic across open spans.
Cable tray is the bridge that allows for safe transport of wires across open spans.
Therefore, think of cable tray as the structural component of a building's electrical system.

What standards / guidelines are available for cable tray systems?
1. The National Electrical Code publishes the standards for all types of electrical applications. Articles 318, 250, and 800 cover various aspects of cable tray systems.
2. NEMA, (National Electrical Manufacturers Association), is an association comprised of the major cable tray manufacturers in the industry. This committee has published three documents to date: NEMA VE1, FG1 and VE2.
NEMA VE1 covers general cable tray definitions, manufacturing standards, performance standards, test standards, and application information. Free download of this document is available on the NEMA website.
NEMA FG1 addresses the standards for fiberglass cable tray systems. Free download of this document is available on the NEMA website.
NEMA VE2 is a cable tray installation guideline which covers receiving and unloading material, storage of material, and general installation practices. Free download of this document is available on the NEMA website.
CTI, (Cable Tray Institute), is a trade association comprised of the major cable tray manufacturers in the industry and was formed to provide specifiers, designers, and installers information on the advantages of using cable tray systems over other types of products. (i.e. conduit, ladder rack, etc.)

What types of Cable Tray are available?
1. Ladder
2. Solid Bottom
3. Trough
4. Channel
5. Wire Mesh
6. Single Rail

How do I know what type of cable tray is right for my application?
1. Ladder Cable Tray provides:
a. Solid side rail protection and system strength with smooth radius fittings and a wide selection of materials and finishes.
b. maximum strength for long span applicationsstandard widths of 6,12,18, 24, 30, and 36 inches c. standard depths of 3, 4, 5, and 6 inches
d. standard lengths of 10, 12, 20 and 24 feet
e. rung spacing of 6, 9, 12, and 18 inches

Ladder cable tray is generally used in applications with intermediate to long support spans, 12 feet to 30 feet.

2. Solid Bottom Cable Tray provides:
a. Nonventilated continuous support for delicate cables with added cable protection available in metallic and fiberglass.
b. Solid bottom metallic with solid metal covers for nonplenum rated cable in environmental air areas
c. standard widths of 6, 12, 18, 24, 30, and 36 inches
d. standard depths of 3, 4, 5, and 6 inches
e. standard lengths of 10, 12, 20 and 24 feet
Solid Bottom cable tray is generally used for minimal heat generating electrical or telecommunication applications with short to intermediate support spans of 5 feet to 12 feet.

3. Trough Cable Tray provides:
a. Moderate ventilation with added cable support frequency and with the bottom configuration providing cable support every 4 inches. Available in metal and nonmetallic materials.
b. standard widths of 6, 12, 18, 24, 30, 36 inches
c. standard depths of 3, 4, 5, and 6 inches
d. standard lengths of 10, 12, 20 and 24 feet
e. fixed rung spacing of 4 inch on center
Trough cable tray is generally used for moderate heat generating applications with short to intermediate support spans of 5 feet to 12 feet.

4. Channel Cable Tray provides:
a. an economical support for cable drops and branch cable runs from the backbone cable tray system.
b. standard widths of 3, 4, and 6 inches in metal systems and up to 8 inches in nonmetallic systems.
c. standard depths of 1¼-1¾ inches in metal systems and 1, 1 1/8, 1 5/" and 2 3/16 inches in nonmetallic systems
d. standard length of 10, 12, 20 and 24 feet
Channel cable tray is used for installations with limited numbers of tray cable when conduit is undesirable. Support frequency with short to medium support spans of 5 to 10 feet.

5. Wire Mesh Cable Tray provides:
a. A job site, field adaptable support system primarily for low voltage, telecommunication and fiber optic cables. These systems are typically steel wire mesh, zinc plated.
b. standard widths of 2, 4, 6, 8, 12, 16, 18, 20, and 24 inches
c. standard depths of 1, 2, and 4 inches
d. standard length of about 10 feet (118")
Wire Mesh tray is generally used for telecommunication and fiber optic applications and are installed on short support spans, 4 to 8 feet.

6. Single Rail Cable Tray provides:
a. These aluminum systems are the fastest systems to install and provide the maximum freedom fort cable to enter and exit the system.
b. Single hung or wall mounted systems in single or multiple tiers.
c. Standard widths are 6, 9, 12, 18, and 24 inches.
d. Standard depths are 3, 4, and 6 inches.
e. Standard lengths are 10 and 12 feet.
Single Rail Cable Tray is generally used for low voltage and power cables installations where maximum cable freedom, side fill, and speed to install are factors.

What materials / finishes are available for the various cable tray systems?
1. Steel (Min. Yield = 33KSI) (35 KSI for Stainless)
a. Plain: hot rolled pickled and oiled steel per ASTM A569 (Commercial Quality) or A570 (Structural Quality)
b. Pre-Galvanized: mill galvanized steel per ASTM A653 CS (Commercial) or SS (Structural) G90
c. Hot Dip Galvanized After Fabrication: plain steel which is hot dipped after fabrication per ASTM A123.
d. Stainless Steel: type 304 or 316L fully annealed stainless steel

2. Aluminum (Min.Yield = 23 KSI)
a. 6063-T6 or 5052-H32 alloy per ASTM B209

3. Fiber Reinforced Plastic (FRP)
a. Polyester and Vinyl Ester resin systems available
b. meet ASTM E-84 smoke density rating; Polyester 680, Vinyl Ester 1025
c. Class 1 Flame Rating and self-extinguishing requirements of ASTM D-635.

Now that I know what types of cable trays are available, what configurations are available?
1. Straight sections are available to route cables in a horizontal or vertical plane.
2. Fittings are available to route cables in various directions in either the horizontal or vertical planes. Typical examples of fittings include elbows, tees, crosses, and risers. Each of these fittings are available in various radii and bend angles.
3. Covers are accessories and shouldn't be in here unless splices etc. are included.

After selecting the type of cable tray and configuration required, what support methods are available?
1. Trapeze Support (Single or Multi-tier)
2. Hanger rod clamps, "J" hangers
3. Center Hung Support
4. Wall Support
5. Underfloor Support
6. Pipe stanchions or other structures
Each of these support methods are preferable in different applications. For instance, trapeze supports may be desired in an application where cables will be pulled through the cable tray. Center hung supports, on the other hand, are generally used when cables will be installed from the side of the cable tray. Center hung supports are especially useful when future cable additions are desired. Wall supports and underfloor supports are useful when ceiling structure is not available or undesired. Outdoor installations are controlled by the structures available to support the cable tray.
Before selecting the type of cable tray, cable tray configuration(s), and support method desired, what additional information do I need to supply to the cable tray manufacturer for them to best understand and satisfy my needs?

Where? Job site and installation considerations
a. Outdoor
1. supports available affect the length and strength requirements
2. environmental loads, ice, wind, snow, and possibly seismic
3. corrosion requirements affect the materials and finishes
4. classified hazardous locations affect the cable types acceptable
b. Indoor
1. support locations available affect the length and strength of the system
2. industrial installation may require a 200 lb. Concentrated Load
3. commercial or institutional installation may make system appearance, system weight, and space available important factors
4. environmental air handling area may affect cable types, cable tray material, or cable tray type and need for covers
5. classified hazardous locations affect the cable types acceptable

What?
a. Type and number of cables to support
1. NEC cable fill requirements dictate size, width and depth, of system
2. cable support requirement may control bottom type
3. largest bending radius of cable controls fitting radius
4. total of cable weight determines load to support
b. Future requirements
1. cable entry / exit freedom
2. design partially full or an expandable system
3. support type to allow for needs

Cable tray selection & application
Is it common practice to use cable trays in the vertical position? Do they maintain their integrity during a 25 or 30 year life of a plant? Is the percent fill of a vertical tray the same as a horizontal cable tray?
Answers:
(1) It is common practice to use cable trays in the vertical position. I have many photos of such installations. There is no problem. Cables must be fastened securely, see NEC392.8(b).
(2) Yes, they do maintain their integrity. I have inspected installations that are over 40 years old. The only aging problem could be the tie wraps, especially for single conductor cables.
(3) There are no differences between the cable fill requirements for vertical and horizontal cable tray installations.

Question : What are the rules for installation clearances for the telecommunication cables in cable trays?
Answer: The 2005 NEC in section 392.6(I)indicates that there shall be sufficient space maintained around cable trays to allow adequate access for installing and maintaining the cables and that cable trays shall be exposed and accessible. Adequate room should be provided around the cable tray to allow for the set-up of cable pulling equipment and to provide easy access for the installation of or removal of cables. Where cable trays are installed one above another, allow 12 to 18 inches between cable trays and the ceiling. This is a guide for installation.
Cable application

Question: Can mechanical utility piping or tubing containing water or compressed air be installed in cable trays with electrical cables?
Answer: No. Cable trays are a support system for electrical cables, power, signal, and communication and optical fiber cables. NEC section 300.8 does not permit any tube, pipe, or equal for water, air gas, drainage, steam, or any service other than electrical in raceways or cable trays containing electrical conductors.

Question: I am in the process of establishing guidelines for raised floors in communications facilities and plan to mandate that all cabling under raised floors be installed on an appropriate type cable tray. Are you aware of any industry standard that may mandate the use of cable trays under raised floors, particularly, power and signal cables?
Answer: We are not aware of such industry standard, but cable trays offer significant advantages for this type of installation and in other computer, telecommunications, and power installations. The telecommunications industry is a very strong cable tray user.

Question: We are using ladder type cable trays at many of our facilities for telecommunications wiring. Do you have any information available for recommended installation clearances for this type of cable tray?
Answer: The NEC does not have a specific installation clearance, but indicates in section 392.6(H) that cable trays should be exposed and accessible. Telecommunications standard TIA/EIA-569 recommends a minimum of 12-inch access headroom above the cable tray.

Question: Are there required code grounding practices regarding cable tray used only for telephone cable? A contractor has just installed a new phone system at my location and he utilized cable trays in the switch room. I did not see any deliberate attempt to ground the system. Our existing cable tray system is heavy bonded and grounded. If this is a code violation, could you refer me to the publication?
Answer: Low energy systems may not be required to be grounded for shock or arcing, ut should be grounded for noise, lightening protection and electromagnetic interference. See CTI Technical Bulletin No. 15.and NEMA VE -2 section 4.7.

Question: Are there any requirements for separation and segregation of various types of cables (i.e. Power, instrumentation, signal, telecommunications, etc.) in cable tray systems?
Answer: Yes, there are NEC rules. Instrumentation, signal, and telecommunications cabling should be separated from power cabling. There are NEC requirements, but also for noise and electromagnetic pick-up from adjacent power cables. This can be accomplished by a separate cable tray system or by a divider within a cable tray.
NEC section 392.6(E)indicates that multiconductor cables rated 600 volts or less are permitted in the same cable tray, however, separation of power and control cables is necessary as indicated in other sections of the NEC and for cross-talk noise reasons. NEC section 392.6(F) provides the criteria for cables rated over 600 volts. The types of cables usually used in cable trays are type TC (article336), PLTC (article 725), ITC (article 727), MC (article 336) and Communication Cables (800-52 (d)), MI (article 332). Fire Alarm Systems (article 760), Emergency Systems (article 700), Optical Fiber Cables (article 770) and Intrinsic Safety (section 504-30). The requirements in these sections are complex. We will discuss them in detail and the general noise problem in the next CableGram.
The requirements for cables that have an outer metal armor are less than for plastic jacketed cables. The general rule is separate communication, control, signal, and instrumentation cabling from power cabling. Power cabling includes 460-volt motor power, 120-volt power, and lightening circuits. Note 120-volt circuits can generate noise. Generally, a separation of two inches is minimum, but the individual circuit and cable are the determining factors in separate requirements.

Question: What types of cables can be installed in Cable Tray systems?
Answer: The types of cables permitted by the 2005 NEC are indicated in Section 392.3 uses permitted, (a) Wiring Methods. They include:
Power and Control Tray Cable (Type TC) - NEC Article 336
Power Limited Tray Cable (Type PLTC) - NEC Sections 725-61© and 725.82(E) Instrument Tray Cable (Type ITC) - NEC Article 727
Optical Fiber Cables - Article 770
Fire Alarm Circuit Conductors - Article 760
Communication Cables - Article 800
Mineral Insulated (MI)Cable - Article 332
Metal Clad (MC) Cable - Article 330
and other cables, including those specially approved for installation in cable trays. Medium voltage (type MV) and single conductor cables in sizes 1/0 and larger are permitted with some restrictions in Industrial Establishments where qualified persons service the installation.
National Electrical Code

Question: We have a customer who would like to install the majority of cable tray in his new industrial facility in what I call an “Edge-Wise” orientation. That is, each cable tray rung would point in a vertical direction as opposed to the usual horizontal direction.
The local electrical inspector has stated that he has no issues with this as long as the manufacturer’s specifications have guidelines in how to install it this way. I have searched and can find no indication in any vendor’s literature that acknowledges the possibility that cable tray would ever be installed in this orientation.
Answer: There is no NEC or other limitation on cable trays that would prevent the “Edge-Wise” orientation. The CTI needs to develop guidelines for this installation. This type of installation minimizes dust accumulation in dust locations and could be advantageous in other situations.

Question: It appears that the NEC doesn’t address the maximum allowable fill area for a solid bottom, channel cable tray. It does however, address ventilated channel cable tray (Article 392.9(E)What is your opinion regarding the maximum fill area for solid bottom channel, given that multiconductor or signal cables only are installed?
Answer: The CTI has submitted a proposal to amend the 2002NEC to provide this information.
Question: Does the NEC apply to telecommunication cabling installations?
Answer: Yes, in the following articles: 645 Information Technology Equipment 725 Class 1, Class 2, and Class 3, Remote-Control, Signaling, and Power-Limited Circuits 770 Optical Fiber Cables and Raceways 800 Communication Circuits 810 Radio and Television Equipment 820 Community Antenna Television and Radio Distribution Systems The sections of these articles that may apply depend on the installation; location; cable selection and equipment. There are other NFPA standards that may apply which include: NFPA 75 Protection of Electronic Computer/Data Processing Equipment NFPA 780 Installation of Lightning Protection Systems
Question: Is it necessary to provide tie-down cables installed in a cable tray?
Answer: Yes; cables are tied down in cable trays to keep the cables in the cable tray, to maintain spacing between cables, or to segregate or confine certain types of cables to specific locations. The last two items can also be accomplished with a solid fixed barrier. The NEC in section 392.8(B)indicates that in other than horizontal runs, cables shall be securely fastened to transverse members of the cable trays.
For vertical installations, the cables may hang away from the cable tray if not tied down. Although this section of the NEC does not require cable tie down in horizontal, it may be necessary to meet other requirements. For instance, it may be necessary and appropriate to space power cables at least a diameter apart to approximate the free air amperage rating of a cable. In hazardous dust locations (class II, division 2), it is required to space type MC and TC cables at least the larger cable diameter apart and arrange the cables in a single layer.
Multiconductor power cables, 4/0 and larger, rated 2,000 volts or less, are required to be installed in a single layer by the NEC [Section392.9(A)(3)Tying down these cables is one way to insure this requirement.
Where single conductor cables are installed it is highly desirable to tie the cables down to keep them in the tray.
There are other situations where tying down the cables is important. The selection of the type of cable tie is also very important. For further information, see CTI Technical Bulletin No. 5, Tie Down Practices for Multiconductor Cables in Cable Trays.

Question: Are Cable Trays listed?
Answer: Metallic cable trays are not required to be listed because they are a support system. Metal cable trays can be U.L. classified with regard to suitability for use as an Equipment Grounding Conductor. Compliance with other appropriate NEC cable articles is required. CTI recommends compliance with National Electrical Manufacturers, NEMA, Standards Publications Nos. VE1 and VE2, and the manufacturer’s recommendations.

Question: Are there cable fill requirements for cable trays?
Answer: Yes — NEC Sections 392.9, .10, .11 and .12, and Tables 392.9, 392.9(F)) and392.10(A), describe the fill in terms of area and cable diameters. The key issue is ampacity. The ampacity criteria in article 392 is based on not exceeding these fill values. The number and type of conductors that can be installed in a cable tray is also limited by the weight of the cables and other load factors for the cable tray for a given load rated cable tray. See NEMA VE-1 and manufacturer’s data. Size the width of cable tray and the load rating for expansion and additions. Adding six inches to the width of a tray increases its price by approximately 10%.

Question: Can high voltage cables be installed in cable trays?
Answer: Yes — NEC permits type MC (Article 330) and type MV (Article 328) in industrial establishments where qualified persons will service the installation. Multiconductor cables rated over 600 volts shall be separated from lower voltage cables by a separate cable tray or a solid fixed barrier. Type MC cables can be mixed with lower voltage cables. See NEC 392.6(F)
General

Question: Can a person walk on an installed Cable Tray System?
Answer: No; walking on cable trays is not to be permitted. It violates the new version of NEMA standard VE-2, manufacturers marking and recommendations, and the intent of the NFPA70 Electrical Safety in Employee Work Practices. Walking on electrical equipment, conduits, cables or other electrical systems should also be avoided. In addition to the fall hazard, there is the risk of damage to equipment and possible contact with conductors.

Center of Gravity

Center of Gravity (cog) satu parameter yang tidak penting dalam Instrument design. Namun begitu ia merupakan parameter penting bagi Structural Dpt. Structural Dpt perlu mendapat maklumat dari Instrument Dpt berkenaan dengan COG bagi Instrument.Jadi sebagai seorang Instrument designer yg berkualiti dan faham bidang disiplinya, beliau bertanggungjawab untuk memberikan maklumat kepada Structural Dpt.

Berikut sedikit maklumat berkenaan dengan cog

http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html

The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object. We can completely describe the motion of any object through space in terms of the translation of the center of gravity of the object from one place to another, and the rotation of the object about its center of gravity if it is free to rotate. If the object is confined to rotate about some other point, like a hinge, we can still describe its motion. In flight, both airplanes and rockets rotate about their centers of gravity. A kite, on the other hand, rotates about the bridle point. But the trim of a kite still depends on the location of the center of gravity relative to the bridle point, because for every object the weight always acts through the center of gravity.

Determining the center of gravity is very important for any flying object. How do engineers determine the location of the center of gravity for an aircraft which they are designing?
In general, determining the center of gravity (cg) is a complicated procedure because the mass (and weight) may not be uniformly distributed throughout the object. The general case requires the use of calculus which we will discuss at the bottom of this page. If the mass is uniformly distributed, the problem is greatly simplified. If the object has a line (or plane) of symmetry, the cg lies on the line of symmetry. For a solid block of uniform material, the center of gravity is simply at the average location of the physical dimensions. (For a rectangular block, 50 X 20 X 10, the center of gravity is at the point (25,10, 5) ). For a triangle of height h, the cg is at h/3, and for a semi-circle of radius r, the cg is at (4*r/(3*pi)) where pi is ratio of the circumference of the circle to the diameter. There are tables of the location of the center of gravity for many simple shapes in math and science books. The tables were generated by using the equation from calculus shown on the slide.

For a general shaped object, there is a simple mechanical way to determine the center of gravity:
If we just balance the object using a string or an edge, the point at which the object is balanced is the center of gravity. (Just like balancing a pencil on your finger!)
Another, more complicated way, is a two step method shown on the slide. In Step 1, you hang the object from any point and you drop a weighted string from the same point. Draw a line on the object along the string. For Step 2, repeat the procedure from another point on the object You now have two lines drawn on the object which intersect. The center of gravity is the point where the lines intersect. This procedure works well for irregularly shaped objects that are hard to balance.

If the mass of the object is not uniformly distributed, we must use calculus to determine center of gravity. We will use the symbol S dw to denote the integration of a continuous function with respect to weight. Then the center of gravity can be determined from:

cg * W = S x dw

where x is the distance from a reference line, dw is an increment of weight, and W is the total weight of the object. To evaluate the right side, we have to determine how the weight varies geometrically. From the weight equation, we know that:

w = m * g

where m is the mass of the object, and g is the gravitational constant. In turn, the mass m of any object is equal to the density, rho, of the object times the volume, V:

m = rho * V

We can combine the last two equations:

w = g * rho * V

then
dw = g * rho * dV

dw = g * rho(x,y,z) * dx dy dz

If we have a functional form for the mass distribution, we can solve the equation for the center of gravity:

cg * W = g * SSS x * rho(x,y,z) dx dy dz

where SSS indicates a triple integral over dx. dy. and dz. If we don't know the functional form of the mass distribution, we can numerically integrate the equation using a spreadsheet. Divide the distance into a number of small volume segments and determining the average value of the weight/volume (density times gravity) over that small segment. Taking the sum of the average value of the weight/volume times the distance times the volume segment divided by the weight will produce the center of gravity.

COMPARISON OF GRP/FRP CABLE TRAYS V/S METAL CABLE TRAYS

1.Corrosion Resistance
GRP/FRP - Excellent corrosion resistance against sea water and most chemical fumes

METAL - Poor corrosion resistance, pitting takes place even in SS or aluminum in sea water. Galvanic Corrosion takes place between Stainless Steel trays and MS supports

2.Hot Working
GRP/FRP -No Hot working, all assembly by Nut & Bolts

METAL -Hot working , welding cutting and joining requires special permission in hazardous areas

3.Light weight
GRP/FRP -Sp.gr. 1.9, 1/4th that of steel, even lighter than Aluminum(sp.gr.2.8), just one person required to lift a big size cable ladder, so very easy and fast Installation, saving time and money

METAL -Very heavy, 4 times to GRP/FRP, hence crane or 3 people required to lift a cable ladder

4.Fire Retardant
GRP/FRP -Meets most stringent offshore fire resistance norms as per British, ASTM and UL specifications

METAL -In heavy fire even steel de-shapes and needs replacement

5.Installation Cost
GRP/FRP -Very low, as lighter in wt hence only one man can lift, and very easy to cut and fabricate at site, thus faster installation and easy site adjustment and modifications

METAL -HIGH, needs 2-3 persons or special equipment to lift, also difficult to cut and refabricate as per site requirement as cutting takes time.

6.Risk of cable damage
GRP/FRP -Very low, as being plastics have much less hardness and even its sharp edges cannot damage a cable

METAL -When any site modification is done, sharp edges are created in metal which can cause damage to cables and pose risk of current.

7.High insulation ands Safe
GRP/FRP -No earthing of cable tray is required as it has high Insulation value. In case of any cable stripping, the tray being Insulated is safe for the Humans.

METAL -Earthing is must, hence cost of earthing increases overall cost, which is not generally considered when evaluating.

8.Part consolidation
GRP/FRP -As GRP/FRP is extruded section, inbuilt ribs for reinforcement, collars for fixing covers are possible in single section

METAL -This is not possible in metal, and if welding is done it looks very bad.

9.U.V. resistance
GRP/FRP -All GRP/FRP cable Trays are made from very high U.V. additives, in addition to special surfacing Veils for glass blooming prevention, and carbon black for additional U.V protection

METAL -N/A

10.Antistatic
GRP/FRP - Cable trays are available in antistatic option as well for oil hazardous areas on demand, thus safe even in hydrocarbon atmosphere.

METAL -N/A

11.First Time COST
GRP/FRP - FRP/GRP cable trays are competitive to SS trays with all above advantages

METAL -SS trays are very costly compared to GRP/FRP Trays

FRP or GRP Cable Ladder/Tray

General
Known as glass-reinforced plastic (GRP) in Britain, fibre-reinforced plastic (FRP) in the USA, or by the trade name fibreglass (after the manufacturing company Fibreglass Ltd.), GRP has been used for a wide range of applications from car body panels and boat hulls to furniture and tennis rackets. It has the virtue of a good weight to strength ratio, rust resistance, and ability to be moulded in a wide variety of ways. It became increasingly widely used in the post-Second World War period, a pioneering design being the celebrated DAR Armchair by Charles and Ray Eames for the 1948 Low-Cost Furniture Design Competition at the Museum of Modern Art in New York. Very much paralleled by the organic forms found in much contemporary product, train, and automobile design in Italy, the flowing, sculptural form of the seat (supported on a metal frame) expressed the creative possibilities of the new medium. These were realized in subsequent designs such as Eero Saarinen's elegant Tulip armchair of 1956. Verner Panton was another designer to explore the expressive qualities of the medium in his moulded, cantilevered chair of 1960 first manufactured in West Germany. Many furniture designs first manufactured in GRP have subsequently been manufactured in ABS plastic. Early use of GRP in automobile manufacture included the roof of the Citroen DS (1955) and the body panels of the Chevrolet Corvette (1953). From the 1970s improved production processes engendered more widespread uses in architecture and interior design, whether in terms of weather resistant details and services or bathrooms.





Definition of FRP Composites


Not all plastics are composites. In fact, the majority of plastics today are pure plastic, like toys and soda bottles. When additional strength is needed, many types of plastics can be reinforced (usually with reinforcing fibers). This combination of plastic and reinforcement can produce some of the strongest materials for their weight that technology has ever developed...and the most versatile.

Therefore, the definition of a fiber-reinforced polymer (FRP) composite is:
A combination of

- a polymer (plastic) matrix (either a thermoplastic or thermoset resin, such as polyester, isopolyester, vinyl ester, epoxy, phenolic)

- a reinforcing agent such as glass, carbon, aramid or other reinforcing material


such that there is a sufficient aspect ratio (length to thickness) to provide a discernable reinforcing function in one or more directions. FRP composite may also contain:

- fillers

- additives

- core materials


that modify and enhance the final product. The constituent elements in a composite retain their identities (they do not dissolve or merge completely into each other) while acting in concert to provide a host of benefits ideal for structural applications including:

High Strength and Stiffness Retention - composites can be designed to provide a wide range of mechanical properties including tensile, flexural, impact and compressive strengths. And, unlike traditional materials, composites can have their strengths oriented to meet specific design requirements of an application.

-Light Weight/Parts Consolidation - FRP composites deliver more strength per unit of weight than most metals. In fact, FRP composites are generally 1/5th the weight of steel. The composite can also be shaped into one complex part, often times replacing assemblies of several parts and fasteners. The combination of these two benefits makes FRP composites a powerful material system- structures can be partially or completely pre-fabricated at the manufacturer's facility, delivered on-site and installed in hours.

-Creep (Permanent Deflection Under Long Term Loading) - The addition of the reinforcement to the polymer matrix increases the creep resistance of the properly designed FRP part. Creep will not be a significant issue if the loads on the structure are kept below appropriate working stress levels.

-Resistance to Environmental Factors - Composites display excellent resistance to the corrosive effects of:
-Freeze-thaw: because composites are not attacked by galvanic corrosion and have low water absorption, they resist the destructive expansion of freezing water.

-Weathering and Ultra-Violet Light: FRP composite structures designed for weather exposure are normally fabricated with a surface layer containing a pigmented gel coat or have an ultraviolet (UV) inhibitor included as an additive to the composite matrix. Both methods provide protection to the underlying material by screening out UV rays and minimizing water absorption along the fiber/resin interface.

-Chemicals and Temperature: Composites do not rust or corrode and can be formulated to provide long-term resistance to nearly every chemical and temperature environment. Of particular benefit, is composites ability to successfully withstand the normally destructive effects of de-icing salts and/or saltwater spray of the ocean.



-Fire Performance of Composites - FRP composites can burn under certain conditions. Composites can be designed to meet the most stringent fire regulations by the use of special resins and additives. Properly designed and formulated composites can offer fire performance approaching that of most metals.

Cable Block Diagram

Cable Block merupakan satu cara untuk menerangkan hubungkait keseluruhan cable yang terdapat dalam Process Plant.
Ia perlu dihasilkan dengan penuh tanggungjawab bagi memastikan ia dapat difahami dan diterjemahkan dengan mudah oleh setiap orang yang melihatnya.

Ia merupakan pemudah cara untuk menyemak setiap cable yang terdapat dalam sistem kawalan (DCS/IPS/FGS, dsbnya) , antara fakta yg perlu ada ialah:
· Cable Type
· Cable Size
· Originate
· Destination
· Maklumat goegrafi yang jelas

Bagaimana hendak menghasilkan Cable block Diagram:
· Perlu memahami system secara keseluruhan
· Perlu memahami jenis signal yang digunakan
· Perlu tahu jenis cable yang hendak digunakan
· Perlu tahu destinasi signal dan punca signal
· Perlu tahu kedudukkan setiap komponen dengan jelas

Installation Detail/Hook-up

Installation Detail/Hook-up

Installation Detail atau sketch perlu memaparkan apa yang diperlukan untuk memasang sesuatu instrument.

Ini termasuk kesemua bahan (material) yang yang diperlukan untuk melengkapkan satu proses pemasangan (installation).
Maklumat dari pihak vendor adalah amat perlu bagi memastikan segala bahan yang diperlukan adalah sesuai dan sepadan dengan saiz connection yang diperlukan oleh instrument.

Designer jga perlu mengandaikan situasi dimana Tukang Pasang tidak biasa atau pertama kali memasang Instrument tersebut.

Location Plan/Plot Plan

Location Plan/Plot Plan
Instrument Location plan memasukan semua Instrument yang berhubungkait dengan instrument lain.
Selalunya terbahagi kepada dua fungsi iaitu:
1. Untuk kegunaan pemasangan (Installation) Instrument dan pemasangan tiub untuk Process connection dan tiub untuk signal pneumatik. Ini selalunya melibatkan kerja-kerja paip dan tiub (pipe fitter). Jadi rajah Location Plan dan Tube routing layout perlu dihasilkan.
2. Untuk kegunaan pendawaian. Ini melibatkan kerja-kerja perancangan utk laluan kabel dan lokasi kotak pencawang (JB). Jadi rajah Cable Tray/Ladder perlu dihasilkan untuk panduan pemasangan Cable Ladder/Tray. Manakala rajah Cable Routing pula diperlukan sebagai panduan kerja pendawaian dilakukan dengan cekap.

Rajah dasar (back ground) biasanya diambil dari rajah Piping atau Equipment Layout.

Wire & Cable Cross reference

BS5308 Cables

---

BS5308 Part 1 / Type 1 Unarmoured Construction

1. Plain annealed copper wire conductors to BS 6360

2. Polyethene insulation to BS 6234

3 Individual pair screen (optional):-
a) Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire
b) Polyester isolating tape(s) numbered for identification

4 Polyester binder tape

5 Collective screen (optional) - Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire

6 PVC sheath to BS 7655

---

BS5308 Part 1 / Type 2 Armoured Construction

1 Plain annealed copper wire conductors to BS 6360

2 Polyethene insulation to BS 6234

3 Individual pair screen (optional):-
a) Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire
b) Polyester isolating tape(s) numbered for identification

4 Polyester binder tape

5 Collective screen (optional) - Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire

6 Black polyethylene bedding to BS 6234

7 Single layer galvanised steel wire armour to BS EN 10257-1

8 PVC sheath to BS 7655

---

BS5308 Part 2 / Type 1 (unarmoured)

1 Plain annealed copper wire conductors to BS 6360

2 PVC to BS 7655

3 Individual pair screen (optional):-
a) Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire
b) Polyester isolating tape(s) numbered for identification

4 Polyester binder tape

5 Collective screen (optional) - Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire

6 PVC sheath to BS 7655

---

BS5308 Part 2 / Type 2 (armoured)

1 Plain annealed copper wire conductors to BS 6360

2 PVC to BS 7655

3 Individual pair screen (optional):-
a) Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire
b) Polyester isolating tape(s) numbered for identification

4 Polyester binder tape

5 Collective screen (optional) - Aluminium/polyester tape, metallic side down, in contact with minimum 0.5mm2 tinned copper drain wire

6 PVC bedding to BS 7655

7 Single layer galvanised steel wire armour to BS EN 10257-1

8 PVC sheath to BS 7655

How Fiber Optics Work

How Fiber Optics Work

by Craig C. Freudenrich, Ph.D.
http://electronics.howstuffworks.com/fiber-optic.htm

1.Introduction to How Fiber Optics Work

You hear about fiber-optic cables whenever people talk about the telephone system, the cable TV system or the Internet. Fiber-optic lines are strands of optically pure glass as thin as a human hair that carry digital information over long distances. They are also used in medical imaging and mechanical engineering inspection.

In this article, we will show you how these tiny strands of glass transmit light and the fascinating way that these strands are made.

2.What are Fiber Optics?

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.

Parts of a single optical fiber

If you look closely at a single optical fiber, you will see that it has the following parts:

• Core - Thin glass center of the fiber where the light travels
• Cladding - Outer optical material surrounding the core that reflects the light back into the core
• Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket.

Optical fibers come in two types:

• Single-mode fibers
• Multi-mode fibers

See Tpub.com: Mode Theory for a good explanation.

Single-mode fibers have small cores (about 3.5 x 10^-4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers have larger cores (about 2.5 x 10^-3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).

Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.

3.How Does an Optical Fiber Transmit Light?

Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway -- light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.

Diagram of total internal reflection in an optical fiber

The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

4. A Fiber-Optic Relay System

To understand how optical fibers are used in communications systems, let's look at an example from a World War II movie or documentary where two naval ships in a fleet need to communicate with each other while maintaining radio silence or on stormy seas. One ship pulls up alongside the other. The captain of one ship sends a message to a sailor on deck. The sailor translates the message into Morse code (dots and dashes) and uses a signal light (floodlight with a venetian blind type shutter on it) to send the message to the other ship. A sailor on the deck of the other ship sees the Morse code message, decodes it into English and sends the message up to the captain.

Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have a fiber-optic communication system in place between the two ships. Fiber-optic relay systems consist of the following:

• Transmitter - Produces and encodes the light signals
• Optical fiber - Conducts the light signals over a distance
• Optical regenerator - May be necessary to boost the light signal (for long distances)
• Optical receiver - Receives and decodes the light signals

Transmitter
The transmitter is like the sailor on the deck of the sending ship. It receives and directs the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a light signal.

The transmitter is physically close to the optical fiber and may even have a lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).

Optical Regenerator
As mentioned above, some signal loss occurs when the light is transmitted through the fiber, especially over long distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical regenerators is spliced along the cable to boost the degraded light signals.

An optical regenerator consists of optical fibers with a special coating (doping). The doped portion is "pumped" with a laser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal with the same characteristics as the incoming weak light signal. Basically, the regenerator is a laser amplifier for the incoming signal. See Photonics.com: Fiber Amplifiers for more details.

Optical Receiver
The optical receiver is like the sailor on the deck of the receiving ship. It takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's computer, TV or telephone (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light.

5. Advantages of Fiber Optics

Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibers are:

• Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cable TV, Internet) and you money.

• Thinner - Optical fibers can be drawn to smaller diameters than copper wire.

• Higher carrying capacity - Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.

• Less signal degradation - The loss of signal in optical fiber is less than in copper wire.

• Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.

• Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.

• Digital signals - Optical fibers are ideally suited for carrying digital information, which is especially useful in computer networks.

• Non-flammable - Because no electricity is passed through optical fibers, there is no fire hazard.

• Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber-optic cables take up less space in the ground.

• Flexible - Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes:
  • Medical imaging - in bronchoscopes, endoscopes, laparoscopes
  • Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)
  • Plumbing - to inspect sewer lines

Because of these advantages, you see fiber optics in many industries, most notably telecommunications and computer networks. For example, if you telephone Europe from the United States (or vice versa) and the signal is bounced off a communications satellite, you often hear an echo on the line. But with transatlantic fiber-optic cables, you have a direct connection with no echoes.

6.How Are Optical Fibers Made?

Now that we know how fiber-optic systems work and why they are useful -- how do they make them? Optical fibers are made of extremely pure optical glass. We think of a glass window as transparent, but the thicker the glass gets, the less transparent it becomes due to impurities in the glass. However, the glass in an optical fiber has far fewer impurities than window-pane glass. One company's description of the quality of glass is as follows: If you were on top of an ocean that is miles of solid core optical fiber glass, you could see the bottom clearly.

Making optical fibers requires the following steps:

1. Making a preform glass cylinder
2. Drawing the fibers from the preform
3. Testing the fibers

Making the Preform Blank
The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).

Image courtesy Fibercore Ltd. MCVD process for making the preform blank

In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from the torch causes two things to happen:

Photo courtesy Fibercore Ltd. Lathe used in preparing the preform blank

• The silicon and germanium react with oxygen, forming silicon dioxide (SiO2) and germanium dioxide (GeO2).

• The silicon dioxide and germanium dioxide deposit on the inside of the tube and fuse together to form glass.

The lathe turns continuously to make an even coating and consistent blank. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control (index of refraction).

Drawing Fibers from the Preform Blank
Once the preform blank has been tested, it gets loaded into a fiber drawing tower.

Diagram of a fiber drawing tower used to draw optical glass fibers from a preform blank

The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity. As it drops, it cools and forms a thread.

The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mechanism. Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (2.2 km) of optical fiber.

Testing the Finished Optical Fiber

Photo courtesy Corning Finished spool of optical fiber

The finished optical fiber is tested for the following:

• Tensile strength - Must withstand 100,000 lb/in² or more

• Refractive index profile - Determine numerical aperture as well as screen for optical defects

• Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform

• Attenuation - Determine the extent that light signals of various wavelengths degrade over distance

• Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-mode fibers)

• Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)

• Operating temperature/humidity range

• Temperature dependence of attenuation

• Ability to conduct light underwater - Important for undersea cables

Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.

7. Physics of Total Internal Reflection

When light passes from a medium with one index of refraction (m1) to another medium with a lower index of refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through m2 bends further away from the line.

At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the surface between the two media (sine [critical angle] = n2/n1 where n1 and n2 are the indices of refraction [n1 is greater than n2]). If the beam through m1 is greater than the critical angle, then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may be transparent!

In physics, the critical angle is described with respect to the normal line. In fiber optics, the critical angle is described with respect to the parallel axis running down the middle of the fiber. Therefore, the fiber-optic critical angle = (90 degrees - physics critical angle).

Total internal reflection in an optical fiber

In an optical fiber, the light travels through the core (m1, high index of refraction) by constantly reflecting from the cladding (m2, lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle the fiber itself gets bent at, even if it's a full circle!

Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

Cable Tray

Cable Tray

A cable tray system, according to the US National Electrical Code, is "a unit or assembly of units or sections and associated fittings forming a rigid structural system used to securely fasten or support cables and raceways." Cable trays are used to hold up and distribute cables.

Types

• Ladder
• Solid Bottom
• Trough
• Channel
• Wire Mesh
• Single Rail

Materials used

• Steel
• Aluminium
• Plastic
• Fibreglass reinforced plastic

The choice of materials is a matter of the physical and mechanical properties produced by each, compared against the intended function, as well as the environment, in which the trays are to be installed.