2016 Part 14.2.15 Recommended Design Criteria for Resistors Revised 2016 (6 Pages) A. Purpose z00381hn 3/12/15 8:34 AM z00381hn 3/12/15 8:37 AM This Manual Part recommends design criteria for resistors suitable for railway signaling circuits other than electronic. B. Design 1. Resistance element should be one of the following general types for operation in free still air: a. Wire wound, encased in molded phenolic should conform to ANSI/EIA Standard EIA-155B Fixed Wirewound Power Resistors. (1) Cylindrical. (2) Flat. b. Carbon film or rod encased in molded phenolic should be in accordance with EIA-196A Fixed Film Resistors - Precision and Semi-precision or EIA-172B Fixed Composition Resistors. (1) Cylindrical. c. Open coil on porcelain base with 14-24 binding posts. d. Wire wound with vitreous enamel coating should conform to EIA- 155B. (1) Cylindrical ceramic tube. (2) Flat ceramic tube on metal plate supports. e. Wire wound, cement coated on ceramic tube should conform to EIA-155B. (1) Cylindrical. (2) Flat. f. Vitreous enamel grid should conform to EIA-155B. g. Wire wound, metal clad should conform to EIA-155B. z00381hn 3/12/15 8:43 AM Deleted: Electronic Industries Alliance - 1
Part 14.2.15 2016 2. Nominal values for resistors specified in Section B.1.a.(1) or B.1.b shall conform to one of the following: EIA-155B, EIA-172B or EIA-196A. z00381hn 3/12/15 8:42 AM 3. Metal parts for mounting, when required, should be protected against corrosion. Zinc or cadmium plating shall not be used on parts carrying current. 4. Open coil windings using bare wire should be so designed and assembled that adjacent turns will be uniformly spaced. Expansion of the turns should not make contact between adjacent turns or other parts associated with the mounting of the assembly. C. Temperature 1. Maximum temperature for all types should be based on ambient temperature of +104 F (+40 C) unless other value is stated by purchaser. 2. Test should be made by continuous operation at the required current stated by purchaser in free air, shielded from draft, with a minimum of 1 ft (305 mm) clearance to the nearest object, other than the mounting. 3. Temperature test should be made on resistance element only or on assembly complete with mounting if so furnished. 4. Temperature should be measured by thermocouple in contact with the hottest portion of: a. The resistive conductor on open coil types. b. The surface of the coating on phenolic, vitreous enamel, cement coated or metal-clad type. 5. The final temperature, under conditions of maximum ambient and continuous load at current stated should not exceed the following for the various types in Section B.1: a. Phenolic molded, wire wound or metalized, +194 F (+90 C). b. Open coil on porcelain base, +235 F (+113 C). c. Vitreous enamel on ceramic tube, +400 F (+204 C). d. Cement coated on ceramic tube, +400 F (+204 C). - 2
2016 Part 14.2.15 e. Vitreous enamel grid, +400 F (+204 C). z00381hn 3/12/15 8:34 AM D. Terminals f. Metal clad, +400 F (+204 C). 1. Cylindrical phenolic type resistors should have axial terminal wire leads 1-3/8 in (35 mm) or more in length, of oxygen free copper, tinned, solid wire not smaller than No. 21 AWG (0.41 mm 2 ). Each terminal should withstand bending through 90 at point 1/4 in (6.35 mm) from the body of the resistor and rotated through 360 in alternating directions for three rotations at the rate of approximately 5 s per rotation. 2. Flat phenolic type resistors should have tinned soldering lugs, unless otherwise specified. 3. Vitreous enamel, cement coated and metal clad resistors should have terminals as follows: a. Soldering lugs, when used, should be fully tinned. b. Flexible leads, when used, shall consist of not less than 7 strands of 0.0159 in (0.404 mm) copper alloy wire, twisted and suitably protected against corrosion. c. When brackets are used they should be of metal conforming to Manual Part 15.1.5 Recommended Developmental Criteria for Various Types of Non-Ferrous Metals and Alloys. One bracket should be slotted perpendicular to the axis of the tube and the other shall be slotted parallel to the axis of the tube suitable for mounting on 14-24 binding posts. d. Flexible leads should be suitably brazed to brackets. 4. Terminal wires shall not be used on open coil type resistors; the coil ends should be clamped securely between two clamp nuts with suitable washers or under heads of binding posts or soldered or brazed to terminals made of copper or metal conforming to Manual Part 15.1.5. 5. Grid type vitreous enamel resistors should have suitable terminal screws or tinned soldering lugs for fastening circuit wires. - 3
Part 14.2.15 2016 E. Mounting z00381hn 3/12/15 8:42 AM 1. Open coil elements should be assembled on dry process porcelain. 2. Cores for vitreous, metal clad or cement coated type resistors should be porcelain or equivalent ceramic having electrical characteristics similar to good quality dry process porcelain. F. Adjustable Resistors Adjustment of resistance, when required, should be provided by taps or slide as follows: 1. Taps 2. Slide a. Each resistance element of open coil resistors should be clamped securely between two clamp nuts with suitable washers or under the head of 14-24 binding posts, or soldered or brazed to terminals made of copper or metal conforming to Manual Part 15.1.5 Recommended Developmental Criteria for Various Types of Non- Ferrous Metals and Alloys. b. Types other than open coil should have terminal lugs or wire leads located as specified, and when mounted should have the taps connected to 14-24 binding posts. Flexible connections should be of suitable cross-section and insulated to prevent contact with other leads or metal parts. a. The resistance element on open coil resistors should have each turn of the winding equally spaced and retained in position by grooves in the porcelain body. b. Types other than open coil should have a portion of the coating removed to expose a track for the operation of the slider along the bare wire embedded in the enamel or cement. c. Slider should make positive continuous contact with the turns of the winding and be designed for fastening securely in the desired position. Ernie 4/8/15 2:55 PM Formatted: Indent: Left: 1", Hanging: 0.5" Ernie 4/8/15 2:55 PM Deleted:. - 4
2016 Part 14.2.15 G. Binding Posts and Terminal Blocks z00381hn 3/12/15 8:34 AM 1. When binding posts are used, they should be spaced no less than on 1 in (25.4 mm) centers. 2. Binding posts for external wires should be located adjacent to wire inlet and duct spaces provided. 3. Binding posts, nuts, and washers shall conform to Manual Part 14.1.11 Recommended Design Criteria for Binding Posts, Nuts & Washers, Details & Assemblies. Ernie 4/23/15 4:35 PM Deleted:, Ernie 4/23/15 4:36 PM Deleted:, 4. Binding posts shall conform to Manual Part 14.1.12 Recommended Design Criteria for Standard Binding Posts. 5. Binding Posts shall be mounted so they cannot be turned in the base or frame to which applied. They shall be properly insulated from each other and other metallic parts. 6. Terminal Blocks shall conform to Manual Part 14.1.2 Recommended Design Criteria and Functional/Operating Guidelines for Solderless Screw- Type or Screwless-Type Spring Terminal Blocks Used in Wiring Signal Apparatus with Copper Wire Only. H. Terminal Boards Terminal boards, when required, should be made of high quality insulating material capable of withstanding the dielectric test specified in Section J. and should be dimensionally stable under the temperatures encountered in continuous operation at rated current of the resistance elements. I. Conductors 1. Wire for resistance elements for open coil type should be made of highgrade iron-free nickel-chrome or nickel-copper alloy. 2. Wire for resistance elements for covered type (phenolic, vitreous, cement or metal clad) should be made of high-grade nickel-chrome with maximum iron content of 25% or nickel-copper alloy. 3. Wire should be of uniform cross-section not smaller than No. 44 AWG (0.002 mm 2 ) plus or minus commercial tolerances, free from flaws, scales or other imperfections, without weld, splice or joint throughout its length between terminals. Ernie 4/8/15 2:56 PM Formatted: No bullets or numbering z00381hn 3/12/15 8:40 AM Deleted: Binding Posts... [1] - 5
Part 14.2.15 2016 4. Size of wire should be such as to afford not less than one-third coverage of the winding length of the core to insure largest possible diameter wire for a given unit. z00381hn 3/12/15 8:42 AM 5. Wire should be uniformly spaced and assembled so that there will be no damage to the coil that will result in shorts between windings. 6. Joints between wire and lead-in terminals should be securely made and in such manner as to prevent injury to the wire because of expansion. J. Environmental and Dielectric Recommendations 1. Resistors shall conform to Manual Part 11.5.1 Recommended Environmental Requirements for Electrical and Electronic Railroad Signal System Equipment. Test to be conducted between resistor and mounting bracket. 2. A surface leakage distance of not less than 1/4 in (6.35 mm) shall be provided between any exposed metallic part of the apparatus carrying current and any other metallic part hereof. 3. Separate windings, which are insulated from each other, should withstand for one minute an insulation test of 1,000 volts ac between their terminals. K. Identification Each resistor should be permanently marked in so far as practicable, with the following information: 1. Manufacturer's reference. 2. Resistance in ohms. On certain miniature insulated types, this may be shown by use of the Color Code for Resistors. 3. Resistance between taps. 4. Maximum continuous rating in amperes or watts (except on miniature resistor). - 6