Reference designators for electrical and electronics parts and equipment
Appendix B: Class Designation Letters
Clause 0, for use in assignment of reference designations for electrical and electronics parts and equipment.
Clause 0. Class Designation Letters
For use in assignment of reference designations for electrical and electronics parts and equipment as described in ANSI/ASME Y14.44, Reference Designations for Electrical and Electronics Parts and Equipment.
Clause 0.1 Class Designation Letter
The letters identifying the class of an item shall be selected in accordance with the list in clause 0.4.
Certain item names and designating letters may apply to either a part or an assembly.
Clause 0.2 Special Considerations for Class Designation Letter Assignment
Clause 0.2.1 Actual versus intended function
If a part serves a purpose other than its generally intended one, the function actually performed shall be represented by the graphic symbol used on the schematic diagram; the class letter shall be chosen from the list in clause 0.4 and shall be indicative of its physical characteristics. For example, a semiconductor diode used as a fuse would be represented by the graphic symbol for a fuse (actual function), but the class letter would be D (class of part). If a part has a dual function, the class letter for the principal physical characteristic of the part shall apply.
Clause 0.2.2 Assembly versus subassembly
The term subassembly as used herein shall apply equally to an assembly.
Clause 0.2.3 Subassembly versus individual part
A group of parts shall not be treated as a subassembly unless it is one or more of the following:
a) A plug-in item.
b) A significant item covered by a separate schematic.
c) A multiapplication item.
d) Likely to be handled as a replaceable item for maintenance purposes.
Clause 0.2.4 Specific versus general
The letters A and U (for assembly) shall not be used if more specific class letters are listed in clause 0.4 for a particular item.
Clause 0.2.5 Inseparable subassemblies
Potted, embedded, riveted, or hermetically sealed subassemblies, modular assemblies, printed circuit boards, and IC packages and similar items which are ordinarily replaced as a single item of supply shall be treated as parts. They shall be assigned the class letter U, unless a more specific class letter is applicable.
Clause 0.4 Class Designation Letters: Alphabetical List
Parts not specifically included in this list shall be assigned a letter or letters from the list below for the part or class most similar in function.
A (see also U and 0.2.4)
amplifier (other than rotating)
D or CR
D or VR
breakdown diode (voltage regulator)
alphanumeric display device
current limiter (for power cable)
fiducial. An SMT optical target for automatic placement of part
hardware (common fasteners, etc)
disconnecting device (receptacle connector)
contactor (magnetically operated)
coil (all not classified as transformers)
converter (rotating machine)
miscellaneous [tape, chemicals, RTV, labels, etc.]
non-plated through hole
disconnecting device (plug connector)
plated through hole
head (with various modifiers)
semiconductor controlled rectifier
reference [usually to a document or DWG]
contactor (manually, mechanically, or thermally operated)
solder jumper pads [SMT]
electrical contact ring
U (see also A and 0.2.4)
VR (see also D)
induction voltage regulator
artificial line (other than delay line)
 The class letter A is assigned on the basis that the item is separable. The class letter U shall be used if the item is inseparable.
 For economic reasons, assemblies which are fundamentally separable may not be so provisioned but may be supplied as complete assemblies. However, the class letter A shall be retained.
 Not a class letter, but used to identify a subdivision of an equipment in the Location Numbering Method.
 Not a class letter, but commonly used to designate test points for maintenance purposes.
 Not a class letter, but commonly used to designate a tiepoint on connection diagrams.
 This class letter is added.
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1. Reference designators for electrical and electronics parts and equipment
2. Appendix A: A list of pertinent applicable standards
3. Appendix B: Class Designation Letters
4. Appendix C: List of nonconforming class letters
5. Appendix D: System subdivision
6. Appendix E: KiCad Anomalous Handling of Suffix Letters.
7. Appendix F: Scenario Questions and Answers.