Reference designators for electrical and electronics parts and equipment


Appendix D: System subdivision

Fig 7 from ASME Y14.44


A system subdivision diagram, or often called a product structure diagram, shows the makeup of a project or product.

Appendix D System Subdivision diagram comes from ANSI/ASME Y14.44, Figure 7. At the top of the diagram is the first level labeled “System designation”. Beneath the System designation level is the “Set designation” level. And below the Set designation level are the “Units” that make up the Set. Reference designation assignment begins below the set level, with the units. On my diagram I indicate the complete reference designation prefix at the upper left outside corner of the rectangle for that level.

An example is that Doppler Radar equipment that I worked on when assigned to the Intermediate Maintenance Department onboard the USS Hancock (CVA-19) aircraft carrier when I was in the U.S. Navy. This equipment was used to feed ground speed and drift angle information to an analog navigation computer in an aircraft.

For this piece of equipment the system subdivision diagram would be labeled something like this:

  • 1st level (System designation): Doppler Radar for Navigation
  • 2nd level (Set designation): AN/APN-153 (V) Doppler Radar
  • 3rd level (Units): Unit 1 Antenna Assembly, Unit 2 Control-Indicator, Unit 3 Receiver-Transmitter with Mount. The reference designation prefix for a unit is simply the number of the unit, such as 3.
  • 4th level: Assemblies that make up the Unit. The complete reference designation prefix for an assembly would be the unit number plus the reference designation of the assembly, such as 3A1.
  • 5th level: Subassemblies that make up the Assembly. The complete reference designation prefix would be the unit number plus the assembly reference designation prefix plus the subassembly reference designation prefix, such as 3A1A1. For this product the Receiver-Transmitter unit had a dozen printed board assemblies (PBAs), the military calls them circuit card assemblies (CCAs), and so there was 3A1A1 through 3A1A12.

And so on for how far down or how far across you need to go.

Things are simple for a single board project/product though. There is only one unit and so you drop the use of the reference designation prefix “1”.

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About the author

The articles in this series are written by Lawrence W. Joy (Larry)
[email protected]
Michigan USA.

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