4.2 Identification and Labeling

4.2.1 Label Types and Material Selection

The label format must match the wire diameter and the operating environment. A label that falls off or fades to white is a Latent Failure.

The industry standard for high-reliability (Class 3) aerospace and military applications.

Structure: Flattened, printable heat shrink tubing (Polyolefin or Fluoropolymer).
Advantages: Permanent, captive (cannot fall off), low profile, highly resistant to fluids and abrasion.
Application: Must be slid onto the wire before termination. It requires a specific shrink ratio (e.g., 3:1) to grip the wire firmly without splitting.

The most versatile solution for general industrial use.

Structure: A white printable zone followed by a clear, transparent tail.
Advantages: The clear tail wraps over the printed text, sealing it against oil, solvents, and abrasion. Can be applied after termination.
Mandate: The clear tail must overlap the printed area by at least 1.5 turns to ensure a permanent bond.

Used for thin wires (fiber optics, small gauge signal wires) where the diameter is too small to support legible text on a wrap-around label.

Structure: A "P-style" or "T-style" label that folds around the wire and sticks to itself, creating a flat surface for text.
Risk: Flags can be torn off during routing or bundle pulling. Avoid in high-abrasion or tight-routing areas.

Consistent placement allows technicians to identify connections instantly without hunting or twisting wires.

Standard Rule: Labels should be placed 15 mm to 50 mm (0.5 to 2.0 inches) behind the connector strain relief or backshell.
Visibility: The label must be visible without unplugging the connector. It should not be buried inside the backshell or hidden under the bundle tie-wraps.
Consistency: All labels on a breakout must be aligned at the same distance from their respective connectors for a professional visual standard.

"Read Like a Book": Text must be oriented to read from Left-to-Right or Bottom-to-Top.
Connector Reference: A common convention is that the top of the text points toward the connector. Whatever convention is chosen, it must be standardized across the entire facility.
Bundle alignment: On main trunks, labels should be aligned so they can be read from the primary service access point (e.g., the front of the rack).

Information density is high, but legibility is paramount.

Connector ID: (e.g., J1, P2) Matches the system schematic. Mandatory on all ends.
Harness ID: (e.g., Part Number & Revision) Typically placed on a "Master Label" on the main trunk.
Serialization (Traceability): A unique Serial Number (SN) or Date Code is required for Class 3 traceability (linked to the MES record).
2D Barcodes: For high-volume tracking, a Data Matrix code is preferred over 1D barcodes as it remains readable on curved surfaces.

Resolution: Minimum 300 dpi thermal transfer printing is required for barcodes and small text. Dot-matrix is generally obsolete for high-density labeling.
Durability (Rub Test): The ink must withstand the MIL-STD-202 Solvent Resistance Test. The print cannot smear or fade when rubbed with Isopropyl Alcohol (IPA) or the specific hydraulic fluid used in the application (e.g., Skydrol).
Defects: Smudged text, wrinkled lamination, or cut-off characters are causes for rejection.

The label must survive the same environment as the wire insulation.

Fluid Resistance: In automotive/aerospace, labels must resist fuels, oils, and cleaning solvents. Heat shrink markers are superior here.
Temperature: Standard vinyl labels fail at >80˚C. High-temperature applications (engines) require Polyimide (Kapton) or Fluoropolymer labels rated to 200˚C +.
UV Stability: For outdoor equipment, labels must be UV-stabilized to prevent yellowing and cracking.

Mandate	Criteria	Verification Action
Legibility	Text and barcodes must be crisp, with no smears or voids.	Visual check (1x) and barcode scanner verification.
Placement Consistency	Labels located 15–50 mm from the connector interface.	Ruler check on First Article; visual consistency on production.
Self-Lam Integrity	Clear tail must overlap the text by ≥ 1.5 turns.	Visual check: No "flagging" or lifting of the label edge.
Material Match	Label material rated for the operating temp and fluids.	BOM check: Ensure label spec matches the wire insulation rating (e.g., High-Temp label on PTFE wire).
Orientation	Text reads "Left-to-Right" pointing toward the connector.	Consistent orientation verified across all breakout legs.
Solvent Resistance	Ink does not smear under solvent wipe.	Rub Test performed during label stock/ribbon changeover.