A coach or locomotive harness can carry hundreds of circuits through several branches. If the marking is missing, wrong or worn off, fitters probe wires one by one, repairs go on the wrong circuit and nobody can trace a failed batch. Marking is cheap at the time of manufacture and expensive afterwards, so it belongs in the drawing and the purchase specification from the start.
Three levels of marking
| Level | What to mark | Purpose |
|---|---|---|
| Wire | Cable grade, size, voltage and applicable standard, printed along the insulation | Shows on inspection that the right cable was used. APAR rail cables carry this printed identification. |
| Circuit | Circuit number at both ends of each wire, matching the wire list | Lets a fitter match a wire to the diagram without probing. |
| Branch and connector | Branch name or number, connector reference and cavity numbers | Tells the installer which leg goes to which equipment. |
| Harness | Part number, drawing revision, batch or date code, supplier reference | Gives traceability for repair, warranty and corrective action. |
Rules that keep labels readable
- Mark both ends. A number at one end only is half the information.
- Place the label where it can be read after installation. Near the connector, outside any overmould or tape wrap, not on the underside of a bend.
- Match the label to the environment. Heat, oil, cleaning chemicals and vibration all destroy cheap labels. Print or sleeve materials should be compatible with the harness sleeving and temperature class.
- Use one numbering system. The circuit number on the wire, the wire list and the schematic must be identical strings.
- Record the revision. A harness built to revision C and a diagram at revision D is a repair waiting to go wrong.
What to specify in the drawing
State the marking method, the character height where the application needs one, the label positions, and the information required on the harness label. If the buyer needs batch records, say which: material lot, crimp tool, test result. Without an instruction, only the minimum is supplied.
Marking and testing together
A continuity tester programmed from the wire list proves that each circuit joins the right two points. The label proves the same thing to a person on site. See the wiring harness testing guide for the tests, and the harness RFQ checklist for the data a supplier needs to apply the marking you specify.
APAR builds harnesses to customer drawings for automotive, EV and rail use. See the automotive and EV harness page and the railway coach harness page.