A cost benefit analysis can help you determine when a wiring harness should and should not be used.
By Darrell Fernald
Most electronic boxes and equipment racks have in common a large number of interconnections. These interconnections run the gamut of signal wires (shielded and unshielded), power connections, coaxial cables, and ground bus wiring.
In the early stages of equipment development, the boxes and racks are often wired point-to-point to quickly prototype them to check out the electronics and the design. But the accurate wiring of any electronic system is critical to its performance. One loose or mis-wired connection will prevent the system from operating properly and can even severely damage the equipment.
Wiring point-to-point can be hit or miss, and the dress and routing of the wires are extremely operator-dependent. Repeatability of wiring in a point-to-point system is difficult, if not nearly impossible.
Wiring harnesses are often not recognized as the most consistent and cost effective method for wiring the equipment, but a properly laid out harness lends itself to automatic testing for continuity, correct wiring, insulation resistance and hi-pot. It is relatively straightforward to program an automatic tester to check for proper wiring and lack of shorts.
Harness types
Depending on the end use and the environment to which the harness is to be exposed, there are at lease three types of harness:
Open bundle. Wires are attached to connectors, terminal lugs, etc., and are tied into bundles with various breakouts by means of plastic tie wraps or waxed lacing twine. (Fig 1.)
 FIGURE 1. An open harness is most commonly used for internal wiring of units. |
Closed bundle. Wires are bundled with a covering, such as pulled-on braided tubing, braided-on Nomex or nylon, or in some cases, metal braid. (Fig 2.)
 FIGURE 2. This combined harness is an example of a harness that can be used to connect multiple units together in a benign environment. |
Waterproof harnesses. Legs are covered with tubing, such as neoprene. The junctions between the legs and backshells of the connectors are overmolded with a material such as urethane or rubber compounds. In some cases, PVC tubing and molding are used. (Fig. 3.)
Typical usage for open bundles is internal wiring in chassis or racks. Closed bundle harnesses normally will be used when hooking multiple chassis together, such as in a training system where the system does not experience harsh environments. The most common place to find a waterproof-type harness is in field-deployed military equipment or in industrial facilities where fluid, dirt and debris are present.
Braiding over wire harnesses is a very skill-intensive process. A harness is inserted one leg at a time through a hole in the base of a specially modified New England Butt Braider, and a metal or fabric braid is applied over each leg of the harness in turn. When braiding shielding over the harness, special care must be taken not to leave voids at the junction points of the breakouts. Any voids in the braid will cause EMI shielding effects to be adversely affected.
The base fixture for a wiring harness is known as a harness board. A drawing of the harness in a 1:1 scale is affixed to a substrate (usually a sheet of plywood), and special headless harness nails are driven into the board at specific locations to route the wires. Cut lengths and connector positions are detailed on the drawing. Tie positions (for tie wraps or lacing twine) are also marked. These manufacturing aids assure repeatability of the harness from one to the next.
Between automatic testing and harness board layouts, each harness will be virtually identical to all others made with the same board. This repeatability assures ease of assembly into the chassis.
Troubleshooting problems in systems that have wiring harnesses installed is significantly easier than in systems that were wired point-to-point. If the harness has been proven and a record of the automatic electrical test is available, the wiring harness can easily be eliminated as a potential problem leaving one major component of the system out of the troubleshooting tree.
When to specify a wire harness
Several factors should be considered when specifying a wiring harness, including:
- Is the quantity of expected items over their lifetime large enough to justify the cost of preparing the documentation necessary to specify the harness? (1:1 drawings, parts list, notes, etc.)
- Are there enough discrete interconnections (wires) in the harness to require harnessing?
- Will a harness aid in troubleshooting the unit?
- Can a harness be assembled into the unit without damaging either the harness or other parts of the unit?
- Does your time to market allow for the proper design of a harness for the first units?
- How much time savings will result from the application of a harness versus point-to-point wiring?
You can use a cost/benefits analysis to determine the desirability of using a wiring harness in any particular application.
The following example shows one straightforward method. (A score below 50 indicates that a harness is not desirable, while a score above 80 indicates that a harness should be used. Any score between 50 and 80 indicates that a harness is desirable, but not mandatory; this becomes a matter of cosmetics and personal preferences as well as past history with harnesses.)
Each of the six specification items listed earlier are assigned points as follows:
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Your decision to layout a harness for construction in a unit is better made as part of the design process, since it may well affect the final layout of the components in the unit.
Once a score is determined for the cost/benefit analysis and your decision is made, a few more design consideration should be weighed during the documentation/drawing process.
Design considerations
Harness drawings can be produced in either 2D or 3D format using CAD software; however, a 2D format lends itself better to harness construction since the 1:1 drawing is normally attached to the harness board as a template. A 3D format, while very explicit, does not lend itself to this application. There are, however, drawing packages that produce both 2D and 3D versions from the same native file.
Some of the basic considerations for harness design include component placement, wire runs, electrical considerations and space considerations. A checklist can be a handy way to determine if all of the considerations for your harness have been looked at. The following is a sample of the types of items that you should check and plan for in your design:
- Have I left enough room for the harness to run without interfering with other components and not contacting heat sources? A good rule of thumb is to calculate the wire bundle diameter (including any shielding, etc.) then multiply that diameter by 1.25 to assure ease of installation.
- Have I left the wires long enough at the ends to allow for service loops when installing the lugs, connectors, etc., that are the end terminations of the wire?
- Have I allowed a turn radius in the harness for installation around corners in the unit?
- Have I separated any wires (signal vs. power) that might cause crosstalk or interference during operation?
- Have I kept in mind the minimum bend radius for coaxial cables and fiberoptic cables in the harness?
- Is my grounding scheme consistent with best practices?
- For an open harness, have I specified tie points that are sufficient to keep the shape of the harness intact during handling and installation?
- Are all of the components (lugs, etc.) properly sized for the wires that are being used?
If these items are carefully considered, there is a much better chance that the harness will fit the intended unit and will be producible.
Once you have decided to design a harness and have done a layout keeping in mind the above considerations, a good practice is to manufacture one harness using your drawing and attempt to install the harness in a prototype unit.
If at first you don’t succeed...
Unless you are extremely lucky, your first attempt will not fit properly and you will need to “go back to the drawing board” to make adjustments. But once you have a design that works and lays in the unit easily, you should have a cosmetically pleasing, functional harness that will last for the life of the unit.
DARRELL FERNALD is business development manager at WPI Incorporated, and national president of the International Institute of Connectors and Interconnect Technology (IICIT), PO Box 20002, Sarasota, FL 34276; Tel: (941) 739-8984; Email: dfernald@iicit.org
