Harnesses – A System Approach

To design sophisticated harnesses for use in the defence environment calls for suppliers with an understanding and knowledge of the key application requirements of these industries and what it takes to operate successfully in such challenging markets. The importance of delivering high reliability and long-term performance is essential as is managing the cost budget.

At the center of any vehicle is network of electrical harnesses; the vehicles virtual electrical “backbone”. Over time, mission and system requirements may change to meet new threats, with upgrades to incorporate new technologies, but the basic vehicle remains constant. This creates a need for an electrical harness strategy that can evolve with each phase of the vehicles life cycle.

Adopting a system approach to harnesses allows the designers to ensure only compatible components are selected to operate within the expected operating environment. Typical examples of these are as follows,

  • Rugged applications with exposure to high temperatures and fluids.
  • Very high temperatures and fluid immersion.
  • Low-fire hazard materials for confined habitat.
  • Extreme temperatures and fluids and abrasion resistance.

Harness Special Materials

The advancements that have been made in harnessing materials now deliveries wires and cables that are smaller, lighter and tougher and meet the wide range of challenging conditions, temperatures and fluids referred to above.
A range of cable jacketing materials and adhesives have been specifically developed to provide a balance of properties to protect the harnesses mechanically, thermally and in fluids that would normally act as solvents. The concept of heat shrink and adhesive bonding between components achieves the ultimate in environmental sealing. This method does not impose loads and stress on the wiring or cable jacket materials, thereby achieving exceptional resistance to mechanical abuse, thermal shock and fluid immersion.

Harnesses A system approach

The importance of delivering high reliability and long-term performance is essential as is managing the cost budget.

Compact size and weight

Heat shrink components combined with multicore cables lead naturally to the most compact harness system for a given signal requirement. The best use of space and significantly weight savings are characteristic of this approach. Machined layed multiconductor cables combine the special benefits of,

  • Space saving from custom design and configurations.
  • Built-in ruggedness due to the controlled lay and mutual support of conductors.
  • A range of controlled and dependable screening levels by inclusion of optimised braids in the construction.

Strain Relief

The design of system components provides full mechanical protection and strain relief of the signal wires. Heat shrink moulded parts and connector adaptor components behave as a continuation of the cable jacket up to the connector. The design and choice of moulded part should also allow for wire loops, which are important in isolating connector pins from any strain and providing a reserve of wire for repair.

Ruggedised

A high degree of mechanical protection for the signal wires is inherent in multiconductor cables. In addition to high crush resistance due to the cables supported cross section, the near absence of relative movement between the “layed” conductors prevents cutting or fretting through of each others primary insulation by the action of compression or vibration.

EMI Screening

Electromagnetic interference can result in unacceptable risks and affect the performance of electronic equipment. It is essential to manage any electrical interference and reduce all risks to acceptable levels. 
Careful consideration is required at the harness design stage to ensure the correct levels of protection are chosen that will provide superior EMC and EMP protection.
To protect against specific threat levels, a choice of cable shield designs is available that cover a wide range of frequencies. The range of screen types typically offered covers, Aluminised Mylar, Single Braid, Single Optimised, Double Optimised, Superscreened and Double Superscreened. When protecting for sensitive and highly sensitive circuits it is advisable to specify optimised braids, which typically offer an optical coverage of 90%. The typical method used for measuring screening efficiency – surface transfer impedance (Zt) is the line-injected method as described in IEC 1196-1.

System Testing

Ensuring harnesses perform reliably over many years requires system testing. This involves mechanical, thermal, fluid and screen tests performed on an entire harness, simulating severe “in service” conditions. Many of the tests are carried out sequentially on the same termination, to produce a cumulative performance record.
Mechanical test typically cover Fatigue flexing, Torque, Tensile Load, Impact, Cantilever and Vibration.
Thermal shock, involves subjecting the harness to rapid changes in temperature, -40 to +60 to stress the cable to connector termination junction.
Further tests exposing the harness to fluids such as Aviation, Hydraulic and lubricating oils at 70.

Designed for repair and maintainability

A benefit of adopting a heat shrink system design is that it allows for components to be repaired. The elastic memory of the components permits re-entry to terminations without breaking environmental seals. This is achieved by undoing the coupling nut of the connector adaptor and heating the termination boot until it becomes elastic, at which point, it can be rolled back on itself providing the necessary access to the rear of the connector for repair. The procedure is reversed for recoupling. The special properties of the materials allow for repeated repairs if required without damage to the system. The smooth profile of all standard harness systems presents no trap areas for fluid or dirt to accumulate.

Conclusion

Interconnection systems for military systems have to last for decades and allow for maintenance and upgrades. Adopting a system approach to your interconnect harness policy enables designers to provide cost effective solutions, confident in the knowledge the designs will perform long term in the most demanding applications, be this Aerospace, Defence, or Marine.

To find out more about what HPL Technology can do for you, please get in touch