Today’s automobile manufacturers are actively working toward reducing vehicle weight in an effort to help reduce CO2 emission levels and increase fuel efficiency. Design engineers are exploring new technologies and design techniques that will help lower wire harness weight without sacrificing system reliability. Current industry imperatives have led them to revisit their approach to protecting automobile power functions against damage from high-current fault conditions by using a decentralized harness technique and PPTC (polymeric positive temperature coefficient) devices for overcurrent protection. This article presents the significant benefits of this approach, as compared to using a traditional centralized architecture with fuse protection.

Trends in Harness Protection

Although a decentralized approach to harness protection that utilizes PPTC devices has been available since the 1990s, its adoption has been slow. In fact, as electrical and electronic content has continued to add functionality, many wire systems in today’s automobiles have become bigger, heavier and more complex than ever.

In addition to the industry’s resistance to changing traditional design methods, the benefits of using PPTC devices may have been hampered by the thicker wires historically used in vehicles. In the past, mechanical strength dictated that the smallest wire used in the vehicle was 0.35 mm² (22 AWG), which could carry current from 8 to10 A. This limitation worked against some of the benefits of using PPTC devices for low-current signal circuits (e.g., <8 A). Today, emerging wire material technologies are enabling wires with much smaller diameters and with more current-carrying capacity, including wires as small as 0.13 mm² (26 AWG) with a maximum 5 A capability. This advancement has led to an opportunity to achieve additional weight savings when used with a PPTC-protected distributed architecture.

One study, employing a decentralized architecture and Tyco Electronics' PolySwitch devices, on a mid- to high-range passenger vehicle showed an estimated 50% savings in the weight of copper wires alone. Additionally, by using a decentralized architecture and replacing fuses with resettable PPTC devices, system reliability and design flexibility were significantly improved.

Automobile Harness Protection

In a car, current flows to the various electrical loads through several major and minor wire assemblies, which are distributed throughout the vehicle. Circuits typically carry 0.10 A to 30 A of current at system voltages of 14 V for 12-V battery systems (28 V for 24-V battery systems found in most trucks and buses). The wiring harnesses must be protected from damage caused by catastrophic thermal events, such as a short circuit.

The challenge for designers is to add circuit protection devices that help protect against potential overload conditions in the electrical system, while simultaneously reducing total cost and weight. Since a typical vehicle may contain hundreds of electrical circuits and more than a kilometer of wire, the complexity of the wiring system can make conventional circuit design techniques difficult to use and may lead to unnecessary overdesign.

Traditional Approach: Centralized Architecture and Fuses

The conventional solution to protect an automotive wiring system has been to use centralized multiple-load fusing, as shown in Figure 1a. In this type of centralized — or “star” — architecture, each function requires a separate wire. Where a single wire supports multiple functions both the wire and its fuse must also support the sum of the currents of those functions.

With so many circuits emanating from an electrical center, it has become almost impossible to route all the wires in and out of a single junction box and place the box in a driver-accessible location. As a result, system designers have resorted to harness design solutions that negate some of the desired end-benefits, such as:

1. sacrificing wire size optimization and fault isolation by combining loads in one circuit
2. locating electrical centers where they are only accessible by trained service personnel, at increased cost
3. routing back and forth between various functional systems, increasing wiring length, size and cost. For example, due to the need for fuse accessibility, a conventional door module may have separate power feeds for windows, locks, LEDs and mirror functions, each potentially protected by a separate fuse in the junction box.

The traditional centralized approach to a vehicle’s wiring architecture relies on a limited number of larger fuses to protect multiple circuits against damage from high-current fault conditions.  Although single-use fuses are relatively inexpensive, replacing them when they “blow open” can lead to customer inconvenience and dissatisfaction. Fuses are also required to be mounted in a somewhat accessible area, which can dictate system architecture and potentially force less desirable packaging layout compromises. Since a fuse’s form factor is not dependent on its rating, it is easy for the user to substitute a fuse for one with a higher current rating than intended for that circuit. This can potentially cause overloading of the wiring in the harness and possibly cause a thermal event (fire).

Figure 1a. Typical centralized architecture

Figure 1b. Typical decentralized architecture