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:

sacrificing wire size optimization and fault isolation by combining loads in one circuit
locating electrical centers where they are only accessible by trained service personnel, at increased cost
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).

An Alternative Approach: Decentralized Architecture Using PPTC Devices

A more optimized harness scheme has a hierarchal, tree-like structure with main power “trunks” dividing into smaller “branches,” with overcurrent protection at each node. This architecture uses smaller, space-saving wires that can reduce weight and cost. It also helps improve system protection and provides fault isolation, which ultimately enhances reliability. Figure 1b shows a decentralized architecture, where several junction boxes (illustrated in yellow) are supplied by power busses. The wires exiting the junction boxes to supply power to different functions can each be protected by a resettable circuit protection device. Figure 2 shows a greatly simplified version of a partially distributed architecture, with each junction box either directly feeding a module or another nodal module that supplies peripheral loads.

A decentralized approach to the electrical system architecture can be implemented with PPTC overcurrent protection devices. Given the availability of automotive-grade devices and the reliability that can now be expected from relays, modules can switch and protect their own output loads and can be located in inaccessible areas.

Since PPTC devices obviate the need to route electrical power through user-accessible central fuse blocks, power can be routed via the most direct path between the power source and load. This translates to shorter lengths of lighter gauge wire and results in significant size, weight, and cost savings, as well as a reduction in the number of terminals, contacts, switches, and electronic drive circuits for each vehicle.

Furthermore, a decentralized architecture can reduce the required number and size of connectors and junction boxes. By incorporating PPTC devices in the door module itself, for example, a single power feed can be used, saving wire and reducing the cost and size of the junction box. Table 1 illustrates the weight savings that can be achieved by using a decentralized architecture and PPTC devices, as compared to conventional fusing techniques. (Note that the minimum wire size used in this example is 0.35mm², although some applications may be able to use smaller gauge, as noted earlier.)

Using resettable circuit protection devices that do not need to be driver accessible offers designers a number of solutions that may be used separately or in combination. A single junction box located in the instrument panel may still be employed. Unlike fuses, which must be positioned on the top of the junction box in order to be accessible, PPTC devices may be embedded inside the box or located on another face, which can reduce frontal area requirements, as shown on Figure 3.

Moreover, by placing protection devices closer to the connectors, the trace length can be reduced and the overall junction box can be downsized. Alternatively, the junction boxes can be divided into smaller units and relocated around the vehicle without considering user accessibility. In these cases, the PPTC devices help designers achieve an electrical architecture that more closely reflects the optimized tree structure and its attendant benefits.

PPTC devices are available in a wide array of form factors, facilitating a variety of interface options with the junction box or electronic module. Through-hole and surface-mount devices lend themselves to installation in fuse boxes or modules using printed circuit boards. Strap devices can also be used in metal fret boxes. A new generation of bladed devices can also be inserted like a bladed fuse or bi-metal breaker in the junction box. Even though these devices are resettable and do not need to be user accessible, the bladed form factor allows designers to replace a fuse or a bi-metal device without waiting for the next redesign of the junction box.

Conclusion

Employing a decentralized architecture combined with PPTC overcurrent protection devices can significantly reduce weight in automotive designs. Although a decentralized approach has been understood for many years, the recent availability of thinner wires that can carry higher current, as well as new industry incentives, makes this approach clearly superior to conventional fusing techniques. Using PPTC devices in a decentralized harness protection scheme offers many important design benefits. The resettable functionality, low-resistance characteristics, and a wide array of current ratings provided by these devices can help automotive designers reduce wire length and weight while facilitating design flexibility and system reliability.

PPTC Device Principle of Operation

The PolySwitch PPTC device is made of a composite containing a conductive filler, such as carbon black, that provides conductive chains throughout the device. This device exhibits low-resistance characteristics under normal operating conditions, but when excessive current flows through it, its temperature increases and the crystalline polymer changes to an amorphous state.

As illustrated below, this transition causes the polymer to expand, breaking the conductive paths inside. During a fault event, the device resistance typically increases by three or more orders of magnitude. This increased resistance helps protect the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady-state level. The device remains in its latched (high resistance) position until the fault is cleared and power to the circuit is cycled; at which time the conductive composite cools and re-crystallizes, restoring the PPTC device to a low-resistance state in the circuit and the affected equipment to normal operating conditions.