Truck/Bus Harness Protection in 24V Battery Systems

TE Circuit Protection has developed overcurrent protections that can also be used as a protection for truck/bus harness. This application note discusses how overcurrent protection products of TE helps protect the wiring-harness architecture found in trucks, buses and other vehicles with electrical systems based on 24 V technology.

Optimized vehicle harness architecture has a hierarchical structure resembling that of a tree with its main power trunks dividing into smaller and smaller branches that use overcurrent protection at each node. Because a hierarchical architecture can use smaller wires and relays on its “smaller branches,” the resulting harness is smaller and lighter, resulting in a cost savings — both in materials and fuel. In addition, a hierarchical or distributed architecture can provide system protection together with fault isolation, thereby reducing warranty costs and increasing customer satisfaction.

Figure 1 shows a simplified version of a partially distributed architecture with each junction box either directly feeding a module or feeding another nodal module which supplies peripheral loads. Unfortunately the sheer number of circuits found in today’s vehicles has made the optimized system hard to realize in practice. With many tens of circuits emanating from the primary power distribution center, it has become almost impossible to position all the subsequent junction boxes so that they are readily accessible and close to the electronics they are intended to feed. 

Figure 1: In a partially distributed architecture, where a module supplies a number of peripheral loads, PPTC devices can allow shorter and smaller wires to be used. Door and HVAC module examples are shown here.

As a result, system designers have resorted to harness design solutions that negate some of the desired end-benefits, such as:

1. (1)  sacrificing wire size optimization and fault isolation by combining loads into one circuit;
   
2. (2)  locating electrical centers where they are only accessible by trained service personnel, at increased cost; and
   
3. (3)  routing back and forth between various functional systems, increasing wiring length, size and cost.

For example, due to the necessity for fuse accessibility, a conventional door module would have separate power feeds for windows, locks, LEDs (light emitting diodes) and mirror functions, each protected by a separate fuse in the junction box. By incorporating PolySwitch resettable PPTC (polymeric positive temperature coefficient) devices in the door module itself, a single power feed can be used. This helps save wire and reduce the cost and size of the junction box. 
 
Using a resettable circuit protection design that does not need to be driver accessible offers a number of solutions that can be used separately or in combination to better optimize harness designs. For example, a single junction box located in the instrument panel can still be employed. Instead of positioning the PPTC devices close to the conventional fuses on the front panel, they can be located inside the box, close to the connectors or on the bottom face of the box. This saves both frontal area as well as reducing the box’s volume, as shown in Figure 2. 

Figure 3 illustrates yet another advantage of replacing conventional fuses with resettable PolySwitch devices. Indeed, using a PPTC device in a dedicated manner (delocalized or not) can allow wire and relay downsizing, thus helping save cost, space and weight. 

To learn more about Truck/Bus Harness Protection, click here.