Polyswitch fuse use in automobile, app note from Littelfuse. Link here (PDF)
A vehicle’s ideal harness scheme has a hierarchal, tree-like structure with main power “trunks” dividing into smaller “branches,” with overcurrent protection at each node. This architecture allows the use of smaller, space-saving wires that can reduce weight and cost. It also optimizes system protection while providing fault isolation, which ultimately improves reliability.
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. However, rather than being positioned close to the conventional fuses, resettable PolySwitch devices can be located inside the box or on another face, which can save frontal area. Moreover, placing them closer to the connectors allows the trace’s length to be reduced. As a result, the overall junction box can be downsized.