White paper | High voltage fuses in heavy-duty electric vehicles

Historically, electrical protection against short circuits was provided by fuses. New components such as pyroswitches and other derivates have emerged and are gradually replacing fuses in certain specific applications, such as the protection of battery packs. These pyrotechnic solutions offer numerous advantages, such as the ability to provide a current-free disconnection within a fixed time, regardless of the expected fault current. You still need to weigh the advantages of the pyroswitch against its disadvantages. As it is an active device, it requires infrastructure (current sensor, on-board electronics) to function, unlike a fuse, which operates passively. The competitive advantage of a pyroswitch needs to be validated case by case.

This last point has given rise to a new requirement for fuses. When a short circuit occurs in a branch of the system, the nearest safety device is expected to be the first to trip and prevent the fault from spreading to the rest of the system. Take the example of a faulty pump: we want the upstream fuse to handle this fault. As pyros have a fast response time, it is essential to ensure that the fuse reduces the current below a certain threshold to prevent the upstream pyro from triggering. Indeed, for the driver, it is easier to bring their vehicle to a safe stop if only one pump is disconnected rather than if all the battery packs are disconnected.

Let's take a closer look at what's happening.

The pyro control system is fairly straightforward. If, after a certain period, the current is still above a certain threshold, the BMS will send a signal to activate the pyro. This design requires the fuse not only to blow quickly, but also to reduce the current as rapidly as possible so that it falls back below the threshold value.