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

Battery systems are evolving:  the increase in system voltage goes hand in hand with a rapid increase in charging capabilities toward megawatt charging (MCS). These changes significantly increase fault energy, transient current levels, and thermal stress within electrical protection devices.
High-voltage (HV) fuses remain one of the most critical last-line safety devices in EV platforms. However, conventional fuse technologies originally developed for industrial or passenger car applications face major limitations when exposed to the operational profile of heavy-duty electrified platforms.

Even though alternative technologies are becoming more widespread in the market, we will focus on HV fuses here as they still play a significant role as protection solutions in the high-voltage (HV) network inside eHDV.  The HV fuse acts as the final line of defence against overcurrent and short circuits in a high-voltage battery system. In heavy-duty applications, their critical importance is amplified by the substantial energy stored in the battery packs and by intensive cycling.        
As seen above, the voltage of such application is slightly higher than for passenger cars and mainly in the range of 600 V and 1000 V DC. New developments for eTrucks already show a trend to higher voltage up to 1.500 V DC

Depending on the fuse’s position in the vehicle, its goal will vary.

As mentioned earlier, fuses are the last line of defence in case of electrical hazards. Indeed, as a passive device, they will always answer even if the active protection systems (pyro, contactors, BMS,..) are failing. Depending on the application, they will be used to protect the components against short-circuits and over-loads occurring in the vehicle. 

Once the fault has been cleared, they will enable the affected areas to be isolated to prevent the fault from spreading throughout the system; depending on the area that has been isolated, this will allow the driver to bring their truck to a safe stop and ensure the safety of people in the vicinity whilst they await the emergency services.

In addition to the usual challenges associated with fuse selection – such as selectivity, coordination with contactors or pyrotechnic devices – they face a harsher environment and higher breaking requirements than those found in a car.

Let us first consider the second requirement. Unlike cars, which have only a single battery pack, the batteries are distributed throughout the tractor and, depending on the options, there can be up to 10 battery packs per vehicle. Although the short-circuit current capacity of a single pack is lower than that of a battery pack (8kA vs 16kA), connecting the various packs in series and in parallel significantly increases the short-circuit current a fuse might have to withstand. Manufacturers therefore require that the fuse be capable of interrupting at least 50kA or even 100kA.

In addition to this, it must not be forgotten that the operating conditions for fuses are far more demanding in a heavy-duty vehicle than in a car. Fuses are subject to vibrations at much higher frequencies and for much longer durations. This vibration can have a negative effect by causing mechanical fatigue, which could lead to the fuse blowing unexpectedly. This results in a reduction in the vehicle’s performance or the disconnection of certain equipment. As well as testing the fuse against the specified profile, it is critical to test it within its subsystem. During this test, it is important to measure the actual vibration experienced by the fuse. An unsuitable mounting arrangement could exacerbate the vibration profile and lead to failure.

The climate and operating conditions are also far more demanding than in a car. These factors must be taken into account when selecting the fuse.

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.

Drawing on over a century of expertise in fuse sizing, combined with the development of digital tools and three in-house test centers, Mersen is your ideal expert partner.

Every application is different; profiles vary, environmental conditions differ and the space available for the fuse is becoming increasingly limited. It’s to meet these expectations that MERSEN has developed a range of techni-cally and economically efficient products to address the challenges of e-mobility, and in particular those relat-ing to heavy duty vehicles.

The wide variety of fuse elements already in the Mersen portfolio allows us to quickly combine components to deliver initial prototypes in response to our customers’ challenges. Before delivering the prototypes, we will have undergone an extensive digital validation phase to verify the thermal performance of the fuse under the customer’s operating conditions and to estimate its service life. Following these prototypes, Mersen's test centers based in China, Europe and the United States will take over to conduct test campaigns ranging from risk mitigation to full qualification.