To avoid electric shocks and high-energy short circuits, Mercedes-Benz has developed a multi-stage high-voltage safety concept comprising seven key elements. Alongside the battery, part of the high-voltage system includes all components with a voltage level higher than 48 volts. The protection concept provides a high degree of safety when driving, both during and after a crash.
- Separate positive and negative lines
A conventional 12 V on-board electrical system uses the vehicle body as negative terminal ("earth"). The high-voltage (HV) system, on the other hand, is fully insulated down to the vehicle structure: all HV components are connected to each other with both a positive and a negative line. The HV lines can be identified by means of their orange sheathing. Even if damage should occur, there is no danger of an electric shock or short circuit, since even in this case there is no closed electric circuit.
- HV system self-test
The entire HV system, in particular the battery, continually monitors itself. By means of continual temperature, insulation and short circuit measurements, fault currents are detected and displayed at an early stage. A so-called interlock switching circuit incorporates all of the HV components and monitors whether all of the components are connected correctly. Errors in the system are displayed and if there is any doubt the HV system cannot be started, or is even automatically switched off.
- Protection zones
Based on more than 50 years of accident research by Mercedes-Benz involving thousands of investigated accidents, a protection zone concept has been specially developed for electric vehicles. As part of this the vehicle is divided into three areas.
- Outer zone: in the case of minor damage, the HV system is mostly unaffected and therefore does not need to be automatically switched off. That's because the high-voltage components are either located outside of the area affected by such minor damage or are safeguarded by additional measures (see 4. Inherent stability).
- Inner zone: if the vehicle is affected by more severe accident damage, the HV system is automatically switched off (see also point 6): in an accident of such severity the airbags are generally deployed. Depending on the severity of the accident and degree of damage, the HV system is switched off reversibly or irreversibly. Either the customer can therefore switch it back on again himself/herself, or activation is only possible again after replacing some parts.
- Core zone: in the third vehicle area, usually no or only slight deformation occurs in crash tests. This protection area is suitable for accommodating the HV battery or particularly sensitive components, for example.
- Inherent stability
In the case of HV components in the outer deformation areas, rigid housings in particular make a contribution towards protecting components. For this, during vehicle development, damage patterns and load levels are derived from crash simulations and crash tests. Affected HV components must also be protected against contact after the crash. The requirements are particularly high for the mechanical properties of the HV batteries. Here, alongside standard crash tests, further load cases are also used to provide even further cover of actual accidents conditions.
- Reinforcement measures
All HV components are connected to each other with HV lines. HV cables are flexible lines which in some cases can also be routed inside structural areas. Although in any event this usually involves two separate lines, they are additionally sheathed in particularly sensitive locations in order to prevent a loss of insulation if crushed. With other HV components, too, alongside inherent stability the protection level can also be further increased, for example through specifically guide surfaces or protective shields.
- Automatic crash shut-off of the HV system
As soon as a specific accident severity is detected in an impact, the HV system is switched off. As part of this, relays are opened in the HV battery which prevent further power from being fed into the HV system. Components which are connected to the battery are discharged in just a few seconds such that only a non-critical voltage level remains present.
In less severe accidents, a reversible shut-off only takes place preventively by means of a simple shut-down indication. When the driver tries to re-start the vehicle, an insulation check automatically takes place prior to switching back on. If no insulation error is detected as part of this, switching back on is permitted. In severe accidents, after which in any event it is no longer possible to continue driving, the HV system is irreversibly switched off through the ignition of a pyrofuse. The vehicle can then no longer be started.
A particular highlight is the "stationary crash detection": when switched off while charging, the EQS is also able to detect a severe impact and quickly interrupt the charging process. This helps to achieve a particularly high level of protection for the high-voltage system.
- Manual shut-off option for emergency services
For the emergency services the vehicles feature additional shut-off options for the HV system, so-called rescue separation points. The installation locations are recorded in the rescue datasheets. For towing away, too, for example, a manual shut-off is useful when the vehicle is only slightly damaged and it cannot be clearly determined whether an automatic crash shut-off has occurred.