Another great advantage of the electronic fuse is apparent in the configuration of the power network. Electronic fuses can be placed virtually anywhere in the car; unlike thermal fuses, they do not need to be gathered in one fuse box. One benefit of this is shorter cable runs, and a consequent reduction in cost and weight.
It also enables the power system designer to implement for the first time tree topologies, which are easy to manage, and even ring topologies for advanced safety features. Tree topologies support the use of very much thinner cables, because they allow the power controller to switch off certain portions of a circuit for a certain period to keep the load on the entire circuit below a pre-determined maximum value.
Quantifying the potential for weight reduction
It is clear, then, that replacing thermal fuses with electronic fuses offers the potential to reduce the size and weight of the cable harness. But by how much?
A couple of decades ago, cables in cars mainly carried loads rather than signals. In the case of an indicator, for instance, the stalk at the steering wheel closed a contact which directly connected the indicator’s relays to the battery.
In the 1990s, car manufacturers began to introduce power networks. The goal was to keep load-carrying cables as short as possible, and to use thinner signal cables whenever possible. So today’s indicator stalk does not close a load contact: instead, a sensor detects that the indicator has been switched on, triggering a pair of electronic control units (ECUs) to send this information to the main front and rear body units. It is these body units which switch power on and off to the indicator lights.
Had no other developments occurred in the car, this would have reduced the total weight of the cable harness. In practice, however, cars today include so many more electrical and electronic functions than cars did in the 1990s that the weight saving has been more than eaten up by the requirement for additional network connections. In fact, a modern mid-range car carries more than 1.5km of cable weighing more than 40kg.
And of course, these cables require protection. This often leads the system designer to struggle with the best way to trade off improvements in safety, comfort and functionality against the desire to reduce weight and cost.
Take the example of an electric sun-roof. The design specification requires that the roof will open even when its seal is frozen. Breaking the grip of ice calls for a high current through the sun-roof’s electric motor, as much as 30A.
But the specification for the circuit’s thermal fuse cannot be for 30A, since ageing reduces the fuse’s current rating*: a margin of 20% has to be added. But 36A is not a standard rating for off-the-shelf fuses, so the sun-roof designer is forced to specify a 40A fuse.
This in turn affects the cable specification. For the motor’s 30A maximum current, a 2.5mm² cable would be sufficient. The power network design must also be specified to withstand a maximum 70°C operating temperature. Assuming it takes 50ms to melt the fuse at 400A – ten times the nominal 40A rating – the temperature of the cable might rise far above the maximum temperature rating of 105°C for a standard cable in a car in the event of a current surge.
As a result, the sun-roof designer is forced to specify the next thicker grade of cable: in this case, 4mm², which is 40% heavier, and 40% more expensive.
By contrast, an intelligent, application-oriented electronic fuse, which does not age, could be designed to be triggered at an exact current value of 30A (or higher), enabling the use of 2.5mm2 cable.
The authors’ estimate is that some 5-8kg of copper can be removed from the car by replacing thermal fuses with electronic fuses. At the time of writing, the cost of copper was around €6.50/kg, providing an estimated cost saving in copper alone of up to €52. In addition, the weight saving improves the vehicle’s fuel efficiency, helping the car manufacturer to avoid the proposed €95/g of CO2 levy to be imposed by the European Union when fleet-wide fuel consumption rises above a certain threshold.