This time – a story about the F10 series vehicle.

The defect of the comfort access system appeared, and it almost finished tragically.


Here, several images:

As you can see, the handle has melted from inside! No doubt – in the electronics of the handle, a significant heat has been released.

The power wire of the comfort access system was burned. Its insulation is melted, in some places burned completely!

The connection of the large (black) connector has withstood the overload, but the white connector (in the previous picture) – already started to burn!

Here, the fuse. It has no visible marking, so the first conclusion – probably, some cheap China analog has been used (apparently, even inappropriate current range).


I usually like to study situations more detailed.

Let’s start with a schematics:

One of the possible options – in the image above. Not taking into account possible different power connections (cars of different setup/different years), all versions are unified by:

  • range of the fuse: 5A;
  • the diameter of the power wire: 0.35mm2.

Let’s calculate the basic parameters of this solution.
Let’s assume the length of the wire is 5m. This is only a very fair assumption, but I think that it will not be far from the truth, taking into account the size of the car.
The resistance of 5m 0.35mm2 cable is around 0.25 Ohm. At least the same value will be in the connections (there are 2 of them in the connectors, at least two more – at the fuse holder and at the PCB, additionally there will also be GND connections). So, total resistance around 0.5 Ohm.
In the case of a perfect short-circuit, the current of 20A will flow via this circuit (by the supply voltage 10V). Sounds good – the fuse as if should blow.

But – let’s check the picture with the melted handle once again – the short-circuit was NOT perfect! A lot of heat was released in the handle itself!
My next assumption – let’s choose the worst scenario: the amount of the heat, which is released in the handle, is the same as in wires. This is the worst scenario because it ensures the largest amount of heat in each of the sources of ignition (risk of ignition). Hence, both sources of burning are risky equally risky – in both, the fire can start equally effective.
To meet this condition as well, the resistance of the handle should be 0.5 Ohm, the resistance of the wire + connection should be identical.
In this case:

  • the total resistance of the circuit “wires + connections + damaged handle” is around 0.5 + 0.5 = 1 Ohm;
  • the current, which flows via circuit by Ubar = 10V: 10A;
  • emitted heat: 100W.

So, in this scenario, till the moment, while the fuse blew:

  • 50W of heat in wires + connections was released;
  • 50W of heat in the handle was released.

50W power is quite impressive – without illusions: the wires would start to burn with an open flame if the fuse hasn’t blow after some time!
Yes, it is possible (most likely), that the amount of heat, which was released in both ignition sources, was not entirely equal. But such an asymmetric situation is even more dangerous – for example, if more heat was released in the handle than in wires, the total current was smaller, the fuse could not blow at all. Instead, even when 20 .. 30% of the full heat was dispersed in a wire of 0.35mm2, it was enough for its insulation to start to burn.


And finally, shortly about fuses.
For example, the fuse of 5A rating means that this fuse can handle the current of 5A for an unlimited amount of time. Depending on the model; manufacturer; the current item (technical tolerances), it would take at least 2x .. 3x overload of blowing a fuse in a reasonable time (several seconds).

Taking into account all previously mentioned, we have to conclude that in case of such damage, the fuse of 5A could not even be able to save the situation!
Yes, everything may be fine according to formulas used by BMW AG, but we have to take into account:

  • the wires in the automotive industry are bound in thick bundles, which significantly worsens the situation with their cooling;
  • the resistance of all connections increases (due to aging) during the time;
  • the defect (short-circuit) can be “partial” – as it is in this case.

Allow me to say that the BMW AG should also take part of the blame. Such a respectable manufacturer, however, could not skimp and install a 0.5mm2 wire. It would be a good start to improve car safety!