In most cars and trucks, a thick cable connects the battery directly to the starter (solenoid). Why don't these circuits have have any kind of overcurrent protection?
It is an accepted practice throughout the motor vehicle industry. Here are two separate standards that specifically exempt starter circuits from overcurrent protection. (These are both for watercraft where the consequences are even greater; you can't walk away from a fire at sea.)
Each ungrounded output conductor from a storage battery must have a manually reset, trip-free circuit breaker or fuse, unless the output conductor is in the main power feed circuit from the battery to an engine cranking motor.
These are just examples; I have no doubt that organizations such as SAE and ABYC have similar provisions in their standards. Millions of vehicles are wired this way.
All circuits, except the main supply from the battery to the starter motor and electrically driven steering motors, should be provided with electrical protection against overload and short circuit, (i.e. fuses or circuit breakers should be installed).
I am asking for the engineering rationale behind these exemptions. Even though the cable is much thicker than the starter windings, a mechanical failure or impact could still create a short to ground. The resulting current can easily exceed 500A and is enough to weld thick steel.
I understand that the starter demands far more current than any other circuit, but surely a cost-effective solution could be found -- such as a fusible link. Or am I mistaken?
Here are some possible reasons that don't make sense to me:
The cable is so thick (relative to the size of the battery) that it doesn't need protection. The starter will burn or the battery will explode before the cable melts. While this may certainly be true from a "protect the wire" standpoint, I believe this an even stronger reason for overcurrent protection on the starter circuit... to protect the entire vehicle.
The risk of a fault in this circuit is extremely unlikely. It is true that starters are sturdy devices and thick cables have more mechanical strength. However failures are still possible, and do occur from time to time in the real world. Furthermore the impact of a failure in this circuit can be catastrophic, leading to total loss of the vehicle or death. Therefore I'd expect the severity of the problem to overwhelm the (admittedly) low likelihood in a failure mode analysis.
Edit for future readers: Most of the answers focus on availability. That is very important, but a secondary reason coupled to the choice of a fuse as the protective device. A breaker would mitigate the risk of being stranded due to a nuisance fault. (Someone mentioned the possible loss of steering, but all production vehicles including the Infiniti Q50 still have mechanical backup.) Fortunately there is a concise answer that explains why even a breaker or fusible link wouldn't be suitable.