Edit: Also, wanted to mention that there is such a thing as a "battle short", which is a switch than can be thrown during battle that shorts out nearly every single fuse and breaker on a weapons platform. Makes sense if you think about it militarily: you'd rather the equipment runs until it bursts into flames than to nuisance trip offline during combat; repair cost means little if your ship is sunk or tank is blown up.
While this may be or have been true for relatively simple and largely mechanically controlled weapons systems it certainly is not the case for todays high-tech electronically controlled systems. These sensitive devices simply fail and cant operate under the shunt conditions described above. You have to have all electronic systems working perfectly to control the weapons system you are using - there is no mechanical or human control possible.
