The amount of protection you have really comes down to economics. The more protection, the more cost. As engineers, when designing a product we have to consider the what is to be expected in the application. In my applications I expect 24vdc to be accidently applied to the RS485 comms - without protection the poor old 75176 would fry so I add protection. This protection consists of transzorbs and polyswitches. The polyswitches protect the transzorbs from going up in smoke if 24vdc is applied. The cost of these devices exceed the cost of the 75176 many times. If I had to design it to withstand 415vac I have two choices - is it to recover from the abuse or should it fail gracefully - limiting the damage? To fail gracefully is probably the cheapest and easist route. For brute force protection there are MOVS (varistors) or transzorbs (and gas discharge tubes for hi voltage) and use a fuse or some means of breaking the circuit in case of a fault. MOVs like to explode when hit with too much energy and transzorbs go short circuit.

As other respondants mention - prevention is the best first line defence.

Features like protection make the difference between a amateur design and a professional product.