Pardon me if I'm jumping into a discussion a little late (and a dollar short), but there might be some confusion on how and why relays are designed for intermittent duty.  As an example, take a standard master relay.  It is rated for continuous duty and something like 80 amps.  That means the coil won't overheat if left on forever and that the contacts also won't overheat when passing 80 amps and that the contacts won't weld when interrupting 80 amps.  It is often the limitations when opening the circuit that determines the rating.  Current will try to keep flowing and if the contacts don't separate rapidly and with enough force the inevitable arc could create enough heat to weld the contacts and prevent opening.  If it is desired to design a relay in the same package (for cost, size and weight reasons) that can be used intermittently to pass a much higher current the design parameters are different.  More force is required to increase the rate at which the contacts are opened so a higher force spring is installed.  Then more force is required to close the contacts, so fewer turns of heavier wire are used in the coil (that's right, FEWER turns, but that's another discussion).  The result is that the relay can now successfully switch maybe 300 amps, but the coil will overheat after a few minutes of operation.  That's the difference between a master and a starter relay.  And I have never, ever heard of using a relay as the "weak link" in the system.

Gary

ps:  My Lancair list emails now come all messed up (see below).  Why is that?  Am I doing something wrong?