hamvil:
The wireless routers are powered using a POE injector (48V max, 25W) and for some logistic reasons I need to put the relay on the DC line. Moreover I need the relay to be closed by default so if the Arduino breaks the router remains powered.So far the best choice I've found is the Drydom MPDCD3-B SSR. However, I've also found this Panasonic relay:
Why do you need a SSR? How about a normally-closed regular relay? Those are cheap, have reasonable reliability (easily 50,000 operations on the contacts) and most of all, have no appreciable resistance when closed, compared to SSRs which always have some voltage drop.
In fact, some SSRs are built for AC and won't even work right when switching DC (for example, zero crossing detection ones.)
This one is $2.25 for 200V / 500 mA: DigiKey - Electronic Components Distributor
There are many, many, (many,) others. You probably want a "NC" (normally closed) version.