I have a very expensive 500va 1:1 toroid transformer with to isolate workbench from ground. But I'm afraid to use it for fear the winding may burn up when something shorted. I'd like to know what is the behavior when the secondary shorts. Would it heat up faster than the primary winding able to draw current and tripping the primary breaker? What value must be the primary breaker, and can you use secondary breaker too (and what value)?
Overcurrent does not damage a transformer, it's over temperature that's the problem.
When a transformer output is short-circuited, there's a large increase in current in both secondary and primary windings. If this is allowed to continue, it will eventually overheat it, and destroy the insulation. However, it will take many, many seconds to reach a dangerous temperature, plenty of time for a fuse or breaker to respond, so it can be completely protected by using a suitable fuse or breaker.
It's more of a problem to protect against 50% overcurrent, as a fuse that's going to blow at 50% above nominal will run very hot, and may nuisance blow on switch-on transients. A time delay fuse is essential to resist the switch-on transients.
A breaker can be set more accurately than a fuse, and is a better bet if you also want protection from small amounts of overload.
Small overloads will take 10s of minutes to overheat the transformer, so the best protection against small overloads is the use of a thermal breaker attached to the transformer.
Primary needs a slo-blow due to excitation current with worst case remanence and no-load excitation currents 1/2 cycle.
Secondary depends on surge current of loads like diode bridge caps so it also needs to be a slow blow for rated current.
This is to prevent false fuse failures.
Alternative protection could be an ICL slow start for rated current with fast blow fuses.
a 500 VA torroid will have a Zo= <5%pu so short circuit current is 20x rated current.