You may find alternate practice is better. For example, a "dim bulb test" or other limited-impedance source. This way, power applied to a short simply can't draw dangerous current.
Use a variac and increase it gradually, in case there's a transformer you can't measure through, or a breakdown threshold where current jumps up suddenly.
You can avoid powering shorts by always measuring the load first. Open it up and check likely components. Understand typical circuits (e.g. SMPS input and power sections) so you know what to probe at.
A true electronic fuse is difficult to make here, as the peak power levels are just so high -- you still need to provide some inrush current for loads, say, more than 10A. (You'll have a short-circuit current somewhat over 40A from that transformer, I would suspect.) This is part of the trouble with making even a crude (relay turn-off) one, you have to delay the response for normal loads, but trip at some point, when it's been too much for too long. But that's what an ordinary fuse does, it's just not repeatable.
And doing it with a relay isn't ideal, putting fault current across contacts guarantees maximum arcing. Eventually, it won't open any more; or it has too much resistance when closed, and burns up.
Ideally, you'd use series transistors or something, like what's often used with low voltage loads. But you'd need a hundred of them in parallel to do this the same way, it'd just be crazy. There is an alternative, using a switching circuit to improve the "dim bulb" circuit: it's normally held closed, but the instant the current rises too high (as limited through an inductor), it switches off, dumping some energy into a load resistor. The resistor can handle much more power, at little cost; the downside is, you need a fast-acting circuit that's more complicated to design and test.
So, sticking with traditional methods is probably the best idea.
Regarding breakers, they're going to be quite slow at this current level; and fuses and breakers in general, aren't precise devices, their only purpose is to protect the wiring. So a 4A nominal fuse, might take up to a minute to open at 6-8A -- the wire won't get too hot in that time. Likewise they don't open the instant some hundred amps is drawn, they allow inrush current through. The time grade determines how much is too much; slow-blow or type C curves are used with heavy loads like motors. For general electronics, you probably want the A curve, but yeah, hard to find -- there's simply not enough heat at those currents, to make a breaker move in a short time.
Tim
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