It depends what is your precious equipment like.. The fuses are rather slow devices. Also when talking for example a transceiver, the circuit around the thyristor's gate should be well decoupled (what slows down the response even more). I tried that once upon a time and rejected the idea after some experiments. I put TVS diodes everywhere then..
Definitely, and that's why I plan to put a TVS diode there as well. Even fast-blow fuses are slow (as you've pointed out), but the thyristor will shunt the current from the moment it's activated. I'll use one that's capable of handling more than the fuse current so that shouldn't really be a problem. So, TVS will kick in first, followed by thyristor and eventually fuse will blow in failure. And the thyristor will stop the TVS from dissipating too much energy internally.
The overvoltage seen by the load will be very shortly lived thanks to the TVS. I expect the load would be fine without the TVS there, but more security is better.
In the few supplies that triggered a crowbar that I've seen the fuse often remained intact. A crowbar can destroy a supply in quick order. Then agian it can be hard to tell what failed first. In any event you are right in that if the crowbar triggers the load should not see the high voltage, the dead short shold take care of that.
The big problem with crowbars is unintened triggering. Are you sure your expensive device will never send a transient back to the supply and cuase it to self destruct. Honestly I'm not a big fan of this approach.
This has been worrying me a bit. My design is a bit strange in that the fuse rating (500 mA) is at the very upper limit of what the lt3045 can supply. Max DUT current is 333 mA (15 V). I've also used the current limit functionality of the lt3045 to limit to 385 mA. I figured the crowbar would only trigger if the lt3045 failed (set to trigger at 15.8 V), but maybe it would be possible for this to trigger under normal operation. In that case, the LT3045 would continuously dump 385 mA into a short circuit and never blow the fuse. Also worth mentioning is that the LT3045 isn't spec'd to put out more than 15 V.
The DUT should be pretty quite and I wouldn't expect spikes: it's the combined oven/oscillator supply for a very high quality OCXO.
The manufacturer gives the allowed voltage as 14.25-15.75 V, so that's what I'm trying to meet. I'd be a bit surprised if I can't push the voltage a bit higher without damage, but I really can't say that with any certainty. If I still use the crowbar, I guess I could raise the trigger point and assume the DUT will still be fine if hits 16.5 V or so. Alternatively, I could filter out fast transients with the Miller capacitor as shown in the design a few posts earlier. That one makes me a bit nervous though as it can send a current pulse through to the SCR gate. I doubt it would be enough to trigger it, but still...
Anything you prefer to crowbars? I guess I could go with one of these overvoltage disconnect protection ICs. I tend to put stock in design decisions made in old HP equipment and they used zener+thyristor crowbars all over the place, but maybe that's because there weren't too many alternatives in the 80s.