But how can you then get 150V from multiple directions?

You have one (1) 24V power supply?

And it is this single 24V supply you want to protect, or do you have other - distributed - electronics that needs protection too?

The intention with the crowbar is that the fuse will blow and break away the 150V.

You may have the fuse on the 150V side, in which case you end up with the 150V disconnected from the crowbar (and a possibly still shorted PSU depending on cable resistances and PSU short-circuit protection giving enough voltage to keep the SCR holding). If the 150V side did not have a fuse or other over-current protection, the wiring will continue to hold 150V after the fuse blew, but the device behind the crowbar is protected.

You may also have the fuse between your device and the crowbar, with the result that:
- only your PSU will send current through the fuse - the external 150V source will short through the SCR.
- the fuse will only break if the PSU is powerful enough (and with slow enough short-circuit-protection) to burn the fuse.
- the survival of the SCR will depend on how many amps the external 150V power source can continue to feed through the wiring, and if the 150V source has overload protection.
- if the SCR survives, it will fight to protect any other devices you have connected to the wiring.

It is important that the crowbar is very fast, which is a reason for thinking about complementing the SCR with a TVS (Transient Voltage Suppressor)

With the fuse on the outside of the crowbar (in relation to your 24V PSU), the SCR may continue to short your PSU depending on exactly how the short-circuit protection in the PSU works, i.e. if it just limits the voltage to get a balance in power output, or if it totally deactivate all output. But since you said hte PSU was short-circuit-protected, it will not be damaged by the SCR-induced short-circuit. Without enough voltage over the SCR, it will open again and no longer short-circuit your PSU. Depending on PSU, cabling etc, you may have to power-cycle the PSU to get it up and running again.

You can also decide to make the crowbar bi-directional by having a fuse on each side, in case you don't know which side that will introduce the high-voltage "attack". But everything on the wrong side of the fuse will of course be unprotected by the crowbar if the 150V power source is capable to continue to feed 150V after the fuse has blown and disconnected the crowbar.

So if you have a bus or star with multiple equipment, and the 150V may arrive anywhere on this bus/star, then you should consider having one crowbar for each connected equipment. This can give quite a number of fuses.