Zener Diode + fuse overvoltage protection for LEDs with constant current driver- fast or slow blow fuse?
The current through the zener will be determined by the source impedance of your 12/24 volt supply and the associated wiring. Most supplies will have a low enough internal impedance that the zener diode will fail when it conducts.

A slow blow fuse is not applicable in this situation since by the time the fuse blows, it is likely that the damage has been done to the LEDs or their constant current supply. A fast blow fuse is required.

You may wish to investigate a technique called a crowbar circuit. This uses an SCR that fires when the maximum voltage is exceeded. The SCR can handle substantial current for a sufficiently long enough period to blow a fast blow fuse. These are commonly used in power supply circuits to protect the load in the event the voltage regulation mechanism in the supply fails. The fact that your current requirement for the LED ring is quite small makes it a good candidate for this type of circuit.







simulate this circuit &ndash; Schematic created using CircuitLab

If you have the room, you could simply add a voltage regulator ahead of your LED ring. In the event someone does switch on the 24 volt source, the regulator would step it back down to 12 volts.