What am i missing for this to work? (Zener Diode)
You seem to be confusing the Zener action with a 'crowbar' circuit. A Zener will limit the voltage whereas a crowbar circuit will short out the supply in the event of an over-voltage. The crowbar short can usually only be removed by cycling the power.

The Zener diode will start to conduct in the reverse direction when the voltage across it reaches about 5 V. From then on very small increases in voltage will cause it to pass an exponentially greater current. At the same time the voltage will increase slightly above 5 V.

The problem with your arrangement is that if the regulator can pass, say 1 A, before going into limiting then your Zener has to handle all that power. \$P = V \cdot I = 5 \cdot 1 = 5~W\$. It will get hot.

Crowbar circuit







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Crowbar circuit.

This is a basic crowbar circuit. When the supply voltage rises above 4.7 V the Zener starts to conduct. When the voltage across R1 gets high enough SCR1 will be triggered and pretty-well short out the supply. The thyristor has to pass all the current (1 A in our example above) but this time the voltage across the device is only about 0.5 V so \$P = V \cdot I = 0.5 \cdot 1 = 0.5~W\$. Cool!

The thyristor will stay on until the current though it is reduced to zero by interrupting the supply.

History lesson: the term crowbar protection comes from the railway third-rail electrification. If the lineman saw a problem and needed an emergency isolation of the track power supply he would throw his crowbar to short the third rail to one of the running rails and trip out the breaker.