I want to use a 2N7000 as a switch to protect my circuit for over voltage.
I have a scenario when i feed 5 volts and everything should be fine.
But is i feed 24vdc the fet should start conducting and blow the fuse.
On data sheet of 2N7000 trigger si 0.8 to 3 vdc.
I attached 2 pictures which should explain better.
Even if i make GS 1.92 volts, the fet stay open and the fuse does not blow.
I need help as it is first time when i work with Mosfets.

 

If you want anything even remotely close to precise, you can´t rely on the mosfets Vgs threshold. Use at least a bipolar transistor to trigger it, or even better a tl431.

 

Or an SCR and a zener diode:

 

On data sheet of 2N7000 trigger si 0.8 to 3 vdc.
I attached 2 pictures which should explain better.
Even if i make GS 1.92 volts, the fet stay open and the fuse does not blow.

The behavior is not strange at all.
Your answer is right there.
0.8 to 3V for Vgs means that's the manufacturing range of Vgs(th).
A particular device can be anywhere between those two voltages and even at that voltage, it's only conducting less than a mA.

Below is the LTspice simulation of a circuit using a 9.1V Zener, D1 to turn on the MOSFET starting at about 11V.
The blowing of the fuse is not simulated.
You can use a lower voltage Zener if you want the MOSFET to turn-on starting at a lower voltage.



Edit: Attached is the circuit using a TL431 programmable reverence (zener).
The TL431 starts to conduct when the control-node to anode voltage reaches 2.5V.
Thus with the R1 and R2 resistor values shown, it starts to conduct at a cathode to anode voltage of 1oV nominal (where Vg starts to sharply rise).

 

Even if you get open at 3Vgs, you should turn into the ESD, as you said you're using MOSFETs first time.