Is it tripping when you turn on the main fan oven setting?
Ok this is nearly always down to the fan heater element in there which causes this fault.
If you are looking at fixing this yourself then you need to empty the oven and from there remove the back panel inside the oven.
There you will see the heater element around the fan.
You need to replace that and then this will cure the fault from there.
But if you are not able to do the repair yourself then you need to get an engineer in. Now you can use Neff themselves but you will find they are quite expensive.
But you can try a local repair engineer as they usually do all brands and are a lot cheaper then Neff themselves. So roughly for a heater fitted you are looking at about the £80 mark.
If you get stuck finding an engineer then left me know you postcode and i can see if i can find one for you.
But for the fault you have its more then likely the heater element thats casuing this fault.