Help finding a product: Wireless handheld personal alarm reciver

[Lancie1]

Perhaps you only need to allow the door to open if it is not possible to have a hot dump in the next 10 minutes.

 

[JesperMP]

I need to alert Wheel Loader machin drivers when it`s unsafe to drive near a melting furnace.

Kas, there is no way that a pager or wireless alerter or similar can be an acceptable solution to your problem.
Are you in charge of doing a risc evaluation ? This is not trivial stuff. If someone gets hurt, the police will come and ask who is responsible for the safety.
I aggree that traffic lights, maybe combined with a siren or bell may be the solution.

I am guessing that the problem at the melting furnace is when charging or when unloading. At these instances you can have spills blasting all over the place.

You have to block the trucks from entering the dangerous area. Traffic lights or booms comes to mind.
You also have to block charging and unloading when the trucks are allowed to go into the area. Again traffic lights may be the way to go.
Between the two states, there must be a time-delay to allow the truck to evacuate the area, just like traffic lights do in a street crossing "in real life".
The audible alert may be for the short time when changing states.

By the way, are you in Fredrikstad ?