Crossover Fuse Bulb Tweeter Protector and it scientific level description

[Kiriakos GR]

At ITTSB Europe, there is no section in favor Pro Audio system repairs. 

But I did accept the challenge so to diagnose damages over a  PA speaker EVENT 4000 Series 
This is a 400W continues PA speaker. 

For a first time, I came across of Crossover Fuse Bulb for Tweeter protection.
This monster uses a 24V 70W bulb.

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In my attempt to locate spares, I did find accidentally the scientific level description   

Fuse Bulb/Lamp with positive temperature co-efficient.
As the power to the tweeter increases, the lamp will glow a little, increasing the resistance of the filament, effectively acting as a compressor and reducing harmful power spikes which could damage the tweeter.
Excessive power will cause the lamp to blow, in this instance it acts as a fuse and a 'last line of defence'

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Unfortunately for this PA speaker owner, 'last line of defence' did not hold the enemy away.
And now him faces one large bill, about spare parts and labor. 

Tweeter unit along 15" woofer and 8" mid-range all dead.
The catastrophic event, was so powerful, that all drive coils were not sorted as I would expect, but they are all open instead = Coil wire melted.
And neither Fuse Bulb/Lamp this survived.

[overvolt]

It is a fact that you will never receive such a lesson in a class room full of electricians. 

Do they teach that at the class room for electronics trainee?