Why did my PROTECTED chest plate break?

[MasterNinad]

I got a fire-resistance enchantment book (which gives fire resistance when an armour piece with it is put on). I had a protected diamond chest plate which had shockwave, molten and port 2. Then why did it break when I tried to apply the book of fire resistance to my chest plate??

 

[ataman]

This thread will now be locked to prevent any further grave-digging.