The fridge could have caught fire by friction. There is a little mechanical pump driven by the motor, this pumps the refigerant around the pipes at the back of the fridge to cool it. Mechanical wear could be the cause and it may not be electrical at all.
PAT testing, stands for Portable Appliance Testing.
Basically a little machine is hooked up to your appliance and it tests the effectivity of the electrical insulation and how efficient the earthing is. If figures fall within a certain range, it passes if they fall outside of that range and it fails and is considered unsafe to use. PAT testing also includes a visual inspection of the condition of wiring for example. This os a fairly simplified view of a PAT test. If things pass, a certificate is issued often listing the serial No: of the tested equipment