why is the liquid fuel in a candle lamp considered safe?
Liquid fuels are classified on a scale between combustible and flammable fuels, based on their flash point. A fuel flash point is the lowest temperature at which a liquid "burns". At the flash point the liquid gives off enough vapour to form a flammable air-vapour mixture near its surface.
For example, the flash point of kerosene is 37 ºC (100 ºF). This temperature can be reached in a number of situation, which is why its storage and transport are strictly controller. If you accidentally spills kerosene or if its container leaks, the ethanol can form begin forming a flammable vapor — potentially making the air ready to combust as soon as it is exposed to an ignition source.
On the other hand, the fuel used in a candle is paraffin oil. Paraffin oil has a comparatively high flash point - over 100 ºC (212 ºF) - putting it in the least hazardous combustible liquid class (III B), alongside coconut oil and olive oil.