UVA UVB Mercury Vapor Bulb - 100W Sun Simulating Bulb High Intensity UVA UVB Light Heat Bulb for Reptile and Amphibian Use AUTO SHUT OFF PROTECT - A safety feature called "THERMAL RESET" can intelligent control the temperature and prevent burns by excessive heating.Simply cool down it around 15-20 minutes and then restart it. KEEP YOUR REPTLES HEALTHY  Self-ballasted mercury vapor reptile bulb provides the benefits of natural sunlight, which is one of the most important aspects in keeping reptiles healthy ALL IN ONE - The choice of professional breeders as it provides the proper balance of ultraviolet light (including UVA and UVB), visual light and infrared light (heat), in one easy-to-install bulb. PLEASE CHECK PICTURES FOR SIZING TO ENSURE THE BULB IS NOT TOO LARGE FOR YOUR FIXTURE. A FULL SPECTURM LIGHT - With carefully tuned peaks to ensure appetite, activity, brilliant colours, and optimal calcium absorption through Vitamin D3 production. SELF-BALLASTED - Screws into a standard clamp lamp fixture with a ceramic socket (No ballast needed) Optimal levels of UVB,UVA, visual light and heat in one bulb Provides the benefits of natural sunlight Helps prevent metabolic bone disease Increased UVB penetration distance (+30cm / 12) Fits into a standard ceramic socket (NO Ballast Needed) The power being supplied to your outlet is inappropriate or fluctuating. Small changes in line voltage can trip the bulbs thermal reset mechanism. If the voltage is consistently low then the bulb will not be able to function properly. Check with your local utility or your electrician to verify your voltage. Overdriving the bulb (i.e. higher voltage levels) by as little as 10% can decrease burn life by more than 50%. There is too much heat in or around the terrarium and the bulb. Its thermal reset feature will shut the bulb off for a cool down period if it gets too hot. Using the bulb in a place or fixture that is not well ventilated can cause heat buildup. If the bulb occasionally shuts off to cool down and later restarts (usually about 15-20 minutes) this is normal; however, if the bulb shuts off all the time or for several hours at a time, contact the seller and ask for help.