Here is a little trick that will allow you to rest easier when thinking about the risk of setting off a fire sprinkler.
We recommend using a styrofoam faucet insulator to protect sprinklers and keep the air surrounding the sprinkler below the temperature of the air in the room. And in fact, doing that will give you a few degrees of extra protection. However, if you place a dampened rag or wash cloth inside the insulator cup, you will increase your margin of protection by 10 degrees or more.
We proved it by placing a wireless temp sensor in the insulator cup with the damp cloth. In a 130-degree room, the dampened cloth kept the temperature around the sprinkler at a cool 110 degrees.
Since most sprinklers in residential areas are set to activate at 155-156 degrees (red fuses), 110 gives you a 45-degree margin of protection. Try it and see for yourself on your next job.