Additionally, throwing water on an electric utility is not a good idea either. Using a CO2 fire extinguisher or baking soda will not get one electrocuted like water. Flames can break out when the coffee goes exothermic and allowed to continue cooking beyond burnt. That is the coffee bean's chemical reaction creates heat and coupled with drying out and continued heating will flame. If the coffee is left unattended, this becomes a problem.
Coffee when roasted, gives off a chaff material which is the silverskin.
This material can burn and if it is on fire throwing something on it can spread the small burning chaff, and could spread the fire. The point being, just keep watching the roast. Forgetting about the roast can cause a fire problem. Call the fire department if a fire starts. Negligence, ignorance, and panic are your worst enemies with fire.