Fuses are safety devices used to limit the amount of current through an individual circuit branch, or to limit the amount of current delivered from a voltage source, such as batteries. They will pass current below their rating under normal circumstances and open the circuit when the current rating is exceeded. Limiter fuses are nearly always installed with a protective cover to prevent accidental shorts, shield adjacent components in case of catastrophic failures, and to protect users from electrical shock. The US Coast Guard requires fuse holders installed on marine vessels to have a protective cover in order to comply with the law.