Heat coils, also known as protectors, bugs or carbons serve as a surge protector between the telephone exchange an outside plant. They are commonly the last point of appearance for a telephone circuit before it leaves the office, for example on the outside plant side of the main distribution frame. On some CLEC circuits there are two heat coils, the extra one being at the point of interface between their circuit and where the ILEC or RBOC receives it. Their primary purpose is to protect central office equipment from surges of high voltage.