Heat coils, also known as ''protectors'', ''bugs'' or ''carbons'' serve as a
surge protector
A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect Electronics, electrical devices from voltage s ...
between the
telephone exchange
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
and
outside plant
In telecommunication, the term outside plant has the following meanings:
*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure (such as conduit, cabinets, tower or poles), and any associ ...
. They are commonly the last
point of appearance
Point of appearance is a generic term for any point in a telephone/data circuit from which a technician can test or pull stats. Some appearances are virtual, such as a Digital cross connect system computer terminal. Others are physical, like a pun ...
for a
telephone circuit
In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the common ...
before it leaves the office, for example on the outside plant side of the
main distribution frame
In telephony, a main distribution frame (MDF or main frame) is a signal distribution frame for connecting equipment (inside plant) to cables and subscriber carrier equipment (outside plant).
Overview
The MDF is a termination point within the l ...
. On some
competitive local exchange carrier A competitive local exchange carrier (CLEC), in the United States and Canada, is a telecommunications provider company (sometimes called a " carrier") competing with other, already established carriers, generally the incumbent local exchange carrie ...
circuits there are two heat coils, the extra one being at the point of interface between their circuit and where the
incumbent local exchange carrier or
Regional Bell Operating Company
The Regional Bell Operating Companies (RBOC) are the result of ''United States v. AT&T'', the U.S. Department of Justice antitrust suit against the former American Telephone & Telegraph Company (later known as AT&T Corp.). On January 8, 1982 ...
receives it. Their primary purpose is to protect central office equipment from
surges of high voltage.
If a surge comes down the line it will melt the connection between the central office and outside plant sides, as in a
fuse
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protect ...
, thereby protecting the equipment. Some heat coils have springs in them, so that when a surge breaks the circuit their tension is released and the plastic cover pops off as a visual indicator that the line is somehow defective.
This image fro
The Bell System technical journalprovides a technical schematic and description of '
''a heat coil used to protect telephone exchange equipment against excessive electrical currents that may accidentally come in over the line wires.
References
Telephony equipment
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