Editing Heat coil

{{Short description|Heat coils serve as surge protectors between a telephone exchange and outside plant}}
{{More citations needed|date=April 2022}}
<!-- [[Image:Heat Coil.JPG|right|thumb|350px|The five pins of a heat coil connect the Central Office and Outside Plant (field) and keep it grounded so that the phone will ring.]] -->
'''Heat coils''', also known as ''protectors'', ''bugs'' or ''carbons'' serve as a [[surge protector]] between the [[telephone exchange]] and [[outside plant]]. They are commonly the last [[point of appearance]] for a [[telephone line|telephone circuit]] before it leaves the office, for example on the outside plant side of the [[main distribution frame]]. On some [[competitive local exchange carrier]] 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]] receives it. Their primary purpose is to protect central office equipment from [[voltage spike|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 (electrical)|fuse]], 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 from [https://www.alamy.com/the-bell-system-technical-journal-telecommunication-electric-engineering-communication-electronics-science-technology-712-bell-system-technical-journal-multi-test-machine-automatic-feed-an-automatic-gaging-machine-for-applying-four-tests-to-a-piece-of-telephone-apparatus-is-shown-in-fig-3-the-part-tested-shown-in-fig-4-a-is-a-heat-coil-used-to-protect-telephone-exchange-equipment-against-excessive-electrical-currents-that-may-accidentally-come-in-over-the-line-wires-it-consists-of-a-tiny-coil-wound-around-a-copper-sleeve-into-the-end-of-which-sleeve-is-springs-please-note-t-image234748605.html The Bell System technical journal] provides a technical schematic and description of '<nowiki/>''a heat coil used to protect telephone exchange equipment against excessive electrical currents that may accidentally come in over the line wires.'''<ref>{{Cite web |last=Library Book Collection |first=Alamy Stock Photo |title=. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. 712 BELL SYSTEM TECHNICAL JOURNAL Multi-Test Machine, Automatic Feed An automatic gaging machine for applying four tests to a piece of telephone apparatus is shown in Fig. 3. The part tested, shown in Fig. 4 (a), is a heat coil used to protect telephone exchange equipment against excessive electrical currents that may accidentally come in over the line wires. It consists of a tiny coil wound around a copper sleeve, into the end of which sleeve is SPRINGS. Please note t Stock Photo - Alamy |url=https://www.alamy.com/the-bell-system-technical-journal-telecommunication-electric-engineering-communication-electronics-science-technology-712-bell-system-technical-journal-multi-test-machine-automatic-feed-an-automatic-gaging-machine-for-applying-four-tests-to-a-piece-of-telephone-apparatus-is-shown-in-fig-3-the-part-tested-shown-in-fig-4-a-is-a-heat-coil-used-to-protect-telephone-exchange-equipment-against-excessive-electrical-currents-that-may-accidentally-come-in-over-the-line-wires-it-consists-of-a-tiny-coil-wound-around-a-copper-sleeve-into-the-end-of-which-sleeve-is-springs-please-note-t-image234748605.html |access-date=2022-04-20 |website=www.alamy.com |language=en}}</ref>
<!-- Unsourced image removed: [[Image:Protector Frame.JPG|left|thumb|300px|This is a typical protector frame. Black heat coils are for POTS lines and red are for Special Service. In the upper right corner you can see an empty slot and how those holes correspond to the pins in the schematic on the right.]] -->

==References==
{{Reflist}}

[[Category:Telephony equipment]]


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