On Thu, 28 Oct 2004 20:16:58 UTC, ranck@vt.edu wrote:
> Matt <spammers@are.bad.com> wrote:
>> The phone line for the caretaker and ONE of the two office
>> lines experience a continual hum as well as a click, click, click,
>> click, click, every time the electric fence fires off. The program
>> director does not experience any known issue on his line, and the
>> other line in the office is fine. I find this very odd, since both of
>> the office lines come in (presumably) on the same cable?
>> Any thoughts? Verizon is kinda stumped on this issue, so I'm
>> trying to see if I can figure anything out to help them out. Does
>> this sound like a grounding issue? If so is it at the demarc box? Or
>> on a line some place? Why only one phone line and not the other?
> Many years ago, when dinosaurs roamed the earth, I looked after some
> 4-wire leased circuits that we used for data communications.
> Occasionally, usually after a thunderstorm, one of those lines would
> get noisy. I would call what was then a Bell company and report noise
> on circuit number blah. The trouble ticker writer would ask for the
> phone number, which I didn't have because it was a leased 4 wire, not
> a real phone line, but they'd take down the circuit number and almost
> exactly 20 minutes later I would get a call from a technician asking
> me how I knew there was noise on that cicuit. I learned that if I
> told them the truth, that I had put an oscilloscope on there and could
> see it, they would get upset. After some further conversation they'd
> go and check it out. After the techs got to know me, they would tell
> me more and the noise usually turned out to be caused by what they
> called "bad carbons." These are some sort of grounded protective
> devices that apparently get leaky some times. I am willing to bet
> that is what the problem is for you. Now, good luck getting anyone to
> understand the old Bell System term "bad carbons." I assume these are
> sort of surge suppressors of some sort, but what the modern phone
> company might call them is anybody's guess.
> Bill Ranck
> Blacksburg, Va.
> [TELECOM Digest Editor's Note: The most unusual thing I ever saw which
> was sort of like that was when I worked for a place which used wire
> contacts on their doors and windows for a security alarm company. One
> night when it was time to go home, I could not get the alarm to 'set'
> and had to call the alarm company to get them to come over and fix it.
> The trouble when this happened was usually around the elevator doors
> which would slide open and closed. For continuity in the wire, there
> was a wire grill like thing which had to be put across the elevator
> door opening when you were leaving the premises for the night. The
> repairman came out (he had a buttset with him with clamps on the ends
> of the wires) and he stood there at the elevator where the grill went
> across the doors, clamped on his buttset and *talked* through the
> wires to the office where he worked, where someone else was making
> adjustments as they talked. Now that I think about it, I guess there
> is no reason you could not carry on a conversation over the wires of
> an alarm system. PAT]
OK, you're in luck, I know the origin of the term "bad carbons". A
carbon protector is a "spark gap" like device that uses carbon
electrodes (acually small blocks of carbon) separated by 3 mils of air
for an insulator. This air gap breaks down somewhere between 500
volts and 1000 volts. One carbon block of the pair is tied to local
ground, the other to either tip or ring. The entire protector has two
pairs of blocks (one pair for tip, one for ring). The idea is that if
a surge (lightning induced, typically, but also power company
switching operations on joint use pole lines can also introduce a
surge) exceeds the breakdown voltage of the 3 mil air gap between the
blocks, an arc forms and a low impedance path to ground protects the
premises equipment.
These protectors are used at customer premises entrances, CO entrances
(actually on the MDF), and at aerial to underground junctions (to
protect the underground cable). The NEC requires them on premises
where the circuit runs between buildings (no flames, please, I know
there are exceptions to that general rule). The "problem" is that
when an arc forms it sometimes "pits" the carbon blocks and results in
some carbon granules floating around the air gap. This can cause a
resistance (often highly variable or intermittent) to the grounded
block, unbalancing the line (typically only one of the pairs of blocks
has the problem). Thus the term "bad carbons".
Modern protectors use gas discharge tubes instead of carbon blocks
separated by air. The sealed discharge tube effectively eliminates
the loss of balance due to particles of the electrodes shorting the
gap. Unfortunately, the old carbon blocks are only replaced when they
go bad or a modern NID is installed, so there are still lots of them
out there. They are usually referred to as "station protectors" or
"primary protectors" (to differentiate them from the SCRs and diodes
used to protect the circuit cards. Those are called "secondary
protectors".)
Hope this helps.
Eric Tappert
Eric Tappert (e.tappert.spamnot@worldnet.att.net)