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Phone Line Interfacing - FCC Part-68

Discussion in 'Electronic Design' started by mpm, May 20, 2008.

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  1. mpm

    mpm Guest

    I'm on a data-collection project, and the terminal will download via
    POTS.

    My question is: We're using the MultiTech Socket Modem (v.34, because
    it's low cost).
    In the documentation, they show several ways to interface to the phone
    line.

    We will have a choke in-line for the tip and ring for common-mode.
    And a resettable fuse. So far, so good.

    But I have a question:
    MultiTech shows a paralleled 220pf 5kV cap & a sidactor (transorb),
    with one set each on both the tip and ring. The other side goes to
    "FGND". Any idea what this means, as it's not referenced anywhere
    else in the document??

    I am assuming Earth ground, or at least some ground other than the
    power supply ground driving the rest of the circuity. (There is also
    an analog ground for the modem speaker - which we're not
    implementing...)

    Our box will be just that: A plastic box with a membrane keypad, an
    LCD, a 9VDC 2-conductor wall-wart power supply, and of course, an
    RJ-11 for the phone line. There will not be an earth-grounded
    conductor.

    Should we bother with the (Y2-rated safety) caps and sidactor
    (transorb) protection, or just go with the common mode choke and
    inline fuse? What is "safe", if anything, to connect to power
    supply ground? Will the choke & fuse only arrangement pass FCC -68
    requirements?

    Thanks. (Seems like ages since I did any work with embedded dial-up
    modems!)
    -mpm
     
  2. mjkaras

    mjkaras Guest

    For some reasonable ideas go to a discount electronics store and find
    some low cost POTS product that has a wall wart type powersupply. Buy
    it and then rip that open and it will likely answer almost all your
    questions.

    - mkaras
     
  3. CampKohler

    CampKohler Guest

    I bet FGRD means equipment frame ground, which typically is connected
    to the power grounding conductor, i.e. the third prong. They want
    phone line surges to be shunted to earth. Of course, if you want your
    equipment floating at God knows what when lightning strikes, well,
    that's your choice, isn't it? Now telco will provide a protector that
    is (or should be) referenced to earth, but then they designed it to
    protect their equipment, not yours. And then there's the matter of
    what your modem is connected to and how important it is that that
    doesn't get blown, too. If you can't get at the building power ground,
    I''d go with a nice groundy cold water pipe.
     
  4. mpm

    mpm Guest

    If you can remember the old days (when we all had dial-up modems),
    they weren't connected to an earth ground.
    They had an RJ-11 jack, and a wall-wart power supply. That's it.

    And those modems were built by the millions. So, surely, safety has
    been addressed absent a frame ground.

    I suspect the "answer" is to just use the common-mode choke and a
    resettable fuse.
    The other stuff would be of benefit if you had a frame ground, but in
    this case (as in so many others) we do not.

    I'm probably just going to call MultiTech tech support today to get
    their opinion.
    But you're probably right. FGND likely means frame ground.

    Thanks!!
     
  5. w_tom

    w_tom Guest

    And that is what a properly earthed phone appliance has today.
    Nothing more than a phone line connection and a power supply. Earth
    grounding the phone appliance does nothing useful. An incoming phone
    wire must be earthed where it enters the building - a short
    connection. And yes, that was also standard even in the 1950s -
    before wall warts and modems.

    If not properly earthed where entering a building, then your filter
    choke will easily be overwhelmed - made irrelevant.
     
  6. JosephKK

    JosephKK Guest

    IME, just but a standard DAA line hybrid, with the certifications, and
    forget about it. It moved the demarcation point a bit in your favor.
     
  7. rickman

    rickman Guest


    If your equipment does not have an electrical connection to anything
    else, then you don't need the fancy earth ground and transorbs. The
    transorbs aren't really safety related, IIRC. There are two things
    you need to protect phone line equipment from, lightning caused surges
    (not necessarily a direct strike) and AC mains crosses. The lightning
    stuff is more of an equipment issue. There is not much of anything
    that you can do which will protect in the event of a direct strike to
    the phone line. It will roll in on the wire and fry every thing
    connected, protection or no. But a close by strike will create a
    voltage/current surge in the line which can be protected against
    depending on the strength.

    A power mains cross with the phone line is a different matter. This
    can very easily kill anyone touching the equipment if it is not
    isolated. This is not just the 110 or 220 power coming into the
    house, but can be one of the higher voltages common on the poles. I
    want to say the requirements are for 5kV of isolation. That can only
    be done with transformers, opto-isolators or choppers/capacitors.

    Maybe the requirement is only for 2kV. I seem to recall that on one
    design I did about 8 years ago, the chip I was using chopped the
    signal into a square wave at high frequency which was passed through 2
    kV caps. On the protected side the signal was recovered. BTW, it was
    a CP Clare chip and it has ***NO*** power supply rejection. If you
    had 10 mV of noise on the power rail, you had 10 mV of noise in your
    signal. And believe it or not, you can hear 10 mV of noise on a phone
    line.
     
  8. mpm

    mpm Guest

    You know Paul, that's an astute observation.
    Pin-1 was equipment ground on a DB-25, though a lot of manufacturers
    didn't use it that way (if at all).
    And I can't tell you how many (ground loop) problems I encountered
    with cableing that occasionally connected the electrical ground to
    circuit common...

    This MultiTech modem is a pretty neat device. Basically, an embedded
    block - that will sit as a daughter board on the finished product. It
    doesn't need RS232 levels, because you can "talk" to it with simple
    logic level.

    So, as I was already in the "frame" of mind (no pun intended) to
    ignore all the modem RS232 stuff, I had forgotten about Pin-1. The
    socket modem has a DAA on it, so really, all I need to do is take care
    of the common mode rejection, and fuse the line. Well, that and watch
    where I put the traces on the PCB.

    Thanks.
    -mpm
     
  9. w_tom

    w_tom Guest

    So a telephone Central Office computer; connected to overhead wires
    all over town; that surges maybe 100 surges during every
    thunderstorm. That computer must be replaced after every
    thunderstorm? Of course not. Due to a properly earth surge protector
    where wires enter the building, then direct lightning strikes cause no
    damage.

    Telcos install an earthed protector to protect their equipment.
    Telco's protector on your end may do same for your equipment. But only
    as effectively as an earth ground that you have provided. Surges that
    do not enter a building need not cause damage. Common mode choke is
    easily compromised if a surge has no other path to earth. Common mode
    choke as secondary protection works if surges are diverted to earth
    via the 'whole house' protector. Surge needs a path to earth.
     
  10. w_tom

    w_tom Guest

    Let's see. Verizon only started to install FIOS a few years ago.
    Most every wire connecting to the CO is copper. And for 100 years,
    direct lightning strikes to that copper resulted in no damage when a
    protector was properly earthed where it enter the building. Michael A
    Terrell does not deny this reality.

    FCC Part 68 requires this properly earthed protector at both ends of
    a phone line. What makes a protector so effective? A protector is
    only as effective as its earth ground.. No protector stops, blocks,
    or absorbs the typically destructive surge. A protector that earths
    before those surges can enter a building means protection inside a
    modem is not overwhelmed. That was known even 100 years ago.

    Two 'top of the front page' articles in Electrical Engineering Times
    define what provides protection in "Protecting Electrical Devices from
    Lightning Transients". Not a protector circuit. Protection is only
    as effective as its earth ground and connection to earth ground. How
    curious. Even FCC Part 68 requires a shorter connection to earth -
    contradicting what Michael posts.

    mpm - more could be learned. Not yet posted because engineering
    questions are not being asked.

    Makes little difference whether underground or overhead (Michael
    ignores this reality only to argue). Same protection was required as
    has been routine in telcos for 100 years to avoid surge damage.
     
  11. rickman

    rickman Guest

    To the best of my knowledge, they are removing very little, if any, of
    the old phone lines. The fiber is mostly in new neighborhoods. My
    house is served by sort of copper to a CO that was put in some 50 or
    60 years ago. Somewhere in the last 30 years they were running out of
    copper pairs out this way and they installed a "pair gain amp" which
    is a type of multiplexor. As a result, I can't get DSL or even 56K
    modem connections. I am lucky to get 28 kbps connections... and yes,
    the lines are on poles for most of the mile to the CO. 20 years ago
    someone (this city I believe) came up with the bucks to bury all the
    phone and electric lines in the main part of town. Otherwise, if the
    wires were on poles 50 years ago, they are *still* on poles here.
    I don't know why you think they don't still maintain the poles and
    lines overhead. It costs real money to bury that stuff and I may be
    wrong, but I think they have to get right of way to bury the lines.
    They did here, but that was downtown where they had to tear up the
    sidewalk to bury them.

    I agree with that. The protectors will not protect against direct
    lightning strikes. That can put thousands of volts on the wire and
    hundreds of amps and actually melt the wire. Even a nearby strike can
    induce enough current and voltage in a loop to arc through
    insulation. I have seen this with my own eyes. No sign of a direct
    strike, but split insulation and melted wire at each point that was
    near a ground.

    But new installation is not the same as replacing prior
    installations. Maybe in Florida they have incentive to bury the lines
    because of frequent storm damage. Here the phone company won't even
    consider burying lines with their own dime.

    I only wish they would replace the CO with smaller, more local
    equipment that would support some sort of high speed. You are one of
    the lucky ones.


    Rick
     
  12. mpm

    mpm Guest


    There's just too much bullshit, and not any answers worth replying to.
    For instance, your claim that Part-68 specifies the grounding.
    It doesn't. (Or at least I did not see it?)
    See: http://www.access.gpo.gov/nara/cfr/waisidx_07/47cfr68_07.html

    To my reading, that information is contained within the document
    specified by Rule 47CFR68.7(b)
    In fact, I didn't see the word "ground" anywhere in the entire set of
    rules.

    Now, back to the questions / assumptions / whatever.
    Michael only asked where that much arial cable was installed, I did
    not read that as argumentative??
    Anyway, those of us who KNOW, (and that includes Michael as I am aware
    of his background having spoken to him by telephone) can attest
    that:

    Phones line protectors do not always work. No matter how good the
    protection is, a direct strike can (and usually does) kill it.
    The "problem" is most engineers don't truly know what a direct strike
    is. What a lot of folks think is a direct strike is actually one that
    is some distance away, even though there may still be a slight fault
    current flowing in the Earth. Any ground conductor (ground rod,
    etc..) that is driven into soil that is actively conducting a
    lightning strike is not truly "ground", and is offering somewhat less
    than optimal "protection", if indeed any protection at all. If you
    care to know more: I refer you to any of the excellent texts by Martin
    Uman. (University of Florida Press)

    Whether under/above ground or copper / SLIC-fiber electronics, they
    all suffer from this type of damage... eventually.
    Generally, underground is better (geometry), and optics are better
    (lack of conductivity).

    As for Telco technology, most of the US (which is where we're
    deploying the devices) is rather updated, but this is not universally
    the case. For example, I know a handful of places that still have 5-
    digit permissive dialing on an ESS-5 or earlier switch. And mostly
    copper facilities...
     
  13. w_tom

    w_tom Guest

    Part 68 requires earthing per the National Electrical Code. NEC says
    that phone line must be earthed by a wire of less than 20 feet. That
    is for human safety. For surge protection, we both meet and exceed
    that earthing requirement. For example, better surge protection means
    that earthing wire is less than 10 feet AND is the same earth ground
    used by every incoming wire.

    When a phone line protector does not work, the human has foolishly
    assumed the protector provides that protection. It does not for the
    same reason that lightning rods are also only as effective as their
    earth ground.

    When protection fails, the study (by engineers, not by techs such as
    Michael Terrell) starts with identifying a defective earth ground.
    Again, what does Electrical Engineering Times discuss in an articles
    entitled "Protecting Electrical Devices from Lightning Transients"?
    Earth ground and connections to earth ground. Where popular myth is
    common, somehow the protector will stop what three miles of sky could
    not. Obviously not. Surge protection is about dissipating surge
    energy where it causes no harm. Protector simply connects surge
    energy into earth. Protectors that are too far from earth also
    permits surges to find earth ground, destructively, via household
    appliances.

    You have stated that some "protectors do not always work." Of
    course. The protectors are only connecting devices. Protection is
    determines by the connection to and quality of earth ground.

    The direct strike is when lightning strikes wires entering your
    building. Effective surge protection earths that direct strike
    without damage to the protector. Every phone wire entering the CO
    from so many subscribers Is (was) copper. Why do telco switching
    centers suffer typically 100 surges during every thunderstorm without
    any damage. That CO has the same protector installed at your building
    - and an even better earth ground.

    Let's see what professionals do to eliminate surge damage. For
    example, Orange County FL emergency response facilities suffered
    damage from lightning. Any damage is unacceptable. So Orange County
    fixed the only reason for their surge damage: earth ground.
    http://www.psihq.com/AllCopper.htm

    In Nebraska, a radio station suffered damage from lightning.
    Finally, they decided to stop listening to myths; consulted a
    professional. Well, the professional restored earth ground
    disconnected by technicians who had listened to myths. And
    professionals upgraded the earthing:
    http://www.copper.org/applications/electrical/pq/casestudy/nebraska.html
    Or learn from another professional what an effective surge protector
    does:
    http://www.telebyteusa.com/primer/ch6.htm
    Same principles of surge protection apply to radio stations,
    incoming AC electric, or telephone. Surges are earthed before
    entering a building or will find destructive paths inside that
    building. From another professional:
    http://www.harvardrepeater.org/news/lightning.html
    A benchmark for surge protection is Polyphaser. Polyphaser makes a
    protector that has no connection to earth ground. Why? Because
    increased distance to earth means less protection. That Polyphaser
    protector mounts ON earth ground - zero feet away. But again,
    Polyphaser application notes are considered legendary by those who
    learned the science rather than the many myths that were posted here
    by others. What does Polyphaser discuss? Earth ground:
    http://www.polyphaser.com/technical_notes.aspx

    Anyone who knows or demands surge protection discuss earth ground.
    Sun Microsystems Planning Guide for the Server Room (contrary to the
    myths posted by others) also says what provides surge protection:
    Section 6.4.7 Lightning Protection:
    Surge protection is determined at the building level by earth ground
    and having all utilities enter at a common service entrance. If the
    building was constructed erroneously, a utility explains how to
    compensate for that defect:
    http://www.cinergy.com/surge/ttip08.htm
    That app note shows wrong, right, and preferred earthing because
    earthing provided the protector. But then QST Magazine (the ARRL)
    says the same thing in July 2002:
    How curious. That short (low impedance) connection to the single
    point earthing electrode (SPGP) is also what both 'top of the front
    page' Electrical Engineering Times articles said. However this
    remains contrary to many technicians with a history of knowing only
    what they were first told.

    Even the US Air Force says the effective protector must be connected
    short to earth and where utility wires enter the building. As does
    lightningsafety.com. As do multiple standards from the IEEE. As
    does Dr Kenneth Schneider in:
    http://www.arcelect.com/lightnin.htm
    Decide based upon the science or because you like someone. These
    are mutually exclusive conclusions. The person you like has
    repeatedly posted in technical error - in direct contradiction to the
    science from even 100 years ago.

    Yes, an earth ground rod is not optimal protection. But the single
    point earth ground rod is massive protection. Then we spend massively
    more money to only achieve a little more protection in telephone
    Central Offices. If earth is conductive, one earth ground rod is
    sufficient. But if in FL sand and if no surge damage is ever
    acceptable, then we spend massively more for that little better
    protection:
    http://scott-inc.com/html/ufer.htm
    http://members.aol.com/gfretwell/ufer.jpg
    Demonstrated is Ufer grounds; originally created to that direct
    lightning strikes to munitions dump cause no explosion. Just more
    examples of how direct lightning strikes must not cause damage. In
    every case, that protection is always about a better conductive path
    to earth. No earth ground means no effective protection.

    The original question is about surge protection for a POTS modem. A
    common mode choke is only supplementary protection. It will be all
    but useless if the shunt mode protector connected to earth ground does
    not exist. Once that 'whole house' protector is properly earthed,
    only then might the common mode choke provide additional protection.
    Without a better path to earth before entering a building, that common
    mode choke will only provide same protection that already exists
    inside a modem - easily overwhelmed by the typically destructive
    surge. A professionals always demonstrate, surge protection started
    by upgrading earth ground to meet and to exceed post 1990 National
    Electrical Code requirements.

    FCC Part 68.215d(4) requires earthing meet NEC Article 800. That is
    the FCC requiring a short connection to earth ground. Article
    800.40.A.4
    As every engineering citation notes, every foot shorter than 20 feet
    (and no sharp bends, separated from other wires, etc) means even
    better protection. The protector is only as effective as its earth
    ground - what provides protection.
     
  14. rickman

    rickman Guest

    It may be that copper is being replaced in some situations. But I
    expect that was trunk lines of some sort and not the wiring direct to
    homes. Also, you are describing installations that are already
    underground. Like I said, there are few poles being taken down to
    install fiber.

    Is this a fact? I understand that 87.4% of all statistics are made
    up. I know for a fact that woodpeckers don't attack phone poles.
    Woodpeckers bore holes to get insects. There are no insects in poles
    unless they are already rotten and need to be taken down.
    In urban areas, it is *not* the same right of way. A phone company
    may have right of way for the poles, but that does not give them the
    right to dig up and bury lines. But the real issue is money. It is
    very expensive to take out equipment and bury it.
    It's not an issue of soil. In an urban area they have to dig up
    streets and sidewalk. That is not cheap. It may work better, but
    they don't rip out stuff that works. But I don't live in FL. Maybe it
    really is worth doing on their own. If so, great. But the rest of
    the country still has *lots* of phone lines.
    I am in an area where there are few if any spare lines. That is why I
    only get 28kbps on my modem. I am not on a copper pair. I share
    copper pairs with a bunch of others through an antique multiplexor. I
    only wish they would replace that piece of crap so I could get high
    speed.

    At least you have it. DSL is only something I hear about... a lot!

    Rick
     
  15. rickman

    rickman Guest

    Please don't be insulting. It is not a matter of belief. It is a
    matter of fact. You can do an incredible amount of repairs to
    equipment before it becomes cheaper than wholesale replacement and
    burying. I think I already said that they did that here in a part of
    town, not because it would save any money, because the city paid for
    it in order to improve the looks of the downtown. The phone and power
    companies would have *never* done it on their own because of the
    enormous cost of tearing up sidewalk and burying lines. Why do you
    think they strung the lines in the first place, because it is so cheap
    to do it that way.

    I have no idea why you say,"everyone shares on copper". If you are on
    copper, you typically have a connection directly to the switching
    office. That is why they call it "copper". Anything else requires
    conversions from A to D and back to A. In my case the A-D-A is done
    between me and the CO. Then it is digitized again in the ISPs modem,
    but the levels have already been quantized and they don't line up with
    the modem's levels. So they can't use the V.90 technology and the
    rate is much lower..

    We really aren't communicating. I don't care how slow DSL is compared
    to cable. It is absurdly faster than dialup and lame dialup at that!
    Around here DSL is 512 and higher for most folks. Of course that
    varies with your distance from the CO. But at some distance they just
    won't give you DSL because of the speed problems.
     
  16. David Lesher

    David Lesher Guest

    Interestingly, I just looked on a walk and found many 1964 poles
    in this DC region....



    You guys are mixing apples and coconuts...and ignoring the important
    ISO layer [below..]

    A) The Old Bitch really is Her bastard children. They don't always do the
    same thing. (But as they get reassimilated into the New Bitch....)

    B) When short of copper last mile, or serving some
    new tickytackytown; the Kids often installed SLC's.
    <http://www.rayvaughan.com/images/telecom/RT/MVC-904Fb.JPG> SLC's save
    copper. Copper co$t$.

    SLC is really a WECO oops Lucent oops Alcatel trademark, but like
    "Kleenex..." it gets misused. It's a mux; it takes low-to-mid hundreds of
    local copper loops and puts 'em on a multiple DS-1's at first, but now
    onto fiber.

    Some Kids [BellSouth] routinely put DSLAMs into their SLC's so you could
    get ADSL. Others refused to.

    C) Occasionally, a Kid used a Pair-Gain [TM]; a 2 {lines} for the price
    of one {pair} solution. It usually ran ISDN internally, but you never
    knew it. The gray NIT on your house was changed and you magically got
    2 lines. But they are rare vice SLC's.

    D) As for protectors, for years She didn't care a whole lot. The
    protector was designed to protect against lawsuits from customers getting
    zapped [AND loud bangs in their ear...]; the 300 series phone didn't have
    anything to get blown up, and 500 series had just varisters. PLUS, they sat
    on insulated feet on your table.

    Later, 2500's and 1A2 came around & She started getting more serious about
    protectors. Three-element gas tubes are often found; they do a good job
    but tend to die while on guard duty.

    E) Lastly; Verizontal has been hyping FIOS; fiber to the home. If you succumb,
    they WILL cut down your copper feed [and lie in House testimony about doing so]
    forcing your "POTS" to require power you supply.

    That's because of the most important ISO layer: political. The Kids
    MUST share existing copper with competitors [COVAD, Speakeasy] but does
    NOT have to rent transit on the glass; so by forcing you off copper and
    cutting it down, they sabotage any competition....
     
  17. David Lesher

    David Lesher Guest




    You're dealing with the telephone company. We are not subject to
    city, state, or federal legislation. We are omnipotent.

    Ernestine the Telephone Operator
     
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