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Telephone protection circuit needed

Discussion in 'Electronic Design' started by Robert Scott, Jan 24, 2007.

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  1. Robert Scott

    Robert Scott Guest

    In the recent ice storm, the 4800 volts from the power lines crossed over into
    the phone lines. It burned out many circuits in our area. It also fried some
    modems and phones in my house. Surprisingly, two connected modems survived.
    These were both protected by whatever surge protection circuit they put in a
    UPS. But it had the side effect of burning out that circuit in the UPS, so I
    cannot use that again.

    Rather than protect phone equipment piecemeal, I would like to protect all phone
    equipement in my house using one circuit that I put at the entrance. I would
    like to use a fuse. I gather that most commercial "surge protectors" do not
    have fuses, but I think they might be a more reliable protection than varistors
    or whatever they put in those commercial units. Has anyone done this? What
    sort of current limit should I set? I guess the ringing with a maximum number
    of phones connected would be the upper limit. But a fuse might not be enough.
    What about a crowbar circuit? Is it possible to detect an overvoltage condition
    with an active circuit and then short all lines to ground through some
    fast-acting power FETs downstream of the fuses, just to make sure the fuses blow
    right away? I don't mind replacing some power FETs and fuses in my own custom
    circuit once in a while if it means I won't have to replace modems and phones.
    I know ice storms and downed power lines are my focus right now, but I am also
    looking toward spring with lightning will be the major risk.

    Robert Scott
    Ypsilanti, Michigan
  2. D from BC

    D from BC Guest

    On Wed, 24 Jan 2007 18:45:31 GMT, [email protected] (Robert Scott) wrote:
    I haven't done any surge suppression/isolation active clamping
    circuits yet...
    I'm guessing some electronic process like:
    1)Slow down the voltage surge risetime (If not slow already from line
    inductance) to give a slow crowbar cct a chance to react
    2)After the fuse blows from the crowbar cct., there's now a big spark
    gap for cct isolation
    3)Find some place to dump surge energy ...another spark gap?

    Without looking, I'll guess this is a well beaten topic.
    Check achieves.
    D from BC
  3. Robert Scott

    Robert Scott Guest

    Well, I had some melted 22-gauge wire. A 500 ma. fuse would have at least
    protected that wire.

    Robert Scott
    Ypsilanti, Michigan
  4. From past experience, in a protective configuration where you have a
    'fuse' protecting active components it is the silcon/germanium/whatever
    that protects the 'fuse' and fails first.
    Read what is time response of fast fuse against overcurrent percentage.

  5. Jim Thompson

    Jim Thompson Guest

    I've designed some crowbar fuse blower circuits that circulated >100A
    thru a 5A fuse... now you see it, now you don't ;-)

    You have to be very careful with the foil pattern on the PCB to avoid
    it being the fuse ;-)

    ...Jim Thompson
  6. Joerg

    Joerg Guest

    4800V is a whole lotta volts. If the fuse is long enough that plus a
    huge MOV could have saved the day. Long in order to avoid arcing over.
    Of course the fuse would have to be mounted in a way that things don't
    arc over to any place that could ignite and burn down the house.
  7. Robert Scott

    Robert Scott Guest

    This isn't going to be a PCB. It is a one-off home-made thing, hand-wired. I
    was going to use wood for the base, but with all this talk about arcing over,
    maybe I ought to use plastic.

    Robert Scott
    Ypsilanti, Michigan
  8. mpm

    mpm Guest

    Part of the problem is the voltages involved in normal phone operation.
    You have to protect against faults, but still allow ring voltage,
    reverse battery, etc...

    There are many providers of Telco protection equip.
    A great many of them rely on MOV's or Sidactors.
    The problem with MOV is they have a limited lifetime.
    They work perfectly for a long time, show no signs of wear, and then
    "poof!": One day they're "Dead & Gone". No warning whatsoever, so
    they're hard to RELY on.

    Gas discharge tubes also work, but they are slow (and probably would
    not have helped you.) Actually, I wouldn't be surprised to see the
    phone company replacing a bunch of these!

    Active circuits are going to void any chance of continued compliance
    with FCC Rules Part-68, unless you want to go through the testing &
    certification procedures.

    You can always just wind the tip & ring (22 ga) around a pencil. Cheap
    & dirty but it does help.
    10 turns or so ought to work wonders. (For lightning frequencies, not
    too sure about 60 cycles)
    now -- I'd consider myself "lucky"!

  9. w_tom

    w_tom Guest

    What happened to that protector installed free by your telco? A
    protector that is defined by its earth ground connection. Was that
    telco 'provided for free' protector earthed by same electrode also used
    by all other incoming utilities? If not, then interior damage would
    result; appliances inside building might instead become an electrical
    return path - destructively.
  10. Maybe not.
    the _length_ of the fuse, that is how many volts it will stop before flash-over
    is also an issue.
    Lightning will jump a across meter distance if iit feels like it, seen that.
  11. Robert Scott

    Robert Scott Guest

    Actually I did have something like that. It was made of enamelled wire wound
    around a ferrite torriod. I don't know if the inductance helped, but a short
    did develop between tip and ring through the enamel, which welded the copper
    wire together. But that short may have helped to protect a modem that was

    Robert Scott
    Ypsilanti, Michigan
  12. I've designed a 4A SMD fuse blower that circulated 150A and blew some SMD
    fuses in 15uS. The others reformed into resistors and limited the current to
    <4A. Go figure.
  13. Robert Scott

    Robert Scott Guest

    On a related topic, I know gas discharge tubes exist for the purpose of
    overvoltage protection, but I was wondering what could be done with an ordinary
    air gap. I seem to recall hearing somewhere that no matter how close you place
    electrodes in air, you cannot reduce the arcing voltage below a certain
    theoretical limit that has something to do with the voltage required to ionize
    air. (which is why they use gas discharge tubes). Can anyone validate that
    concept for me?

    Robert Scott
    Ypsilanti, Michigan
  14. Andy I.

    Andy I. Guest

    Robert Scott wrote:
    I've posted a schematics of a phone line protector in a.b.s.e,

    Subject "phone line protector", Message-ID:

    I've designed this to produce a very low impact on the ADSL signal.

    The (constructive) feedback / remarks are very welcome.


    -- Andy
  15. Robert Scott

    Robert Scott Guest

    Excuse my ignorance, but what is a.b.s.e. and how can I look at your schematic?

    Robert Scott
    Ypsilanti, Michigan
  16. Robert Scott

    Robert Scott Guest

    OK, I found it. It is in alt.binaries.schematics.electronic

    Thanks for the hint.

    Robert Scott
    Ypsilanti, Michigan
  17. AZ Nomad

    AZ Nomad Guest

    sounds like a binaries newsgroups a.b. would be alt.binaries.,
    perhaps the s.e. are sci.electronics?
  18. Andy I.

    Andy I. Guest

    I should have expanded, sorry about that, I meant the
    "alt.binaries.schematics.electronic" newsgroup.

    A few comments:

    - As with any such circuit, a good ground connection is fundamental, it
    goes to the pin 2 of the CN1 connector. The other two pins of CN1 go to
    the phone line, the home phone equipment is connected to CN2.

    - One should probably aim at a higher voltage rating of the fuses F1 and F2.

    -- Andy
  19. joseph2k

    joseph2k Guest

    Given your location you need at least two stage protection. First stage is
    lightning type telephone line protectors. The second stage should be what
    is called a 5-pin (replaceable) protector. You should be able to find them
    easily with Google.
  20. w_tom

    w_tom Guest

    Protectors are not protection. Shunt mode protectors become
    conductive only during a transient. They do not stop or absorb
    surges. They make a temporary connection (a shunt) to protection.

    Reviewing the schemtic posted by Andy, those fuses also do nothing
    useful for a 4800 volt surge. Fuses are only rated for 600 volts -
    remain conductive when trying to stop many thousands of volts. So
    what provides protection? A protector that connects to earth; a shunt
    to earth so that a surge need not find earthing connection via
    household appliances.

    What is two stage protection? A system that has layers of
    earthing. Same exists with AC electric. 'Whole house' protector
    installed in a mains box makes a short connection to building's
    earthing electrode. That is secondary protection. Primary protector
    provided by the utility is demonstrated in pictures in:

    Meanwhile, the telco installs a 'whole house' type protector on all
    subscriber lines where their wires meet a homeowner's. Again, this
    protector is only as effective as its short ('less than 10 foot')
    connection to a single point earthing electrode. A protector
    installed for free because it is so effective and so inexpensive. A
    protector often unknown to those who somehow want to stop or block
    surges (ie those fuses). Surges are not stopped or blocked. Anything
    that would do that stopping is already inside phone appliances.
    Surges must be diverted to what surges seek. Either a surge is
    earthed before it can enter a building OR surge will seek earth ground
    destructively via household appliances. So that protection already
    inside appliances is not overwhelmed, the effective protector makes
    that short connection to earth.

    Telco install that device, for free, in the NID. A protector that
    is only as effective as earthing provided by the homeowner.
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