D. McKinnon, ast technology Labs, Inc. (To be ansi/tia-571-B) Telecommunications Telephone Terminal Equipment Environmental Considerations



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B4. 2-Line and 4-Wire Circuits


The overvoltage tests are applied to a representative pair of tip-ring leads for equipment that has multiple lines. If currents are induced into multiple lines going into a piece of equipment, the induced cur-rent in one pair produces an EMF that induces a reversed current in the other pairs. Therefore, not all pairs will have the maximum current, and worse case condition is likely to be testing one pair with the maximum current.

Digital circuits often use a 4-wire (F-type) circuit, one pair for transmit and another pair for receive. A 4-wire circuit is not two 2-line circuits, e.g., the transmit and receive circuits are interconnected. To test both circuits, a 4-wire test was designed to be used as a single test, instead of having several tests on the various paths possible.


B5. Multiple Sets


The telephone line may be connected to several telephone stations (branches). Current in the main line (unbranched) must be limited to I2t=400 to protect the line cord.

A common installation has a telephone set and an answering machine, each of which can terminate the network in a low impedance after an overvoltage event. . If each branch were fused for I2t=400, the main line could see a much higher current. Assuming fault current is evenly distributed to the branches, each branch (i.e., the telephone and answering machine) needs to limit short duration cur-rent to I2t=400/22= 100.


B6. Wiring Simulation


A composite model of a telephone line cord has a limiting l-t characteristic that is determined by the following:

  1. Long duration current limit is just over 2.2 A.

  2. The current limit is just over 7 A at a 5-second duration.

  3. Short duration (adiabatic) current-time characteristic is about I2t=100 .

Characteristics (1) and (2) are within the test parameters. To provide an indication of whether telephone wiring would be damaged during a short duration fault a fuse that opens at I2t=100 is desirable to use for testing purposes. If such a fuse is blown open during testing, the telephone line cord would be damaged. A fuse that meets these parameters is the Bussman MDL-2.

It is not necessary to use a fuse; the wiring model could be used to evaluate test results obtained with a current probe. Also, 32 AWG copper wire has a suitable fusing characteristic to be used as an indicator.



Not all telephone line cords use tinsel wire. When 26 AWG stranded wire (the same wire gauge as riser cable) is used, equipment does not need to limit I2t to 100 because the line cord is considered sufficiently robust.

B7. Primary Protector Coordination


If telecommunications user premises equipment provides a low impedance path to ground (including operation of arrestors that provide a path to ground during surges), a fault current could by-pass the primary protector and result in excessive current through the telephone building wire and the equipment . The building wire can provide coordination if it has enough resistance, which is not always the case. The equipment’s characteristics should coordinate with the protector operation, which is achieved by having a fusing limit of I2t=100.

B8. Test Points


To minimize testing effort, only the worst case test conditions need be evaluated. These usually occur at maximum voltage and current except when voltage or current limiting devices (usually MOVs and fuses, PTCs or fusible resistors) are used. Then, conditions of maximum voltage and current that are not interrupted by limiting devices need be evaluated.

B9. Test Conditions


  1. Overvoltage conditions can be longitudinal or metallic. Both modes need be evaluated independently when equipment has a grounding conductor.

  2. The test conditions apply to both series and terminal equipment. For series equipment testing, terminal equipment is simulated as both a short circuit and an open circuit in separate tests.

  3. No testing is necessary in the following situations:

  1. Longitudinal tests are not necessary if a dielectric barrier exists between tip-ring and ground. Instead, a simpler dielectric test can be conducted.

  2. For metallic tests, series equipment (note that a line cord can be thought of as series equipment) needs to be tested only to M-2 and M-3 when the terminal equipment is simulated as a short circuit because the terminal equipment provides protection for the M-1 test. (M-1, M-2, M-3, and M-4 refer to tests in UL-1950 based on the conditions described in paragraph B2 above and not to the Table in section 4.3.3.3.)

  3. When current (and possibly voltage) limiting is provided by a secondary protector suitable for the purpose, either

  • the test conditions are adjusted so that they do not exceed the ratings of the protector, or

  • the equipment is tested with the protector in place.

B10. Failure Conditions


  1. Fire hazards are evaluated using a cheesecloth indicator wrapped around the equipment under test.

  2. Shock hazards are evaluated with a leakage current test applied after testing. A simpler dielectric test may be used.

  3. Telephone line cord hazards are evaluated against the wiring model (using an indicator fuse, 32 AWG wire or current probe).

ANNEX C (Informative) RATIONALE FOR SURGES (4.3.3)

C1. Sources of surges


The most common source of surges on telephone tip-ring conductors results from a lightning strike to an aerial or buried cable shield. The lightning current flowing on the shield to earth induces a voltage into the cable pairs within the shield. If the ground path offers a high impedance to the lightning cur-rent, the voltage along the shield may build up enough to produce a side flash to the cable pairs within the shield, especially at wire junctions where the only insulation is air spacing. A side flash can be considered a direct lightning strike to tip-ring that is mitigated by a parallel ground path along the shield.

Another source of surges to equipment is via the power service to the building. Lightning surges may enter a building over the serving power service conductors. Such lightning activity can also result in a local ground potential rise with respect to remote earth, which can cause telephone protectors to operate.




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