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AN34

Secondary Line Protection for T1 and E1 Line Cards

Cirrus Logic 

T1/E1 Line Protection

should be designed to withstand these surges to

insure reliable operation in all environments.

UL1459

UL 1459, Second Edition, establishes the safety

requirements for telecommunications equipment

mandated by the National Electrical Code. This

standard is focused primarily upon fire and elec-

trical shock hazards and includes guidelines on

equipment enclosures, printed circuit board flam-

mability, and user accessable voltages and

currents. (Refer to [12] for an in depth overview

of the requirements in UL 1459 - Second Edi-

tion.)

The overvoltage tests for telecommunication

ports on outside lines are summarized in Table

8. These tests are applied in longitudinal and

metallic configurations through a resistor and a

fuse used to simulate premises wiring. As in the

TR-NWT-001089 AC power fault tests, the

equipment is allowed to fail open circuit as long

as it does not become a fire or safety hazard.

For series type equipment (such as a CSU/DSU),

longitudinal testing is conducted on the network

port with the terminal equipment port open. Me-

tallic testing is conducted on the network port

with the terminal equipment port short circuited.

Test

Min.

Voltage

[VRMS]

Min.

Current

[A]

Duration

M-1, L-1

600

40

1.5 sec.

M-2, L-2

600

7

M-3, L-3

6003

2.2

M-4, L-4

2004

2.2

5 sec.

30 min.

30 min.

L-5

120

25

30 min.

Notes:

1. L = Longitudinal Mode, M = Metallic Mode

2. Primary protectors removed.

3. Test repeated with current set just below the interrupting current

of any secondary current limiting devices.

4. Applicable to equipment with Voltage limiting devices operating

between 200 - 600 V. Voltage adjusted below the operating

threshold of the secondary Voltage limiting devices.

Table 8. UL1459 Overvoltage Tests

8

PROTECTION COMPONENTS

This section provides an overview of the wide

variety of components typically used to provide

line protection for communications equipment. A

brief explanation of the operating principles, ad-

vantages and disadvantages of each component

is provided in order to point out the devices best

suited for T1/E1 line protection applications.

Metal Oxide Varistors

Metal oxide varistors, or MOVs, are devices

made from a zinc oxide material whose resis-

tance varies with terminal voltage. MOVs

provide bipolar over-voltage clamping protection

and are placed in parallel with the circuitry they

are protecting. An MOV is selected by specify-

ing its maximum operating voltage and

breakdown voltage. [13] At and below the oper-

ating voltage, the MOV presents a very high

impedance. However, when its terminal voltage

reaches the positive (or negative) breakdown

voltage, the MOV’s impedance decreases dra-

matically limiting the voltage near the

breakdown voltage.

MOVs are designed to handle large surge cur-

rents, and are commonly manufactured in a disk

form factor with large cross-sectional area to re-

duce point current density. Its large

cross-sectional area unfortunately gives the

MOV a large parasitic capacitance of as much as

10 nF in some models. [11] The parasitic capaci-

tance of an MOV is inversely proportional to

clamping voltage. [11] These devices are fast

acting (with response times of a few nanosec-

onds), but they have a finite lifetime and will

gradually degrade (with a reduction in clamping

voltage and increased leakage) when subjected

to a number of transients. [11] MOVs must be

derated when specified for operation with repeti-

tive transients. [13]

MOVs are not designed for continuous currents

because they exhibit a negative temperature co-

AN34REV1

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