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AN34

Secondary Line Protection for T1 and E1 Line Cards

Cirrus Logic 

AN34

Application Note

Secondary Line Protection for T1 and E1 Line Cards

Roger Taylor

The lower cost of high speed digital T1 and E1

trunk lines has resulted in the increasing deploy-

ment of this technology in place of traditional

analog lines. In the past, T1/E1 trunks were used

primarily in protected telephone company central

office transmission applications, but now they

are used outside the central office to bring higher

capacity service directly to customers. The

broader application of this technology makes it

important for line card designers to include sec-

ondary protection circuitry in their designs.

Line cards are exposed to a wide variety of elec-

trical hazards through the transmission lines

which connect them to the network. Even line

cards installed on inside lines in a protected cen-

tral office or customer premises environment are

subject to line induced hazards capable of dam-

aging line interface ICs. Line cards connected to

outside lines are especially subject to the hazards

of lightning surges and contact with AC power

distribution facilities. The use of appropriate

board-level secondary protection devices can

dramatically improve the reliability and robust-

ness of line cards in all applications.

This application note is organized into four ma-

jor sections. The first is a description of the

hazards line cards are exposed to through trans-

mission lines. This section establishes the

practical importance of protection circuitry. Pre-

sented next, is an overview of the major

transient immunity and electrical safety stand-

ards which establish realistic immunity levels

against these threats. Then, a comprehensive sur-

vey of the many transient protection devices now

available helps explain which components are

best suited to T1/E1 applications. Finally, three

application circuits with line protection are pre-

sented along with explanation of how each

responds to various faults.

LINE CARD THREATS

There are three general types of threats to which

line cards are exposed: lightning surges, AC

power faults, and improper installation or main-

tenance practices.

Lightning Surges

T1/E1 Line cards are exposed to lightning in-

duced damage through surges coupled either

through their transmission lines or through the

AC power system. Although power supplies are

often designed with excellent surge immunity,

many line card designs do not provide adequate

board level protection to prevent damage during

lightning surges through transmission line con-

nections. The type of line protection a card

requires depends upon the nature of the lines it

is connected to.

For outside lines, spark gap or gas tube primary

protectors are installed at the network interface.

Unfortunately, these devices will not limit volt-

ages and currents to levels safe for

semiconductor ICs on line cards. Primary protec-

tors are only designed to limit voltages and

currents below levels where premises wiring

might ignite. Because of the way they are con-

nected, primary protectors often compound the

Crystal Semiconductor Corporation

P.O. Box 17847, Austin, TX 78760

(512) 445 7222 Fax: (512) 462-2723

http://www.crystal.com

Copyright © Crystal Semiconductor Corporation 1997

(All Rights Reserved)

SEPT ’94

AN34REV1

1

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