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AN34

Secondary Line Protection for T1 and E1 Line Cards

Cirrus Logic 

T1/E1 Line Protection

Maintenance Related Hazards

Although line cards connected to inside lines

may not be directly subjected to worst case light-

ning surge and power cross faults, they are

susceptible to hazards associated with installa-

tion and general handling. Electrostatic discharge

(ESD) can damage CMOS components on line

cards, although most CMOS ICs now have inter-

nal protection which insures good ESD

immunity. ESD damage can be prevented during

maintenance or installation if grounded wrist

straps and work surfaces are used. Also, twisted-

pair cables should be discharged before they are

connected to T1 equipment. Cables may become

charged as they are "pulled" through building

walls during installation. T1 equipment which

uses a current source to provide span power can

be damaged by the start-up transient which can

result when a cable is connected to the port and

current starts to flow.

Care should also be used to avoid the installation

of a line card into the backplane of an operating

system unless the card has been designed to al-

low this. Some line cards have an edge

connector with long power and ground traces

which will insure power is applied before other

signals when the card is inserted into a live

backplane. Cards which are not designed this

way may present signals to CMOS ICs before

power is established triggering SCR latchup.

Crystal Semiconductor uses design and layout

techniques to reduce susceptibility to SCR latch-

up.

T1/E1 line cards can be damaged if they are in-

correctly connected to analog telephone lines

which carry ringing and battery voltages. Dam-

age may also occur if line card output ports are

shorted to each other or to ground. Crystal Semi-

conductor’s newest generation of T1/E1 line

interface ICs have transmitter short-circuit cur-

rent limiting to help prevent damage during

output shorts.

TRANSIENT IMMUNITY AND

ELECTRICAL SAFETY STANDARDS

Transient immunity and electrical safety require-

ments for T1 equipment are defined in standards

published by the United States Federal Commu-

nications Commission (FCC), Underwriters

Laboratories (UL) and Bell Communications Re-

search (Bellcore). In addition, standards bodies

such as The American National Standards Insti-

tute (ANSI) and the Telecommunications

Industry Association (TIA) play an active role in

shaping revisions to these standards.

International standards published by the CCITT,

ETSI and IEC together with country-specific

standards establish the requirements for systems

which are marketed in other countries. CCITT

K.20, IEC-801-5 and ETS 300-046-1 outline

some of these requirements for E1 systems.

The major surge immunity and electrical safety

standards which apply to T1 equipment are FCC

Part 68, UL 1459 and Bellcore TR-NWT-

001089. Article 800-4 of the 1993 National

Electric Code requires that telecommunications

equipment be listed by a Nationally Recognized

Testing Laboratory. [12] UL 1459, Second Edi-

tion, establishes the mandated electrical safety

requirements for telecommunications equipment.

This equipment must also be FCC Part 68 regis-

tered before it may be connected to the public

network. [1] Part 68 establishes requirements to

insure that equipment will not harm the network.

Bellcore TR-NWT-001089 presents new guide-

lines that Regional Bell Operating Companies

(RBOCs) may require for equipment they pur-

chase or allow customers to connect to their

networks. The requirements in TR-NWT-001089

are similar to but not identical to those in UL

1459 and Part 68. The following sections sum-

marize the key requirements applicable to T1

equipment in each of these standards.

AN34REV1

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