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AN34

Secondary Line Protection for T1 and E1 Line Cards

Cirrus Logic 

T1/E1 Line Protection

danger of surges on outside T1 lines. Typically,

gas tubes or carbon block arrestors are installed

on outside T1 lines between TIP to ground and

RING to ground. A lightning surge will generate

a longitudinal, or common-mode, transient on

both TIP and RING conductors. Ideally, the lon-

gitudinal surge should not damage components

on the card because the line coupling transform-

ers will not pass a common mode signal.

Unfortunately, the line coupling transformer’s in-

terwinding capacitance does couple fast

transients across the transformer potentially dam-

aging sensitive components. Also, the primary

protection components will convert some of the

longitudinal transient energy to a metallic tran-

sient because the two protectors on each

TIP/RING pair breakdown asymmetrically. [2]

When one of the primary protectors fails or lim-

its at different potential than the other, a smaller

metallic TIP to RING transient results. This

smaller metallic transient will be coupled across

the line transformers and may also damage sen-

sitive components on the line card.

The primary protectors with the highest let-

through energy are 3-mil carbon blocks. These

devices have a maximum 3-sigma limiting volt-

age of 1 kV peak under transient conditions and

600 VRMS at 60 Hz. [17] In the event that one

primary protector fails completely and the other

does not, equipment on outside lines may be ex-

posed to voltages within these limits and should

provide adequate secondary protection to with-

stand them.

Intra-building T1 lines do not have primary pro-

tectors so they will not see metallic surges

created by primary protector asymmetry. How-

ever, they may still be damaged by longitudinal

surges coupled through the line transformer in-

terwinding capacitance. (Note that Bellcore

network equipment standards still require equip-

ment on inside lines have immunity from low

energy, metallic lightning surges so that secon-

dary metallic protection is still necessary.)

2

AC Power Faults

AC power cross faults can occur because of the

close proximity of telephone lines to AC power

distribution lines on utility poles and in building

wiring. Power cross faults result from direct con-

tact between a T1 line and an AC power line.

This type of fault can result in as much as 600

VAC across a T1 line. Unfortunately, in the past

many designers of telecommunications equip-

ment designed only for worst case power cross

faults and overlooked the danger from low volt-

age power cross faults. Such designs typically

used a large fuse (e.g., 1 Amp) in conjunction

with a 200 V to 300 V voltage limiting device

such as a Metal Oxide Varistor (MOV). [12]

This type of interface will open the fuse provid-

ing protection for faults above the MOV

breakdown voltage. However, during sustained

faults below the MOV breakdown voltage, this

design will allow steady state fault currents

which result in component heating and a fire

hazard. Adequate series current limiting protec-

tion should be provided to prevent fire and

electrical safety hazards resulting from all power

cross faults. Most electrical safety standards

have been revised to include additional testing

below the breakdown voltage of secondary shunt

protectors.

Although AC power cross faults pose a more se-

rious threat to telecom lines, AC power

induction can impair performance also. Through

electromagnetic coupling, currents in nearby AC

power lines can induce long-term, common

mode interference on telephone lines. [17] While

digital T1 lines are less sensitive to this type of

interference than analog subscriber lines, proper

longitudinal balance is important in both applica-

tions. Secondary protection circuitry must be

designed to maintain longitudinal balance.

Bellcore TR-EOP-000001 contains additional in-

formation and statistics on disturbances at the

Network Interface (NI) due to lightning, AC

power or RF transmission systems. [16]

AN34REV1

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