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Surgectors for Telecommunications Systems

Posted: 26 May 2003     Print Version  Bookmark and Share

Keywords:power 

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10-149

AN9774.1

1-800-999-9445 or 1-847-824-1188 | Copyright ) Littelfuse, Inc. 1999

Surgectors for Telecommunications Systems

Introduction

This note discusses transient voltages associated with

telephone line applications and highlights the attributes of

the Littelfuse TO-202 packaged Surgector products as a

means to suppress these transients. For information on

Littelfuse Surface Mount DO-214AA Surgectors, see the

SGT Data sheet (File Number 4632) and Technical Brief

373.

System Transients

A telecommunication system can include subscriber stations

linked together through the cable plant and a central office

switching network. Included in the system are repeater

amplifiers, multiplexers, and other electronic circuits.

Supplying the electrical energy to run the system is a main

power source.

The cable plant and the power supply provide a path by

which damaging transients enter the system, to be

transmitted to vulnerable electronic circuitry. The cable plant

consists of conductors in shielded cables, which are

suspended on poles (shared with power lines) or buried in

the earth. A single cable is made up of many conductors,

arranged in twisted pairs (tip and ring). These cables (even

the ones underground) are susceptible to transient energy

from lightning and conducting them to the central office or

subscriber equipment.

The power used by a telecommunication system is usually

obtained from commercial power lines. These lines, like the

telephone cables, are either suspended on poles or buried.

Transient energy can be induced into power lines and

transmitted to the central office by direct conduction or by

induction into the telephone cable plant.

Lightning - Induced Transients

Lightning is a common source of over voltage in

communication systems. Quantitative information on

lightning has been accumulated from many sources [2], with

research centers in the United States, Western Europe and

South Africa. One of the most comprehensive surveys of

available data has been compiled by Cianos and Pierce [3],

describing the amplitude, rate-of-rise, duration, etc., in

statistical terms.

Lightning currents may enter the conductive shield of a

suspended cable by direct or indirect stroke, or it may enter a

cable buried in the ground by ground currents, as shown in

Figure 1.

In the case of a suspended cable, the lightning current that

enters the cable is seeking a ground and will travel in both

directions along the cable. Some of the current will leave the

shield at each grounded pole along its path.

Stroke currents leave a buried cable in a similar way but with

a different mechanism. Since the cable shield has a finite

electrical resistance, the current passing through it will

produce a potential gradient along its length.This voltage will

produce a potential difference between the cable and the

soil, as shown in Figure 2.

At some point (Point A) the shield-to-earth potential will

exceed the dielectric strength of the jacket, causing it to

puncture. Some of the lightning current then flows through

the puncture into the soil, thus equalizing the potential at that

point. The remaining current continues along the shield until

another puncture occurs, providing another path to ground.

The surge voltage that appears at the ends of the cable

depends upon the distance to the disturbance, the type of

cable, the shield material, and its thickness and insulation,

as well as the amplitude and waveshape of the lightning

current in the shield.

Calculations of Cable Transients

The voltage surge induced into the conductors of a cable will

propagate as a traveling wave in both directions along the

cable from the region of induction. The cable acts as a

transmission line. The surge current and voltage are related

to each other by Ohm's law where the ratio of voltage to

I

SHIELD

FIGURE 1. LIGHTNING CURRENT IN BURIED CABLE

INSULATING

JACKET

ISHIELD

VOLTAGE

A

DISTANCE

"TRUE" GROUND

SOIL VOLTAGE

VOLTAGE ACROSS JACKET

SHIELD VOLTAGE

0

FIGURE 2. CONDITION FOR PUNCTURE OF CABLE JACKET

Application Note July 1999

[ /Title

(AN97

74)

/Sub-

ject

(Surge

ctors

for

Tele-

com-

munica

tions

Sys-

tems)

/Autho

r ()

/Key-

words

(Surge

ctor,

TVS,

Tran-

sient

Sup-

pres-

sion,

Protec-

tion,

Tele-

com,

Line

Card,

Sec-

ond-

ary

10-150

current is the surge impedance (Z0) of the cable. Z0 can also

be expressed in terms of the inductance (L) and capacitance

(C) per unit length of the cable by the equation,

The series resistance of the shield and conductors, as well

as losses due to corona and arcing, determine the energy

lost as the disturbance propagates along the cable.

Tests conducted on telephone cables [4] have measured

surge impedances of 80 between any of the conductors

and the shield. Shield resistances between 5 and 6 per

mile were found to be typical. These values and the applied

lightning current waveform of Figure 3 were used to compute

the worst case transient which would appear at cable

terminals in a central office. The computation assumes the

lightning current is introduced into a suspended cable shield

at a point 2.75 miles from the central office. An average

cable span between poles of 165 feet, with a ground

connection on every fourth pole, was assumed. It was also

assumed that the cable will support the voltage without

arcing over.

The resulting short-circuit current available at the central

office is shown in Figure 4.

The open-circuit voltage at the cable end is shown in Figure

5. This analysis shows that if a severe, 100kA lightning flash

strikes a cable at a point 2.75 miles from a central office, a

voltage transient reaching a peak of nearly 18kV may appear

at the cable end, with about 355A of current available.

The open-circuit voltage and available current which would

result from stroke currents of various magnitudes is given in

Table 1. Included in the table is the probability of occurrence,

as given by Cianos and Pierce [3]. It should be realized that

voltages in excess of 10kV probably would not be sustained

due to cable insulation breakdown.

The values in Table 1 are based on the assumption of a

single conductor cable with the stroke point 2.75 miles from

the central station. For closer strokes the peak short-circuit

current at the cable end will increase as shown in Table 2.

These calculations were made assuming a breakdown at the

stroke point, which gives the worst case result.

Z0 L C/ ( )=

100

80

60

40

20

0

0 50 75 10025

TIME (5s)

CURRENT(kA)

FIGURE 3. SEVERE LIGHTNING CURRENT WAVEFORM

(2/505s)

0 20 30 5010

355

284

213

142

71

0

40

TIME (5s)

CURRENT(A)

FIGURE 4. AVAILABLE CURRENT 2.75 MILES FROM 100kA

LIGHTNING STROKE

TIME (5s)

VOLTAGE(kV)

0 20 30 5010 40

18.2

14.7

11.0

7.3

3.7

0

FIGURE 5. OPEN CIRCUIT VOLTAGE 2.75 MILES FROM

100kA LIGHTNING STROKE

Application Note 9774

10-151

Since telephone cables actually have many pairs of wires

rather than a single conductor, the peak currents in each

wire will vary.

Assume a cable of six pairs is struck by lightning, inducing a

stroke current of 100kA into the shield, at a distance of 0.25

mile from the protector. The transient current will be divided

up among the twelve suppressors at the cable ends. Each

protective device must handle up to 852A of peak current in

order to clamp the voltage to a protected level.

Power System Induced Transients

Since telephone cables very often share a pole and ground

wire with the commercial AC utility power system, the high

currents that accompany power system faults can induce

over-voltages in the telephone cables.These faults can have

long duration (compared to the lightning-induced transients)

from a few milliseconds to several cycles of power frequency.

Three types of over-voltage can occur in conjunction with

power system faults:

Power Contact - (Sometimes called "power cross"). The

power lines fall and make contact with the telephone cable.

Power Induction - The electromagnetic coupling between

the power system experiencing a heavy fault and the

telephone cable produces an over-voltage in the cable.

Ground Potential Rise - The heavy ground currents of

power system faults flow in the common ground connections

and cause substantial differences in potential.

TO-202 Type Surgector TVS Suppressors

The need for a surge suppressor stems from the increasing

sophistication of electronics in the telecommunications

industry. For example, the use of medium scale integrated

(MSI) and very large-scale integrated (VLSI) circuits. These

devices are used in equipment that transmits, processes,

codes, switches, stores data, and has multifunction

capability, but may be intolerant of voltage overloads.

The surgector is a monolithic silicon device. It consists of an

SCR-type thyristor whose gate region contains a special

diffused section that acts as a Zener (avalanche) diode.

It combines the continuous voltage protection of the Zener

with the thyristor's ability to handle high current. As a result,

the surgector can provide the much-needed secondary

surge protection for telecommunications circuitry, data links,

and other sensitive electronic circuits that are especially

susceptible to damage from transient voltage.

Littelfuse surgectors are listed as recognized components to

UL497B standard for protectors.

Surgector Characteristics Include

7 High input impedance until breakdown (i.e., low leakage)

7 Repeatable breakdown/threshold voltage

7 High surge current handling capability

7 Responds to rapidly reoccurring surges

7 Bidirectional protection

7 No degradation of characteristics with use

Figure 7 shows the structure in cross section.

TABLE 1. LIGHTNINGTRANSIENTS AT CABLE END

2.75 MILES FROM STROKE POINT

PEAK

CURRENT

(kA)

PROBABILITY

OF

OCCURRENCE

(%)

TERMINAL

OPEN CIRCUIT

VOLTAGE

(PEAK V)

TERMINAL

SHORT-CIRCUIT

CURRENT

(PEAK A)

175 1 32,200 621

100 5 18,400 355

60 15 11,040 213

20 50 3,680 71

TABLE 2. PEAK LIGHTNING-INDUCED CURRENTS IN

VARIOUS LENGTHS OF TELEPHONE CABLE

(100kA LIGHTNING STROKE)

DISTANCE

TO

STROKE

(MILES)

PEAK CURRENTS (A)

AT

STROKE

POINT

AT CENTRAL OFFICE

SINGLE

CONDUCTOR

6 PAIR

CABLE

12 PAIR

CABLE

2.75 630 355 - -

1.50 630 637 - -

1.00 734 799 - -

0.50 1110 1120 712 453

0.25 1480 1480 852 463

1.5K V/5s INDUSTRY STANDARD

LIGHTNING STROKE

TO LIGHTNING SURGE

DEVICE CURRENT DUE

SECONDS (5)

VOLTS

200

150

100

50

0.25 0.5 0.75 1 1.25 1.5 1.75 2

V I

150A

AMPS

250

200

150

100

50

FIGURE 6. INDUSTRY STANDARD LIGHTNING STROKE

Application Note 9774

10-152

Surgectors Provide Transient Protection for:

7 Central Office Equipment

7 Supervisory Equipment

7 Switchgear Equipment

7 Data Transmission

7 Handsets

7 EPABX, PABX, PBX

7 Repeaters

7 Line Concentrator

7 Receivers

7 Headsets

7 Modem

7 PCM

Surgector Operation of TO-202 Type

Surgectors

With its low leakage and low capacitance, the surgector

allows normal operation of the circuit. Surgector devices are

rated at 30V, 60V, 100V, 230V, and 270V. When a transient

voltage reaches the avalanche breakdown voltage, the

Zener instantly clamps the voltage, as shown in Figure 8.

The current flows from the Zener region into the thyristor

gate, switching on the thyristor. The thyristor drops to low

voltage, creating a low impedance in the circuit, and shunts

the excess energy from the circuit to the ground.

While the transient is present, the surgector remains in the

ON state, and the voltage across the circuit is low. Its precise

value depends on the type of pulse and the type of surgector

being used. When current falls to the "holding current," limit

the surgector turns off.

n+

K SHUNT

p

n

p+

ZENER

ANODE

CATHODE

n

p

n

ANODEEPI WAFER

n+ n+

ALUMINUM

METAL

ZENERp

p+

CATHODE

OXIDE

FIGURE 7. SURGECTOR VERTICAL STRUCTURE

REPEATERS

TIP

RING

TIP

RING

CENTRAL

DECODE

CONTROL

PABX

SWITCH

NETWORK

OFFICE

CENTRAL

DECODE

CONTROL

PABX

SWITCH

NETWORK

OFFICE

IT

IH

mA

IDM

VT

A

VBO

VZ

VDMV

FIGURE 8. TYPICAL VOLT-AMPERE CHARACTERISTICS

250

225

200

175

150

125

100

-40 10-30 -10-20 0 20 30 40 706050 80 90

HOLDINGCURRENT(mA)

AMBIENT TEMPERATURE (oC)

FIGURE 9. TYPICAL HOLDING CURRENT vs TEMPERATURE

IT(INITIAL) = 2A

Application Note 9774

10-153

Surgector Types

Littelfuse surgector devices include Variable Clamp,

Unidirectional and Bidirectional types. The variable clamp

type is unidirectional but provides three terminals instead of

two. The third terminal gives the user direct access to the

SCR gate region.With this external gate control circuitry, any

voltage between 5V and 270V can trigger the device

depending on the type.

The Unidirectional and Bidirectional surgectors have two

terminals, and are internally triggered at voltages of 30V, 60,

230 and 270V, depending on the type. See the Littelfuse

SGT Series should surface mount bidirectional types be

desired.

Performance Characteristics of TO-202

Type Surgectors

7 Surgector devices have ratings for transient peak surge

current of 300 to 600A for a 1 x 25s pulse and

appropriately scaled currents at 8 x 20, 10 x 560, and 10 x

10005s. These rated surges can be applied to the

surgector devices repeatedly without degradation.

7 The surgector clamps the transient voltage within

nanoseconds.

7 The surgector is designed not to fail to an open condition

on a 1 x 2 pulse below 450A (900A for the SGT27B27).

This becomes especially important in telecom equipment

designs which are required to meet UL-1459

requirements.

7 Surgector devices switch to the off-state once the pulse

current drops below the intentionally high holding current

threshold. (The holding current of the surgector must be

greater than the normally available short-circuit current in

the circuit to ensure that the surgector will return to the

off-state.)

7 Leakage is low; less than 50nA.

7 The capacitance of surgector devices is also low,

presenting about 50pF.

TIP

RING

SURGECTOR

SURGECTOR

TELEPHONE

LINE PAIR

FIGURE 10. APPLICATION EXAMPLE OFTWO

BIDIRECTIONAL SURGECTOR DEVICES

PLACED BETWEEN THE TIP AND RING LINES

Application Note 9774

10-154

References

For Littelfuse documents available on the web, see

http://www.littelfuse.com/

[1] Bennison, E., P. Forland and A.J. Ghazi, "Lightning

Surges in Open-Wire, Coaxial and Paired Cables" IEEE

International Conference on Communications, June

1972.

[2] Golde, R.II., "Lightning Currents and Related

Parameters," Lightning, Vol. 1, Physics of Lightning,

Chapter 9, ed. R.H. Golde, Academic Press, 1977.

[3] Cianos, N. and E.T. Pierce, "A Ground Lightning

Environment for Engineering Usage," Report No. 1,

Stanford Research Institute, August 1972.

[4] Boyce, C.F., "Protection ofTelecommunication Systems,"

Lightning, Vol. 2, Lightning Protection, Chapter 25, ed.

R.1I. Golde, Academic Press, 1977.

[5] "Connection of Terminal Equipment to the Telephone

Network," Federal Communications Commission Rules

and Regulations, part 68, October 1982.

Ordering Information and Packages for

Leaded Surgectors

Surgector type numbers use the following format: The first

three characters - "SGT" - stand for surgector. The next two

digits represent the maximum off-state voltage divided by

10. Following the voltage is a letter indicating either SCR (S),

Unidirectional (U), or Bidirectional (B). The next two digits

indicate holding current in milliamps divided by 10.

All versions of the leaded surgector are housed in a

modified TO-202 plastic package. (Note 4.)

Surgector Leaded Packages

MODIFIED TO-202

PACKAGE STYLE

PACKAGE A PACKAGE B

SGT 03 U 13

Holding Current in mA divided by 10

Type of Surgector

U: Unidirectional

B: Bidirectional

S: SCR

Off-State Voltage Rating Divided by 10

Surgector

TABLE 3. SELECTION GUIDE

PART NUMBER FUNCTION

VZ MIN

(V)

VBO MAX

(100V/5s)

ITSM

(1 x 25s)

ITSM

(10 x 10005s)

IH

(mA)

PACKAGE

STYLE

SGT10S10 (Note 1) VAR Clamp 100 Note 1 300 100 >100 A

SGT27S10 (Note 1) VAR Clamp 270 Note 1 300 100 >100 A

SGT27S23 (Note 1) VAR Clamp 270 Note 1 300 100 >230 A

SGT03U13 Unidirectional 30 < 50 300 100 >130 B

SGT06U13 Unidirectional 60 < 85 300 100 >130 B

SGT23U13 Unidirectional 230 < 275 300 100 >130 B

SGT23B13 Bidirectional 230 290 300 100 >130 B

SGT27B13 Bidirectional 270 345 300 100 >130 B

SGT27B27 Bidirectional 270 345 600 200 >270 B

NOTES:

1. Dependent on trigger circuit.

2. All leaded surgectors supplied in modified JEDEC TO-202 Package.

Package Style A = 3 lead version

Package Style B = 2 lead version

3. All devices UL recognized to 497B - File Number E135010.

4. See the SGT Series Data Sheet, File Number 4632, and Technical Brief 373 for information on Littelfuse DO-214AA Surface Mount Surgectors.

Application Note 9774





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