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April 2004

AN579

APPLICATION NOTE

PROTECTION CONCEPTS IN TELECOMMUNICATIONS

EQUIPMENT

REV. D2A - 3591

INTRODUCTION

The goal of a telecommunication network (see Figure 1) is to permit data exchange (speech or digital) be-

tween two or more subscribers.

The network is made up of different parts which are subject to various disturbances.

The most susceptible elements are the lines, due to their length and their geographical location.

Disturbances strike the lines and are then propagated to the extremities of the lines at which lie telephone

sets and the subscriber line interface cards (SLIC).

So the lines receive two kinds of overvoltages:

– Surges of short duration with high peak voltage value (a few hundred micro-seconds for a few thousand

volts). These are generated by atmospheric phenomena.

– Surges of long duration with medium voltage value (greater than one second for a few hundred volts

RMS) which are due to the mains AC power networks.

The purpose of this application note is to analyse these two kinds of overvoltages.

Figure 1. Classical telecommunication network topology

OVERVOLTAGES ACROSS TELECOMMUNICATION LINES:

Atmospheric effects:

Lightning phenomena are the most common surge causes. They are mainly due to a voltage difference

between the ground and the clouds (a few 100 kV). Two kinds of strikes may occur:

1. Negative discharge with a peak current of 50kA, rise time of 10µs to 15µs and 100µs duration.

2. Positive discharge with a peak value of 150kA, rise time between 20µs and 50µs and a duration

between 100ms and 200ms.

The lightning effect appears on the lines in two ways.

– Direct shock.

– Induced shock.

MODEM

PABX

EXCHANGE

EXCHANGE

PABX

MODEM