ST AN2064 Application note
AN2064
®
APPLICATION NOTE
Compliance of series/parallel protections for Telecom CO
ST and Cooper Bussmann, experts of wireline networks protection, have jointly prepared this application
note to present a full protection solution based on companion devices from both companies.
Even with all the wireless telecommunication options (GSM, DCS, PCS, UMTS, Wi-Fi, etc…), the wireline
network remains the most cost effective wide range solution to exchange data over the world. The use of
this copper carrier requires system designers to provide adequate protection against overvoltage and
overcurrent events occurring on the line. The goal of this document is to provide telecom system card
designers the necessary information to make proper protection choices.
An example of a wireline telecom network is shown in figure 1. Two different kinds of equipment are connected together by means of a copper line. One line termination is connected to the Central Office (CO)
while the other is connected to the terminal. In the CO, the line goes through the Main Distribution Frame
(MDF), which connects network to the signal cabinet, and is then connected to a subscriber line card.
Figure 1: Classical topology of wireline network subscriber line card
Central office
Terminal
The following pages show how to implement the protection stage utilizing series overcurrent protection
devices and Transient Voltage Suppressor (TRISIL™) devices (figure 2). Both elements work together
during surges, TRISIL acts to suppress overvoltages while the series overcurrent devices protect the circuit from lethal overcurrents.
Copper line
MDF
Subscriber line card
TM: TRISIL is a trademark of STMicroelectronics.
REV. 1AN2064/0705
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AN2064 - APPLICATION NOTE
Figure 2: One wire telecom protection topology
Series element
Line side
Transient Voltage Suppressor
or
TRISIL
Protected side
1. TRISIL™ selection
Transient voltage suppressor (TRISIL) selection has to take into account the two working modes it will
meet during its life. The first mode is the normal operating mode where the protection device has to be
transparent, that means no impact on the speech or data signal. The second mode is the suppression
mode where the TRISIL has to eliminate all dangerous transient voltage surges.
During normal operation, we have to focus on voltages and currents managed in the line. These values
depend on the specific countries where equipment is located and the type of signal being managed
(analog or digital). For example, in the US the nominal battery voltage is -56.6V and the ringing voltage
is 150V
, where the normal operating voltage is between 0V and -56.6V in speech or dialing mode and
RMS
between +155.5V and -268.7V in ringing mode. In digital networks the voltages can be the same as those
used in analog networks, as is the case with ADSL. Frequently, ringing is managed by digital code where
only the battery voltage is present (generally -100V), as is the case in ISDN applications. When the
telephone is picked up, the loop current increases and indicates to the CO to stop the ring signal or to wait
for dialing signals. Call connection occurs when the loop current exceeds a few milliamps. Analog CO
systems may use series resistors while digital systems do not. These resistors (10Ω to 100Ω depending
on the applicable country standard) are used to manage line longitudinal balancing while the use of any
series resistance is forbidden in ADSL system. From these requirements we can conclude that the TRISIL
threshold voltage has to be higher than 268.8V for US analog and ADSL networks (190V for Europe) while
the TRISIL leakage current has to be lower than 1mA. The right choice for US is 270V (200V for Europe)
and the leakage current is less than a few µA.
When considering the suppression mode, we have to take into account that telecommunication lines can
be subjected mainly to two kinds of disturbances. The first disturbance is linked to atmospheric effects
while the second disturbance is produced by contact or proximity with the 50/60Hz mains network. These
disturbances are well defined in standards, which can be worldwide or dedicated to a specific country.
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The table 1 gives the main lightning standards.
Table 1: Main line card lightning surge standards
Country Standard Surge voltage Waveform Current
Worldwide ITU-T K20 1500V 10/700µs 37.5A
Worldwide IEC-61000-4-5 1000/4000V 10/700µs 25/100A
Worldwide IEC-61000-4-5 1000/4000V 1.2/50µs 25/100A
Germany VDE0433 2000V 10/700µs 50A
Germany VDE0878 2000V 1.2/50µs 50A
USA GR-1089 Core (Telcordia) 2500V 2/10µs 500A
USA GR-1089 Core (Telcordia) 1000V 10/1000µs 100A
France I3124 1000V 0.5/700µs 25A
Main worldwide standards for 50/60Hz overvoltage disturbances can be defined by two parameters: the
applied RMS voltage, between 60V and 1000V, and the test duration, between 0.2s and 15min.
From this section we can conclude that the TRISIL current capability must be adapted according to the
specific country (for example, a waveform surge of 100A for 10/1000µs for the US). As TRISIL are
dedicated to manage high currents for short duration surges (in the range of hundreds of ms) the 50/60Hz
disturbances test requirements show the need of a complementary protection stage, implemented with
series protection devices like fuses or positive temperature coefficient resistors (PTC).
2. Protection circuit
In the protection circuit we find particularities linked to the characteristics already mentioned in the TRISIL
selection section and also linked to the CO line interface. Generally the subscriber line interface circuit
(SLIC) is directly connected to the line without any isolation stage when operating in analog mode, while
in digital applications isolation is achieved using a transformer. Please note that, at the CO side,
protection is split into two areas - the primary protection stage located in the MDF and the secondary
protection directly soldered on the subscriber line card.
Figure 3: Analog line card protection circuit
-V bat
-
Rs2
Tip
PTC or Fuse
TRISIL 1
Subscriber line card
Relay
Rs1
TRISIL 2
SLIC
PTC or Fuse
TRISIL 1
Ring
Ring generator
Rs1
TRISIL 2
Rs2
Secondary protection, first stage Fuse or PTC andTRISIL 1, second stage TRISIL 2
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Figure 3 shows the classical protection topology used to protect one section of an analog subscriber line
card. A first stage uses both series overcurrent protection devices (fuses or PTCs) and TRISIL protectors
(TRISIL 1, threshold voltage = +/- 270V for US) allowing the ring relay to be protected against full lightning
and power contact surges. A second level (TRISIL 2, threshold voltage = 0/-Vbat) allows the SLIC to be
fine tuned protected. The presence of longitudinal series resistors makes the current capability rate of this
TRISIL to be adjusted.
Figure 4: CO ADSL modem protection circuit
ADSL CO Modem
Fuse or PTC
Tip
Ring
Fuse or PTC
Fuse or PTC
TRISIL 1
TRISIL 1
Transformer
C
TRISIL 2
Secondary protection Fuse or PTC and TRANSIL 1
Figure 4 shows the protection topology generally used to protect the ADSL modem (or line cards when
use of a line transformer). This modem is connected to the analog line card by means of a splitter stage
and then receives the same operating voltages. Please note that series resistors are generally not permitted in such an application. Protection is provided by the TRISIL (TRISIL 1, threshold voltage = +/270V), which assumes the full lightning current while the series overcurrent protection devices (fuses or
PTCs) allow the module to be well protected against 50/60Hz power contact.
3. Series protection
As previously mentioned, the use of series overcurrent protection devices is mandatory to protect subscriber line cards against 50/60Hz power contacts. As far as power contact is concerned, standards require the equipment to withstand several tests with different acceptance criteria. First level criterion for
US standards (or A criterion for European requirements) requires the equipment to be fully operational
after tests while second level (or B criterion) allows the system to be out of order but no fire or smoke is
permitted. Table 2 shows AC power fault requirements of the US Telcordia GR1089 standard.
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Table 2: Telcordia GR1089 AC power fault test
First Level AC Power Fault Test
Test Applied Voltage, 60Hz Short Circuit Current Duration
RMS
50V
100V
, 400V
1000V
RMS
RMS
RMS
RMS
, 600V
RMS
0.33A 15 minutes
0.17A 15 minutes
1A at 600V 60 Applications, 1 second each
1A 60 Applications, 1 second each
1
2
3
4 (*)
200V
5 NA NA 60 Applications, 5 second each
6
7
8
9 (*)
600V
440V
600V
1000V
RMS
RMS
RMS
RMS
0.5A 30 seconds each
2.2A 5 x 2 seconds each
3A 5 x 1.1 second each
5A 0.5 second each
Second Level AC Power Fault Test
1
120V
2
3
4
(*) Primary protector in place (MDF)
100V
RMS
600V
600V
RMS
, 277V
RMS
RMS
-600V
RMS
RMS
25A 15 minutes
60A 5 seconds
7A 5 seconds
2.2A at 600V 15 minutes
Using series overcurrent protection, two technologies are available, PTCs and fuses.
PTCs are resistive elements that dissipate power when subjected to current, increasing their temperature
and making their resistance quickly increase (10Ω @ 25°C and 100kΩ @ 150°C for example). The nice
feature of the PTCs is that they are resettable but they have two main drawbacks. The first drawback is
its resistance and, as already mentioned, some applications like digital networks do not allow resistive elements. The second drawback is linked to its tolerance, which makes it difficult to achieve line equilibrium
(longitudinal balancing).
Fuses do not have these resistive drawbacks, making them well suited for digital applications.
From the previous discussion we can conclude that the fuse must withstand first level surges for US (or A
criterion for Europe) but may operate for second level surge (or B criterion). As far as the US market is
concerned the fuse has to remain operational for the 10/1000µs 1kV 100A and the 2/10µs 2.5kV 500A
lightning surges. It must also withstand first level AC power faults while it must operate for 277V
and 600V
60A second level AC power faults.
RMS
RMS
25A
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