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

Terminal

Copper line

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.

TM: TRISIL is a trademark of STMicroelectronics.

AN2064/0705

REV. 1

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Figure 2: One wire telecom protection topology

	Series element
Line side	Protected side
	Transient Voltage Suppressor
	or
	TRISIL

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 150VRMS, where the normal operating voltage is between 0V and -56.6V in speech or dialing mode and 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

Subscriber line card	-V bat
Relay
Rs1	Rs2
PTC or Fuse

Tip

TRISIL 2

TRISIL 1

SLIC

	TRISIL 2
	TRISIL 1
PTC or Fuse	Rs1	Rs2

Ring

Ring generator

Secondary protection, first stage Fuse or PTC and TRISIL 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

TRISIL 2

Tip

TRISIL 1

C Transformer

TRISIL 1

Fuse or PTC

Ring

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
1	50VRMS	0.33A	15 minutes
2	100VRMS	0.17A	15 minutes
3	200VRMS, 400VRMS, 600VRMS	1A at 600V	60 Applications, 1 second each
4 (*)	1000VRMS	1A	60 Applications, 1 second each
5	NA	NA	60 Applications, 5 second each
6	600VRMS	0.5A	30 seconds each
7	440VRMS	2.2A	5 x 2 seconds each
8	600VRMS	3A	5 x 1.1 second each
9 (*)	1000VRMS	5A	0.5 second each
	Second Level AC Power Fault Test
1	120VRMS, 277VRMS	25A	15 minutes
2	600VRMS	60A	5 seconds
3	600VRMS	7A	5 seconds
4	100VRMS-600VRMS	2.2A at 600V	15 minutes

(*) Primary protector in place (MDF)

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 277VRMS 25A and 600VRMS 60A second level AC power faults.

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