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Pain prevention

Considerations for the protection of subscriber line interface cards. By Chris Likely.

Subscriber lines can be divided into two main types, analogue and digital. Analogue lines work from a nominal -48V supply but also have ringing voltages of up to 150Vrms. Digital lines on the other hand tend to operate with a much lower supply voltage, as low as 3.3V, and use digital signals to produce ring tones in the terminal equipment.

Both use subscriber line interface cards (SLICs) and both require overvoltage protection, the main difference being in the selection of the transient voltage suppresser (tvs).

There are two main overvoltage threats to SLICs: lightning, and short circuit to the AC utility supply. Protection mechanisms for each event need to be different, after a lightning strike the protection circuit needs to reset and the SLIC must continue to operate.

However, mains crossing events are less common than lightning strikes and potentially more destructive, under these circumstances the protection circuit is allowed to fail in order to prevent damage to the SLIC.

In analogue systems, during ringing a voltage of 150Vrms is superimposed on top of the -48V supply giving a peak voltage of up to 270V. When the telephone is answered the loop current increases and as a result the ringing is stopped and the call is connected.

Call connection normally occurs when the loop current exceeds 3mA. This mode of operation sets the minimum requirements for the tvs: it must not operate at less than 270V and at this voltage level the leakage current must be less than 3mA.

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Graham Pitcher

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