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Vishay BCcomponents

Revision: 13-Nov-15

2

Document Number: 29070

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

OVERCURRENT PROTECTION OF TELECOMMUNICATION LINES

The PTC thermistor must protect the telephone line circuit

against overcurrent which may be caused by the following

events:

• Surges due to lightning strikes on or near to the line plant.

• Short-term induction of alternating voltages from adjacent

power lines or railway systems, usually caused when

these lines or systems develop faults.

• Direct contact between telephone lines and power lines.

To provide good protection under such conditions a PTC

thermistor is connected in series with each line, usually as

secondary protection; see Typical Telephone Line drawing

fig. 1. However, even with primary line protection (usually a

gas discharge tube), the PTC thermistor must fulfil severe

requirements.

Surge pulses of up to 2 kV can occur and in order to

withstand short-term power induction the PTC thermistor

must withstand high voltages. If the line has primary

protection a 220 V to 300 V PTC thermistor is adequate.

Without primary protection, however, a 600 V PTC device is

necessary. Vishay BCcomponents manufacturers a range of

PTC

thermistors

(see Electrical

Data

and

Ordering

Information Table) covering both requirements.

In the case of direct contact between the telephone line and

a power line, the PTC thermistor must withstand very high

inrush power at normal mains voltage. Under such

conditions, overload currents of up to 10 A on a 230 V mains

could occur for up to several hours. To handle this power,

the resistance/temperature characteristic of the thermistor

must have a very steep slope and the ceramic must be

extremely homogeneous.

In case of overcurrent due to short-term induction of

alternating voltages, currents of several amperes with

seconds.

For standard high voltage applications, resistance values

from 25

Ω to 50 Ω are available. However, ISDN networks

which carry high-frequency sound and vision, need lower

line impedance.

Telecommunication designers are therefore demanding

high voltage thermistors with much lower

places even greater demands on the manufacture of PTC

thermistors. For these applications PTC thermistors which

have a

In a typical telephone line application, two PTC thermistors

are used, one each for the tip and ring (or A and B) wire

together with their series resistors. For good line balance it

is important that the thermistor and resistor pairs are

matched.

Fig. 1 - Typical telephone line showing where PTC thermistors can be used for overcurrent protection.

SUBSCRIBER

LOOP

INTERFACE

CIRCUIT

(SLIC)

OR

TRANSFORMER

voltage

clamp

PTC

PTC

MDF

SECONDARY

PROTECTION

PRIMARY

PROTECTION

(OPTIONAL)

subscriber

lines

+

PTC

+

+

+