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Vishay BCcomponents
Revision: 13-Nov-15
2
Document Number: 29070
For technical questions, contact: nlr@vishay.com
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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
voltages as high as 650 V
RMS
can be present for several
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 R
25
values, which
places even greater demands on the manufacture of PTC
thermistors. For these applications PTC thermistors which
have a R
25
value of 10 Ω with voltages in the 300 V
RMS
to
600 V
RMS
range are available.
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
+
++
