A protection arrangement for a telephone subscriber line interface circuit is
disclosed.
The arrangement is particularly useful for protecting an electronic telephone set
from over-voltage and over-current fault conditions. The arrangement provides a
FET that operates in saturation mode to connect an office battery to the subscriber
line under normal operation. The FET also provides isolation capabilities for protecting
the line circuit from an over-current condition on the subscriber line. Over-voltage
protection is provided by way of an isolation relay between the line circuit and
the subscriber line. Both the FET and isolation relay are operated by a controller
that uses timers in the methods of over-voltage and over-current protection that
it performs. A further capability of the arrangement is that it resets itself after
the fault condition has ended. This feature is particularly useful in the case
of fault conditions of short duration.
