Surge Tolerant Fuses
Protection of equipment
Secondary protection
Regardless of the type of interference affecting telecom
equipment, it is imperative that no damage occurs, or only
limited damage whose effects can be calculated.
The secondary protection is normally located directly at
the appliance entry of the telecom equipment and has two
objectives.
As shown below, this requirement can be satisfied by the use
of appropriate protection circuits.
1. It operates as a voltage limiter which ensures that
interference up to a defined amplitude, not yet capable of
activating the primary protection, is absorbed or reduced to
a harmless level for the telecom equipment.
Protection circuits in the telecom branch are usually designed
on the two-stage principle. They comprise of primary and
secondary protection.
2. It effectively suppresses high energy level interferences,
which can no longer be adequately absorbed by the primary
protection (e.g. in case of direct contact
between the signal lines and the power network), by
galvanic decoupling of the circuit. This prevents the
occurrence of serious damage, even fire, in the telecom
equipment.
Primary protection
(e.g. at the
building entry)
Secondary protection
Exchange/
subscriber
Subscriber
The following diagram shows a frequently used, extremely
reliable protection circuit for this purpose.
Secondary protection
Exchange
Telecom fuse
Varistor
Varistor
Primary protection
Primary protection is frequently comprised of a combination of
resistors and surge arrestors and is usually located at the
«building entry» interface.
Telecom fuse
Primary protection
The circuit, which in its simplest form is comprised of two fuses
and two varistors, is characterized by an extremely attractive
cost-benefit ratio. The varistors limit the interference voltage
peaks to a level compatible for the telephone exchange, and
respectively, the subscriber circuit. Under normal conditions,
the fuses remain intact.
Under worst-case conditions, e.g. direct contact with the power
network, where both the telecom equipment components and
the varistors in the protection circuit would be seriously
damaged or destroyed, the fuses interrupt the circuit thereby
effectively and reliably protecting the telecom equipment.
The task of the illustrated primary protection circuit is to
sufficiently reduce the high-energy interference distortion so
that it can be safely absorbed by the following secondary
protection.
Schurter, Inc. • Phone 707-778-6311 • Fax 707-778-6401 • E-mail: info@schurterinc.com • Website http://www.schurterinc.com
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SCHURTER [ SCHURTER INC. ]
