High Speed Transmission Equipment
T1/E1 Circuit Protection
T1/E1 networks offer data rates up to 1.544 Mbps (2.058 for E1) on four-wire systems.
Signal levels on the transmit (TX) pair are typically between 2.4 V and 3.6 V while the
receive (RX) pair could go as high as 12 V. Loop powering is typically ±130 V at 60 mA,
although some systems can go as high as 150 V. (Figure 3.16)
Central Office
T1 Transceiver
Line Regenerator
Line Regenerator
3000 ft
6000 ft
TX Pair
RX Pair
Line powered DLC Four-wire,1.544 Mbps/2.048 Mbps
Figure 3.16 T1/E1 Overview
Protection Circuitry
Longitudinal protection is required at the Central Office Terminal (COT) interface because of
the ground connection used with loop powering. (Figure 3.17) Two P1800SC MC SIDACtor
devices provide overvoltage protection and two F1250T TeleLink fuses (one on Tip, one on
Ring) provide overcurrent protection. The P1800SC MC device is chosen because its VDRM
is compliant with TIA-968 regulations, Section 4.4.5.2, “Connections with protection paths
to ground.” These regulations state:
Approved terminal equipment and protective circuitry having an
intentional dc conducting path to earth ground for protection purposes at
the leakage current test voltage that was removed during the leakage
current test of section 4.3 shall, upon its replacement, have a 50 Hz or
60 Hz voltage source applied between the following points:
a. Simplexed telephone connections, including Tip and Ring, Tip-1
and Ring-1, E&M leads and auxiliary leads
b. Earth grounding connections
The voltage shall be gradually increased from zero to 120 V rms for
approved terminal equipment, or 300 V rms for protective circuitry, then
maintained for one minute. The current between (a) and (b) shall not
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SIDACtor Data Book and Design Guide
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