Electronic Design

Designing T1 Protection Circuitry For GR1089 And UL1950 Requirements

A T1 carrier line is a digital transmission link operating at a DS-1 transmission rate of 1.544 Mbits/s and employing two pairs of twisted-pair wiring. T1 lines are used extensively for connecting networks across remote distances. This digital transmission standard is recognized in the United States, Canada, Hong Kong, and Japan. Outside of these countries, an E1 transmission rate of 2.048 Mbits/s is employed.

Baseband signaling is typically an AMI (alternate mark inversion) linecode technique, although BNZS (bipolar with N-zero substitute) or ZBTSI (zero-byte time-slot interchange) also are used. The nominal pulse voltage is 2.4 to 3.6 V zero-to-peak.

The original repeatered-T1 carriers were developed to work with H88 load coils. This resulted in T1 line drivers being able to drive sections as long as 6000 ft. Repeater locations aren’t limited to these distances, since the cable type, cable gauge, loss requirements, and noise requirements determine the repeater spacing (Fig. 1).

The line regenerators (also known as repeaters) are powered from the central office. Most line-powering equipment can be set to provide 60, 100, or 140 mA of line current at ±130 V. A phantom powering scheme is typically used for this loop powering approach. In this method, +130 V is placed across the transmit pair and −130 V is placed across the receive pair. The resulting 260-V differential powers all of the repeaters on the loop.

Since T1 lines run between the customer facilities and the central office, they’re subject to overvoltage and overcurrent conditions from lightning-induced surges, power crosses, and other noise impairments. Protection circuitry is required to safeguard the terminal equipment from these overload stresses. The protection requirements for T1 are specified in the UL1950 (AC hazards) and GR1089 (both lightning and AC hazards) standards. These documents outline both longitudinal (common-mode) and metallic (differential-mode) stresses.

The Level 1 lightning surges of GR1089 are outlined in the table. The equipment under test (EUT) must pass these surges operationally.

The block diagram outlines a longitudinal and metallic protection scheme for GR1089 compliance for the central office site (Fig. 2). The switching voltage of the surge protectors must be selected so as not to interrupt loop power (±130 V) or disturb normal signaling levels (2.4 to 3.6 V).

The surge devices placed for longitudinal protection must not turn on if subjected to the ±130-V levels. Therefore, a P1800SC is chosen. This device has a VDRM (peak off-state voltage) of 160 V and will withstand the GR1089 lightning surges without any additional series resistance. The metallic protection must not turn on during during normal signaling. It can peak at 3.6 V on the transmit side, but on the receive side can go as high as 12 V due to standing waves.

The P0300SC, P0640SC, or even the P0720SC can be used here. A fuse element is used in series on both lines to protect against overcurrent conditions for adherence to GR1089 power-cross second-level tests and UL 1950 power-cross tests. Under these conditions, the EUT doesn’t have to pass operationally but must not cause a fire, electrical, or safety hazard. This F1250T fuse is robust enough to survive the lightning surges without causing nuisance openings, but will interrupt correctly during power-cross events.

The remote regenerator site may not require longitudinal protection if no ground connections are available. This prevents a reference to the ground plane that’s required for common-mode signals. The block diagram (Fig. 3) shows the metallic protection for this condition. Notice only one fuse per wire pair is required, since all events occur in a metallic mode only.

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