Electronic Design

LAN/WAN Ethernet Overcurrent And Overvoltage Protection

Local-area networks (LANs) are short-distance data communication links typically within a single building or single campus environment. A LAN doesn't use common carrier circuits (i.e., the public switched telephone net work, or PSTN). By contrast a wide-area network (WAN) does employ common-carrier-provided lines to cover a more extended geographical region than a LAN. Ethernet is the physical-link and data-link protocol used for LAN interconnections. This twisted pair and/or coaxial cable link can operate at speeds up to 100 Mbits/s. lt has been known by several different names, such as Thick Ethemet (10Base-5), Thin Ethernet (10Base-2), Twisted Ethemet (10Base-T), and now Fast Ethernet (100Base-T).

Base-T Ethernet was originally designed to operate over telephone cables. Base-T Ethemet links are the most commonly installed LANs today. These networks have a wire connection from each workstation to the Base-T hub, similar to a phone system. Its advantage is that the entire system doesn't go down if one machine on the network crashes. However, the coax Ethernet LANs will go down if one machine on the network crashes, since coax Ethernet is one long wire that loops from one machine to the next.

Both 10Base-T and 100Base-T networks use Category 5 wiring (known as CAT5) with maximum segment lengths of 100 meters on unshielded pairs. The 10 indicates 10-Mbit/s speed capability, while the 100 indicates 100-Mbit/s capability. This CAT5 wiring is either UTP (unshielded twisted pair) or STP (shielded twisted pair) cable that can pass signals from 1 to 100 MHz.

GR 1089, an industry-wide standard for the performance of network equipment, outlines intra-building lightning surge immunity levels for LAN-type equipment. These requirements apply only to network equipment that doesn't interface with outside-plant equipment, such as the PSTN (Public switched telephone network), or serve off-premises equipment. The LAN equipment can't be damaged and must continue to operate safely after application of the "Level 1" lightning-surge conditions (Table 1).

For a WAN connection, the equipment could be exposed to more severe lightning events since its connection extends beyond the confines of a single building. Therefore, these WAN connections need to comply with the GR1089 surge requirements for a telecommunications port (Table 2).

A 10Base-T Ethernet protection solution (Fig. 1) provides both overcurrent (power-cross) and overvoltage protection (lightning-induced). The Teccor F1250T surface-mount fuse provides overcurrent protection without opening during surge events, preventing nuisance openings. It will, however, open appropriately for UL 1950 compliance and/or UL 1459 compliance. These UL power-cross tests are outlined in Table 3. The MDL-2A fuse for UL 1950 and the MDQ-1.6A fuse for UL 1459 isn't allowed to open during these power-cross events. The F1250T will open before the wiring simulator fuses.

The Teccor SIDACtor PG640EC/SC provides either a through-hole or surface-mount technology solution for overvoltage surge-induced protection that complies with GR 1089. The single SIDACtor protector provides metallic (differential) protection. If a ground reference were used, then two more SIDACtor protectors would be required to provide longitudinal (common-mode) protection.

The Motorola MUR1100E diodes are used to compensate for the capacitive-loading effects of the solid-state SIDACtor protectors. Due to the high data rates of 10Base-T, the shunt reactive value of the protector must be reduced. This is accomplished by using a pair of MUR1100E diodes.

For a LAN application, the Teccor P0640SA/EA can be used due to the low-level, intra-building surge immunity requirements of GR 1089. This SIDACtor protector exhibits a capacitance of 50 pF. The MUR1100E diodes have a characteristic capacitance of 10 pF. Their parallel combination results in a capacitance value of 20 pF. This capacitance is in series with the 60-pF capacitance value of the SIDACtor, resulting in an overall shunt capacitance of less than 15 pF.

For a WAN design, the Teccor P0640SC/EC is used due to the higher telecommunication-port surge-immunity requirements of GR 1089. This SIDACtor protector exhibits a capacitance of 100 pF. The compensation diode circuit lowers the shunt capacitance to less than 17 pF.

For 100Base-T applications, the higher data rates require an even lower shunt capacitance value. Therefore, the circuit in Figure 2 uses two bundles of diode pairs to further reduce the effective capacitance to less than 9 pF in LAN applications, and less than 10 pF in WAN applications.

These designs have been successfully implemented in applications up to the 100-meter loop length limits for 100Base-T Ethernet loops. They have been tested for conformance with the GR 1089 and UL 1950 requirements. By using the compensating fast-switching diodes, the bit error rates are well within acceptable limits. Also, the eye-pattern elliptical shape (a visual tool used to determine the quality of an equalized transmission line signal) is still well defined and undistorted. These designs result in a cost-effective and robust solution for the protection of Ethernet connections.

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