In a message dated 5/18/99, [email protected] writes: > I'm sure many of you have seen both topologies described in application > notes for various interface components, and have had to deal with both. > Any light shed will be appreciated by all. >
Dwight: If the interface is a ground-referenced circuit such as a central office SLIC, ground-start FXO, or a PBX DID circuit, it is essential to provide 3-way protection. These circuits are not isolated from ground, so common mode surges will induce large currents to ground. In a terminal device such as a modem, tip and ring are typically isolated from ground with a transformer or other isolation devices (optos, capacitors, etc). For these types of circuits, it is possible to use 1-way (metallic) protection, and rely on the isolation barrier for common mode protection. This saves some cost in the protection circuit, and eliminates concerns about where the common mode surge currents go. In consumer products, there is not always a reliable connection to earth ground to accept common mode surges. The main catch with relying on the isolation barrier for common mode protection is the degree of protection that you get. If you use inexpensive isolation devices, the isolation barrier may only be good for about 1500 volts peak. In theory, this is enough for FCC Part 68 and for lines that are protected by a primary protector at the service entrance, although Bellcore/Telcordia has a 2500 volt common mode surge. Unfortunately, the primary protector at the service entrance is sometimes disconnected or defective, especially on residential installations. Without a primary protector, common mode surges can reach about 4000 to 5000 volts peak, even for induced surges that do not represent direct hits (you can forget about direct hits; nothing will protect you from those). So, a robust protection scheme that relies on the isolation barrier for common mode protection should have an isolation barrier that can withstand surges of 4000 to 5000 volts. Sometimes it is worth it to pay a little extra for high-dielectric isolation devices, in order to avoid having to install 3-way protection. If you use 3-way protection, you will need a fuse in tip and a fuse in ring to pass UL 1950, while 1-way protection only requires one fuse. I hope the above remarks make sense. The overall answer to your question is that 3-way protection is essential on some type of interfaces, and optional on others. If you use 1-way protection when that is an option, you are relying on the isolation barrier to protect the circuit from common mode surges. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (voice) 781-721-0582 (fax)
