Marko Radojicic writes: > Battery Return is also problematic. In NA, it *must* be held isolated from > CG/FG. In the EU, it *must* be bonded to the CG/FG! >
Hi Marko: Thank you for your very informative response. I would just like to clarify any possible distinction between battery return and signal ground when we are talking about isolation from frame ground. You point out that the "requirement" to isolate signal ground from frame ground is now routinely violated in North America, and seems to be fading away as a requirement. Is this also true for the "requirement" to isolate battery return from frame ground? An example of where isolation becomes problematic is an analog PSTN SLIC (Subscriber Loop Interface Circuit) line card. Here we are feeding DC current to the loop directly from the -48V battery. The circuit is typically implemented using an integrated SLIC chip that has inputs for both the -48V battery and various low level signals such as transmit, receive, and some logic control. Since both battery and signal ground coexist on the same physical silicon chip, it is not possible to separate them in the chip. To meet the isolation "requirement," I have seen some designers go to extraordinary lengths to fully isolate the codec-SLIC subsystem from the remainder of the electronics. This is typically done with an array of high speed optical isolators inserted in the PCM highway. I have never implemented this isolation in my own designs, and so far I have not had a problem with any isolation "requirement" for the battery. However, I see the confusion on this subject and I do see some other designers implementing an isolation barrier so that the -48V battery circuits are fully isolated from frame ground within the equipment. Can you clarify whether your earlier remarks are applicable to battery return as well as signal ground? Thanks, Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
