In a message dated 12/15/99, Roger Magnuson writes: > One safety laboratory we know use 56.5 V and 400 ohm as test conditions. 56.5 V is > OK with us (being one possible line voltage) but we don't agree with the 400 ohm as it > seems too harsh to be reflecting Real Life.
Roger: The old electromechanical central offices that were deployed widely in the USA used a typical 48 V battery and a typical 400 ohm feeding resistance (this combination was also used in several other countries). The value of 56.5 volts is sometimes used to represent the worst case voltage for a 48 V nominal central office battery system. Overall, I think that a 56.5V/400-ohm feed is a reasonable representation of a worst-case feeding condition in the USA. With today's modern electronic line cards, most central offices now limit the loop current to a value of 30 to 40 mA. In addition to conserving battery power, this step is necessary to prevent the solid state feed circuits from overheating. Unfortunately, there are probably still some of those old electromechanical central offices in operation in rural areas, so it is still possible to encounter a feed condition similar to 56.5V/400-ohm. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
