On Nov 30, 2010, at 9:45 AM, Jack Coats wrote: > Why 48VDC? I am sure it is in the history books for the Bell System and > Western Electric somewhere. > If someone knows the original reason, I would be interested.
-48V is the "on-hook" voltage for the phone system in the US (though it can be as large as -60V in some countries). It's something of a compromise value. You want it as high as possible to get through miles of wire to the customer's house, but 60V is frequently the limit for safe low-voltage operations. 48V is under the limit enough to have a safety margin that makes it acceptable for low-voltage wiring, and also happens to be a nice multiple of 12 for car batteries. Jason -- Jason Healy | [email protected] | http://www.logn.net/ _______________________________________________ Tech mailing list [email protected] https://lists.lopsa.org/cgi-bin/mailman/listinfo/tech This list provided by the League of Professional System Administrators http://lopsa.org/
