Message text written by INTERNET:[email protected] >You'll see -48 and 60 VDC in CO's, worldwide, typically. AC (various) is also seen. 24VDC, I believe, will be seen at cell sites and the like.<
North American systems and most European systems are 48Vdc systems at this time. That voltage was more or less arrived at by the loop lengths that had to be served by a Central Battery system in the Central Office. Later, for Teletype, they added in many Central Offices +130 Vdc and -130 Vdc and for long loops they connected the -130 on one side, the +130 on the other side of a cable pair, so as to be able to draw a constant 60 mA. Interruption of the 60 mA constituted the teletype signal. (Accidentally causing a short between the two by means of long-nose pliers, quickly made for short-nose pliers. I own several pairs). The 130 Vdc was never used for telephony as such, but in some cases was used to power other systems in a Central Office, that needed to be on uninterrupted power if and when the mains failed. There have been some PBXs that used 24 Volt systems, but they never became very popular. (PBXs serve much shorter loops). I believe that the 60 Vdc systems are mainly in Eastern Europe. As information and communications systems of a digital nature penetrate the globe more profoundly, the need for systems build on being able to push a minimum current over a maximum looplength by means of current interruptions diminishes. However, the infrastructure, with the expensive batteries (accumulateurs for you francophiles) is there in every Central Office. At one time most PBXs were capable of sustaining service on a battery plant, if the mains failed. I note that more and more, the owners of PBXs either do not provide service under those conditions, or rely on a stand-by generator. A keen reading of IEC Publication 60950 will show that it was a stretch to make that fit under the umbrella of permissable voltages. It was a major bone of contention when telecom equipment was brought under the umbrella of iec 60950. The fact that those voltages/currents had been used in the Telecom industry for more than 70 years without being considered dangerous makes one wonder about the people who write horizontal standards on the subject. Happy New Year Vic Boersma
