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|Why telecom power supply has -48V dc power supply?|
In earlier days of telephone networks, 48V DC was once found to be suitably high to be able to make telephone work on long telephone lines and still low enough not to cause serious danger if somebody touches the telephone wires. Telephone central offices (exchanges) so started using 48V DC. Even when newer automatic exhanges were setup, they were designed to make use of existing 48V battery sets and arrays which were available with telecom service providers. This legacy design practice has been continued and over many decades all new telecom devices have been designed to work with 48V.
The positive grounded or -48V system is also from telephone history. The negative voltage on the line was better than positive to prevent electro-chemical reactions from destroying the copper cable quickly, if cables get wet
Ans- It is because of cathodic protection. It reduces the sulphation on the battery terminals.
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