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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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SteveThe voltage applied to the telephone wires is related to the distance at which the telephone can be operated. Higher voltage can signal to greater distance. High voltage, however, presents a hazard to both customers and technicians. Thus 48 volts is a compromise between safety and distance. Usually central office battery is adjusted to between 50 and 52 volts, but the nominal voltage is still used in documents and designations.
In the middle 20th century, long loops in many rural areas of North America used range extenders, which operated at 100 or 130 volts to ensure reliable signaling. Some rural switching systems were designed to apply range extenders internally, while for other lines the extender was an external applique.
Originally, the voltages were positive. Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage, the copper is protected from corrosion.
Telephones run on voltages from 36 v to 50 v. It's a safe voltage and high enough to allow phones to work 2-3 miles from the nearest exchange (switching centre). It is common to earth the positive side, for historic reasons which are probably based on minimising electrolytic damage to the cables.
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How do you get -48v DC?
In a 48V DC system you ground the positive to 0V so the live wire will be -48V.
How do you wire -48v dc power and ground to earth?
always think of any supply system so : RED = MOST POSITIVE BLACK = MOST NEGATIVE in a telecom system the -48V supply is refferenced to ground and earth, so the supply wire is NEGATIVE, this means a DC supply most positive signal the RED wire, go to EARTH = ZERO, the BLACK wire is the MOST negative, so in this case it is -48V
Why is a rectifier needed for a GSM BTS?
This is bcoz we need -48V for Gsm BTS, -48 DC is used to control Fluctuation So the main function of this is 1) To convert AC to DC 2) To control overloading
Why DC is preferred in telecom industry?
One main advantage is that it is easier, cheaper, and more efficient to battery back the system. -48V DC can be made simply of 4 12V batteries (think giant car batteries) in series. The string looks like this: AC POWER --> RECTIFIER --> BATTERIES --> DC LOAD To battery back an AC system you have this string. AC POWER --> RECTIFIER --> BATTERIES --> INVERTER --> AC LOAD (which is usually converted back to DC by the equipment's power supply to run the electronics). No step in this process is 100% efficient, and every part costs $$. DC eliminates a lot of these steps. This is not to say that everything in a Central Office is DC powered -- they still have Air Conditioning, Lights, convenience outlets, etc -- but if you're ever in a Central Office when the power goes out, those things turn off until the generator kicks on and the transfer switch swings. Also -- doing things with Low Voltage (-48V) does have some EXPENSIVE costs associated with it. COPPER power runs are very expensive because of the low voltage used (-48VDC) -- it takes more than twice as much copper to feed the same Wattage/Amperage as a 120V supply. A=V*W W=V/A V=W/A The simplest rating on a wire is how many A it can draw for how many feet (there are other ratings, such as maximum voltage, but most of these approach 600V, so don't apply to this comparison). Most of these additional costs are one-time costs and therefore are justifiable.
What is the main function of auxiliary transforme in a substation?
auxiliary transformer is used to supply low voltage for AC power system inside substation such as lighting ,air conditioners and other AC supply system and DC power system such as protection relays, batteries, SCADA & telecom system and other DC supply system
