Why use negative DC voltage?
Telecommunications - Negative voltages are used as a safeguard against corrosion of the copper lines used in telecommunication systems
Positive grounding has been used in the telecommunications industry for many years, primarily because the grounded positive electrode of a battery bank will corrode at a much slower rate than a grounded negative electrode
It was more efficient because of lower voltage loss over distance (as a % of the operational voltage), smaller-gauge wire requirements
Why is 48V used in telecom?
Telecom networks use -48 volt direct current (VDC) because it's safe, efficient, and can support telecom signals.
Safety
-48 VDC is considered a safe low voltage, and is accepted by electrical codes
It's safer than alternating current (AC) systems, which can cause hum, noise, and other issues
It has a lower risk of electric shock and arc formation in the event of a fault or short circuit
Efficiency
It reduces power distribution losses, lowering overall energy consumption
It allows for the use of smaller gauge wires, while still being capable of carrying power over long wire runs
Support for telecom signals
It provides enough power to support a telecom signal
It's also suitable for battery backup systems
Other benefits
It's compatible with various energy storage technologies
It supports predictive maintenance to optimize performance
It's also beneficial to the automotive industry, especially in electric vehicles (EVs)
VRLA stands for Valve Regulated Lead-Acid, a type of rechargeable battery that is also known as a sealed lead-acid battery
Lead-acid cells consist of two plates of lead, which serve as electrodes, suspended in an electrolyte consisting of diluted sulfuric acid. VRLA cells have the same chemistry except that the electrolyte is immobilized
When a cell discharges, the lead and diluted acid undergo a chemical reaction that produces lead sulfate and water. When a cell is subsequently charged, the lead sulfate and water are turned back into lead and acid.
VALVE:-
Safety valve vent plugs shall be provided in each cell. They shall be explosion resistant, self-resealing and pressure regulating type. They shall not allow gas (air) to enter into the cell but shall allow gas to escape from the cell above a certain internal pressure, which does not lead to deformation or other damage to the cell.
The vent plug used shall be explosion resistant and self re-sealing pressure regulating type. Vent
plug shall be such tat it cannot be opened without proper tool.
The valve shall be so designed that it operates at a pressure between 0.14 Kg / Sq. mm to 0.63 Kg/ Sq. mm to release the excess gas and reseal automatically as soon as the gas pressure within the cell drops to atmospheric value.
All the cells shall be subjected to pressure test upto 0.7 Kg / Sq. mm.
The self-discharge rate at room temperature shall not be more than 5 % of the capacity of each battery per month.
Each valve opening shall be covered with flame barrier capable in preventing the ingress of flame into the cell interior when the valve opens and hydrogen / oxygen gas mixture is released.
NeedofD.C.Power supply
Harmonics of A.C may affect the speech signals.
● Relays used in telecom systems are more sensitive to D.C than A.C
● Transistors and I.C.s etc. being unidirectional devices, the use of D.C has become
necessary.
● Arranging standby source to A.C is difficult compare to D.C for which secondary
cells can be used as standby source.
● Nothazardous to human life.
