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Why is the telecom power supply voltage level specifieded with a wide range: 36 to 72 V?

Is it because the rectified AC voltage would have variations as per AC line variations? We could easily get a regulated output (48 V) with much smaller variations.

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Picture source.

Is it because some telecom boards/ peripherals would need higher voltage (72 V) to operate?

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Picture source.

Does it have anything with battery backup?

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    \$\begingroup\$ re: we could "easily"... you may be able to do so today, but now when the standard was rolled out. \$\endgroup\$
    – Vladimir Cravero
    Commented Apr 19, 2022 at 11:43
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    \$\begingroup\$ You could easily get regulated 48V output, but someone setting up a telephone exchange in the early 1900s would have had considerably more trouble. \$\endgroup\$
    – Mark
    Commented Apr 20, 2022 at 0:54

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It's to allow a telephone exchange to run on batteries during a power cut. Specifically, a 48V stack of lead acid batteries.

While the batteries are charging, the voltage could be around 57V. When they are flat, they could be down to 40V.

Whoever drew up the specification allowed a bit of safety margin on that range.

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    \$\begingroup\$ Negative voltage is also used due to the chemistry causing less corrosion of contacts, which was a big deal back in the days of branch exchange switches having physical metal contacts. The choice of 48 nominal was rumored to avoid paying union wages to early employees since anything above 60 volts was union work. During power failures, I've seen voltages go to just under 40 as Simon B stated. Also, battery chargers sometimes do an "equalize" on the cells, and voltages go even higher than 57 for short periods of time. \$\endgroup\$
    – user3425949
    Commented Apr 19, 2022 at 14:54
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    \$\begingroup\$ Also, some branches of the telephone circuit can run for extremely long distances. You can experience a considerable voltage drop over several miles of wire, so the voltage range necessarily had to be rather wide. \$\endgroup\$
    – bta
    Commented Apr 19, 2022 at 19:09
  • \$\begingroup\$ IIRC polarity indicates whether the phone system is currently running on emergency power. \$\endgroup\$
    – Simon Richter
    Commented Apr 20, 2022 at 11:55
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Sometimes the nominal 48V flooded lead acid battery is "equalized" by placing a current limited higher than normal charging voltage on the series string. This is also done on electric forklifts. The upper limit means that equipment can remain connected while the high voltage is being applied. The lower limit is to allow operation down to the undervolt lockout of typically 1.75 volts per cell.

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