What should the meter read when measuring a POTS with the phone on the hook (the $J31X jack connector)?

What should the meter read when measuring a POTS with the phone on the hook (the $J31X jack connector)?

• A. 40-44 Volts DC
• B. 60-64 Volts DC
• C. 48-52 Volts DC
• D. 70-74 Volts DC

Answer: (C)

On a POTS line, the telephone network provides a DC power supply to the pair to power the handset. When the phone is on-hook, that circuit is open and only a tiny current is drawn, so the line voltage remains essentially at the supply level. That supply is about 48 volts DC, with a typical tolerance, so a meter across the pair at the $J31X jack will read in the around 48 volts DC range—commonly 48 to 52 volts. If you were to take the phone off the hook, current would flow and the voltage would drop significantly, which is why the resting (on-hook) reading is the higher, nearly nominal supply voltage.

On a POTS line, the telephone network provides a DC power supply to the pair to power the handset. When the phone is on-hook, that circuit is open and only a tiny current is drawn, so the line voltage remains essentially at the supply level. That supply is about 48 volts DC, with a typical tolerance, so a meter across the pair at the $J31X jack will read in the around 48 volts DC range—commonly 48 to 52 volts. If you were to take the phone off the hook, current would flow and the voltage would drop significantly, which is why the resting (on-hook) reading is the higher, nearly nominal supply voltage.