What voltage range is expected on a POTS dial tone line when measured on-hook?

What voltage range is expected on a POTS dial tone line when measured on-hook?

• A. 12-24 VDC
• B. 24-36 VDC
• C. 48-52 VDC
• D. 60-70 VDC

Answer: (C)

A POTS line has a nominal DC supply on the two-wire pair, and when the phone is on-hook (resting), no current flows through the loop, so you measure the full supply voltage. That standby voltage is about 48 volts DC, with typical variation that places it in the 48–52 VDC range. So this range matches what you’d expect to see on an idle POTS line. The other options are outside the normal stand-by telephony supply: voltages well below 40 V aren’t enough for the standard loop power, and voltages well above the nominal (like 60–70 V) aren’t representative of the usual POTS standby condition. When the line is off-hook (the handset lifted), current flows and the voltage drops, often to only a few volts, which is why the high value is only seen in the idle state.

A POTS line has a nominal DC supply on the two-wire pair, and when the phone is on-hook (resting), no current flows through the loop, so you measure the full supply voltage. That standby voltage is about 48 volts DC, with typical variation that places it in the 48–52 VDC range. So this range matches what you’d expect to see on an idle POTS line. The other options are outside the normal stand-by telephony supply: voltages well below 40 V aren’t enough for the standard loop power, and voltages well above the nominal (like 60–70 V) aren’t representative of the usual POTS standby condition. When the line is off-hook (the handset lifted), current flows and the voltage drops, often to only a few volts, which is why the high value is only seen in the idle state.