Voltage and Current Handling on the Phone Line
The voltage on the phone line is typically around 20V. When an incoming call occurs, the telephone line voltage increases to 48V, causing the phone to ring. When the phone is off-hook to answer the call, the phone line voltage drops back down to approximately 10V.
Telephone lines have current flow limitations, with fuses installed before the cable enters. The switching equipment supplies power to each module upon activation. The telephone line operates at 48V, with resistance connected in series. The line resistance is typically around 600 ohms at each end. The highest power consumption occurs during ringing. The circuit design considers the normal call current, with an average DC power of 48V per household not exceeding 20 mA. If a user draws 100 mA for an extended period, monitoring equipment will automatically cut off power to the user.
Modern program-controlled switchboards use a DC feed voltage of 48V. The telephone’s actual voltage when on-hook is about DC 50V due to the floating lead-acid battery. When the phone is off-hook, the phone’s DC circuit connects, allowing current flow. The voltage drops to 7-10V depending on the line length and phone model.
Post off-hook, the current varies due to the subscriber line, phone, and intra-office switching equipment, typically ranging from 20-30 mA. When on-hook, the current is approximately 5 microamps.
During ringing, the AC voltage is 75V with a frequency of 16-25 Hz.
Phone Specifications:
- On-hook: 48V
- Ringing: 90V
- Call: 8V
These values can vary slightly within a specified range. The DC voltage when on-hook at the local office is 48V.
Ringing:
- Frequency: 20 Hz
- Voltage: Above 75V
Call:
- Depends on line and phone quality, generally between DC 6-12V.
Small switches typically use either 24V or 48V DC.
Ringing and talking voltages are similar. Voltages above 36V are not harmful to people due to their extremely low current.