電話線中的電壓是20V。當(dāng)有電話呼入時(shí)，電話線電壓上升至48V，話機(jī)便響鈴。當(dāng)摘機(jī)應(yīng)答時(shí)，電話線電壓又會(huì)下降至10V。

電話線路對(duì)電流有一定的限制，進(jìn)入電纜前就安裝了保險(xiǎn)絲。交換設(shè)備供出一個(gè)個(gè)模塊出來(lái)時(shí)都有電，電話線是48V供電，線路上已串接了電阻，每一端大約600歐姆，負(fù)載稍微超過(guò)，交換機(jī)就會(huì)認(rèn)為摘機(jī)，這樣就會(huì)影響通訊。電話響鈴的時(shí)候耗電量是最大的，線路設(shè)計(jì)時(shí)已經(jīng)考慮到正常通話的電流，平均每戶48V的直流電不超過(guò)20毫安。在某個(gè)用戶端長(zhǎng)時(shí)間取電100A時(shí)就會(huì)被監(jiān)測(cè)設(shè)備監(jiān)測(cè)到，自動(dòng)給該用戶閉電。

目前使用的程控交換機(jī)，直流饋電電壓是48V，由于鉛蓄電池是浮充工作的，電話機(jī)掛機(jī)時(shí)實(shí)際電壓約直流50V。當(dāng)電話機(jī)摘機(jī)后，話機(jī)直流回路接通，話機(jī)中有電流，這時(shí)電壓降為7-10V，視線路長(zhǎng)短、話機(jī)不同而異。

　　話機(jī)摘機(jī)后的電流不相同，受用戶線路、電話機(jī)及局內(nèi)交換設(shè)備影響。一般在20-30mA。掛機(jī)時(shí)電流應(yīng)該在5微安左右。

　　當(dāng)鈴響時(shí)，振鈴電壓是75V、頻率為16-25Hz的交流電壓。

原縱橫制交換機(jī) 掛機(jī) 60V
程控交換機(jī)掛機(jī) 48V
鈴流電壓 90V

普通話機(jī)
掛機(jī)48V,震鈴90V,通話8V
但值并不準(zhǔn)確，會(huì)在一定范圍內(nèi)變化。給出的只是一般值

市話局掛機(jī)時(shí)直流48V.

振鈴：20HZ，75V以上

通話：視線路及話機(jī)質(zhì)量，一般在直流6－12V之間

小交換機(jī)：直流電壓一般兩種：24或者48。

振鈴和通話時(shí)差不多。

雖說(shuō)電壓在36V以上，但因其電流極小，對(duì)人是沒(méi)有危害的，放心好啦。

The PTN system had been specified in terms of voltages and I did not see any reference to current.

The maximum (and minimum) voltage that had been defined in the HK standards is a sine wave between +95V and -142V (a sine wave of 75Vrms** with a DC of voltage between 40 and 48V). This voltage is encountered when the phone rings.

The PTN should supply a steady voltage of -40 to -48V when idle (compared to the other wire, which is grounded).

This power should not be used by any other electrical equipment because all equipments that are plugged into the PTN need to be certified or you would have breached the regulations set by OFTA.

** 1Vrms wave is approximately a wave with amplitude of 1.4V, which is about 2.8V from the peak of the wave to the trough of the wave.