kabra437@athenanet.com (Ken Abrams) (02/26/91)
In article <telecom11.143.3@eecs.nwu.edu> floyd@ims.alaska.edu (Floyd Davidson) writes: > In reference to using 117 VAC 60 Hz to ring a phone: > One more time: 100 VAC 20 Hz is JUST as dangerous, if not more so, > that 117 VAC 60 Hz. Using house current to ring the phone is no more, > and no less, dangerous, than ANY other reasonable way you can make the > ringer work. The REAL issue is not the voltage applied but the ability of the source to provide current limiting. 10,000 volts at .00000001 ma is not dangerous (static electricity). 100V, 20HZ and 117V, 60 HZ are both deadly if they are not current limited. The wall socket that provides the 117V certainly is NOT current limited (to any practical degree) without a device in series to accomplish that. A suitable current limiting device would make 117V, 60 HZ suitable for use to ring the phone with little danger. The 88-100V, 20 HZ normally used to ring a phone on the network is current limited at the source AND by virtue of the loop resistance between the CO and the phone (typically 200 to 500 ohms or greater). Standard telco ring voltage will give you a nasty surprise but is not (usually) dangerous because it IS current limited. Ken Abrams uunet!pallas!kabra437 Illinois Bell kabra437@athenanet.com Springfield (voice) 217-753-7965
floyd@ims.alaska.edu (Floyd Davidson) (02/28/91)
In article <telecom11.164.12@eecs.nwu.edu> kabra437@athenanet.com (Ken Abrams) writes: > The REAL issue is not the voltage applied but the ability of the > source to provide current limiting. 10,000 volts at .00000001 ma is > not dangerous (static electricity). 100V, 20HZ and 117V, 60 HZ are > both deadly if they are not current limited. The wall socket that > provides the 117V certainly is NOT current limited (to any practical > degree) without a device in series to accomplish that. A suitable > current limiting device would make 117V, 60 HZ suitable for use to > ring the phone with little danger. To a degree, the last sentence is correct. It is no more dangerous than 20Hz ringing current. But... > The 88-100V, 20 HZ normally used > to ring a phone on the network is current limited at the source AND by > virtue of the loop resistance between the CO and the phone (typically > 200 to 500 ohms or greater). Standard telco ring voltage will give > you a nasty surprise but is not (usually) dangerous because it IS > current limited. Don't believe it. The current required to ding the ringer is enough to kill you dead. Both from electrocution and by triggering a heart attack. The current limiting is protection against damage to the physical plant. A direct short will not burn up the wires. In the first article I posted on this subject I suggested current limiting using a common 120vac lamp, and in email to the person who requested information to begin with I provided exact details down to the math to calculate maximum current for a given size of lamp. And enough detail and emphasis to convince anyone that it is NOT optional. The one real safety feature of a normal 20Hz ring supply is that it is interupted, which lets you loose if you get across it on your phone line. However, there were several suggestions to use a subcylce ring generator, which are not necessarily interupted. Likewise most of the 20Hz ring current which I am exposed to on private line circuits is NOT interupted. Floyd L. Davidson | floyd@ims.alaska.edu | Alascom, Inc. pays me Salcha, AK 99714 | Univ. of Alaska | but not for opinions.