Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uwm.edu!spool.mu.edu!news.cs.indiana.edu!cs.widener.edu!dsinc!casbah.acns.nwu.edu!nucsrl!telecom-request From: floyd@ims.alaska.edu (Floyd Davidson) Newsgroups: comp.dcom.telecom Subject: Re: How to Hook up a Phone For a Play Message-ID: Date: 28 Feb 91 07:28:40 GMT Sender: news@casbah.acns.nwu.edu (Mr. News) Organization: University of Alaska, Institute of Marine Science Lines: 47 Approved: Telecom@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 169, Message 5 of 5 Originator: telecom@delta.eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu Nntp-Posting-Host: hub.eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu In article 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.