Ring trip/blank line problems: cordless vs conventional phones?

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 Question by Carol Percy posted 05 May 2005
 Ring trip/blank line problems: cordless vs conventional phones?
I am a layperson and new to all of this.

In the last month, all 4 of my conventional telephones (Harmony) have ended up causing problems with incoming calls: the phone rings once, then the line goes blank.

As far as I can tell, only the new cordless phone that I have bought works now. It seems to work ok by itself, but I'm now wondering whether it is in fact the cause of what is happening to my conventional handsets. (I am about to buy another $10 phone and use it by itself on the line to see whether it works ok.) Or is the problem more likely to be something else that affects the Harmony phones but not the cordless one? The cordless is a VTech, if that helps.

Thanks for any help you can give.

Carol Percy
 Answer by Joseph Golan posted 06 May 2005
Dear Carol,

I will try and not get too technical here but one thing about your quiry disturbs me. Typically, lines from a telco as designed to handle about 5 ringers or ringer circuits. Ringer circuits are used in things like answering machines, modems and FAX machines to detect an incoming call. Most telephones today have information on the bottom about a "Ringer Equivalence number", If your total ringer equivalence (including all devices above) should not exceed 5, otherwise that may be your problem.

The condition you are describing is called "Pre-Trip) meaning that the line has pre-tripped prior to a telephone going off-hook. Tripping is a signal (DC short) to tell the central office that the customer has answered the call and to stop sending ringing voltage.

The ringer portions of telephones (cordless and conventional) are fairly simple and should be similar in nature.

It is possible for the ringer circuit of one telephone to do exactly what you are describing. Basically there is a capacitor or opto-isolator in the circuit that prevents the ringer (oldstyle was a coil, new styles are electronic circuits) from appearing as a DC short on the line, allowing the AC ringing signal to pass. Typically a DC short is created when you pick up the handset to make or answer a call). If this capacitor was failing, it may act as you described but it would be rare.

Another cause of this situation is wet cabling, most likely outside the home, which causes a short when the higher ring voltage (typically 90 V AC @ 20-30 Hz) is sent down the line from the central office. The higher voltage arcs from one side of the pair to the other causing a temporary DC short thus tripping the ringing.

In older homes, that have not had a Network Interface Devive (NID) installed by the service provider, there may also be a carbon lightning protector, which when mositure gets into it, may cause a lower resistance to ground which will give the same symptom

A user might see either of the above situations come and go as the cabling or protector gets wet (rain storm) and then dries out in a day or two or three.

Joseph Golan, RCDD

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