Locating and Diagnosing a Ring Trip Fault

[DavePEI]

Finding and Diagnosing a Ring Trip Fault in your line:

First of all, I don't pretend to be an expert in finding these faults - I expect there to be all sorts of corrections and amendments, but I hope it will be useful to others.

First of all, what is ring trip?

"In a POTS switching system, ringing is said to be "tripped" when the impedance of the line reduces to about 600 ohms when the telephone handset is lifted off the switch-hook. This signals that the telephone call has been answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call. This is the source of the name of the problem called "ring-trip" or "pre-trip", which occurs when the ringing signal on the line encounters excessively low resistance between the conductors, which trips the ring before the subscriber's actual telephone has a chance to ring (for more than a very short time); this is common with wet weather and improperly installed lines." -- definition from Wikopedia

What made me think of this, is last weekend, Linda told me our lines were misbehaving, getting only a partial ring, then when picked up giving only dial tone. I was able to confirm this using the cell phone. Now, we have a very complicated telephone distribution system, one line providing lines in both the house and in the museum next door. One could dial out and carry on a conversation, but the phone wouldn't ring to signal an incoming call. As it was late in the day, I quickly determined it was the branch line to the museum which was faulty - I temporarily disconnected it which restored service to the front house for the night.

Most often when a phone line fails, it will simply go dead. However, this is a fault known as a ring trip fault, which will allow you to dial out and make calls, but will not allow an outside party to call in. The symptom is one very short ring, and the line when picked up will give only dial tone.

The next morning, I went to work in the museum using the procedure detailed below. First thing I did, putting the cart ahead of the horse, I changed a long section of CAT 3 cable between both houses, and replaced it with a new section of Outdoor CAT5 cable. While this didn't cure the problem, it was needed.

While doing this, I changed the junction block in the museum from a modified 66 block to a TII Network Technologies 68-2 terminal block with insulation displacement contacts. This block requires no tools, and the connected pairs are simply placed into the input terminals in the fully up position. When snapped down, the insulation is pierced in the gel filled rockers, and the connection is made. Why I did this, is I have had the block for a while, and it allows one to very easily temporarily isolate lines within the premises to ease troubleshooting.

Eventually, I narrowed the problem down to a bad phone jack - one of about 30 in the system. Green corrosion had formed across the ring and tip contacts in the socket, resulting in its resistance lowering when it was humid. Further complicating the troubleshooting in its case, when the humidity was low, if didn't conduct, and the ring was normal. Left on its own, this would eventually have become a full time short, and would have been much easier to find but meanwhile, we couldn't have received calls. Most cases of ring trip faults wont be so difficult to locate, but this one was a real pain!

Now, often it will occur on lines belonging to the telephone company, but all lines inside your house must be maintained by you. Your first step should be to connect a known good phone to the NID (Network Interface Device) on the outside of your house where the phone line enters it. Call your number from a cell phone, and see if the phone rings normally. If it does, the problem is inside our house, and you will be charged for any service provided from the phone company.

To track it down, you need to isolate your phone wiring into sections. If you have the main line coming into a junction box, disconnect all wires at the box, noting where they came from, and what colours the wires are. Call the line again from a cell phone. If it works, it is time to go further. If it doesn't, the problem is between the NID and that junction box, a bad wire. Continuing on, re-connect each line, one at a time, placing a call, one line at a time verifying that it works. When you come across one which causes the ring to trip, it is time to trace further.

You need to locate the next junction on that line. Disconnect the lines at that junction, again noting where the wires go, and what colour goes to which terminal. If that junction happens to be a jack with a modular plug, look into the socket portion, and make sure there is no dust or corrosion on the contacts inside. If there is, replace that jack - don't even try to clean it - if it is corrosion it will re-grow. If it appears clean, reconnect the lines attached to that box one at a time, confirming the status of the line by calling as each line is re-connected. If a line appears to be bad, disconnect the telephone at the other end, and try calling again. If that call goes through with the phone disconnected, temporarily replace that phone with a known good one. If it doesn't return the line to normal, replace the wire. If that doesn't bring things back to normal, replace the jack.

It is all a matter of logic and confirming "each link in the chain" works before moving on to the next. Ring trip faults can be a real pain to find, but using a logical approach, they can be found using no special test instruments other than a cell phone and a screw driver.

When having ring trip problems, watch for in particular; sloppy wiring (neaten it up as you go), wires running around the exterior of the house, - be aware water could be entering the line from a misplaced staple causing a situation where the resistance between ring and tip could be low enough to cause a problem, corrosion or dust on anything - in particular phone jacks (especially when the problem is made worse by humidity), and faulty telephones.

I hope this will help you find what can be a very confusing line fault - I would far rather diagnose a completely dead line, but one does need to be familiar with a ring trip fault and how to cure it.

Let me know if you find this helpful -- if it is, I will post other repair related subjects from time to time...

Dave

[rdelius]

Modular jacks can also get carbon between the pins from lightning  or a foreign voltage and can conduct then hit by ringing voltage.
Robby

[GG]

Another cause can be dust or dirt in modular jack contacts or between pins on type 66 connecting blocks.  In damp or humid weather or with any moisture or water, the dust or dirt becomes a high-impedance short that conducts ringing voltage. 

This can also happen in outside plant, so if you disconnect everything and it's still happening, call telco repair and report the trouble and the steps you took. 

[DavePEI]

Another cause can be dust or dirt in modular jack contacts or between pins on type 66 connecting blocks.  In damp or humid weather or with any moisture or water, the dust or dirt becomes a high-impedance short that conducts ringing voltage.  

This can also happen in outside plant, so if you disconnect everything and it's still happening, call telco repair and report the trouble and the steps you took.  

That is why I mention checking modular jacks first, as due to the fact humidity appeared to be triggering it,  was expecting dust to be the problem. I did check them, but missed that one jack. I thought I had checked all of them, but missed that one.

There are about 30 jacks in the museum alone and about 1500 feet of cable running back to the junction box. There are another 4-500 feet of cable used in the Norstar 6x16 system which provides the museum's main telephone service, three operational demonstrators in the main display area, one in the new display area, and one which I use in the new office/workshop area. The Norstar system provides its own ring and dial tone, and is a very low REN. Were all the jacks all in use, I would NEVER get a ring, I would be so far above the ringer equivalency:-) Only two  have phones actually plugged into them, the Norstar System and one Centurion Payphone, but they are available for various demonstrations. The new display area has seven phones hooked up, but to two Teltone line simulators, a TLS-3A and TLS-5A, not to the Telco line.

I do mention above checking your outside line by plugging a known good phone into your NID and calling your number from a cell phone. This will let you know if your problem is in the outside plant or in your local wiring.

Dave