The Phone Repair Company

TDR Testing Information
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Short

The TDR ability to see a short on a twisted copper cable pair is limited to around 1,000 Ohms of resistance Trouble higher than 1,000 Ohms will not reflect sufficient single for the receiver to see it.
Crosses can be seen with a TDR if you know broth sides of the cross and the resistance between the two sides are not higher than 1,000 Ohms. The reading to the cross will not be correct because of the change in the VP.
It has been said that the only measurement that is right when using a TDR is the first large reflection all the rest will be wrong. I find this not to be true. If they are wrong it is so little it will not change the way you run the trouble.
You should have with you a back off wire of 25’ to put between you and the cable pair. This will allow you to see a trouble close to the beginning of your cable.

Cross Pairs

Crosses can be seen with a TDR if you know broth sides of the cross and the resistance between the two sides are not higher than 1,000 Ohms. The reading to the cross will not be correct because of the change in the VP

Open

Opens are a high reflection and will get smaller the more distance you have between the TDR and the open Pair ore open side of the pair.
When using a TDR on an open make sure that you are not looking at a load coil. ( load coil have the same sine wave as a open )
You cannot see through an open in the cable pair. A TDR will show a number of opens on one pair.

The 2nd and 3rd open are harmonies of the first trouble and will show at the distance to the open x 2 or times 3 (if the open is at 1,500’ the 2nd open you will see is At 3,000)

High Opens

A high open is always between you and the central office ( If it was out a leg of cable or to the field it would be a UB (unbalance pair )
High opens are a high reflection sine wave.

To test for a high open

On the office end of the cable pair hook the tip and ring up to a good tested ground (<25 Ohms) if there no test for a good ground, then hook up to a power neutral.
At the field end of the cable pair using an Ohm meter with a digital read out.
On the field end, test the ground with the ground resistance test or use a power neutral hook up.
Test the pair Tip to ground Ring to ground, and Tip to ring using the Ohms. Record each reading.
The Tip Ohms and the Ring Reading should be the same. Halve the amount of the Tip to Ring Ohms reading.
If there is a difference of 3 Ohms or more between the Tip and Ring side to ground there is a high open on the pair. ( Long loops of 700 Ohms or more should be cut in half, and the test done twice)

The presence of a high open of 3 Ohms will cause Circuit xDSL, dsl, ext, ext to drop at times, to produce errors and a number of outer tricks
The TDR will see a high open with an Ohms reading as low as 3 Ohms. You just might have to get close to it.
If you have a high open under a sheath and you would like to open at the trouble I would suggest you call me first or put a large # of closure on your truck.

Bridge Taps

A low reflection single they look like a short.
They are a large drain on the TDR single ( 3db of loss or half the power of the single )
Any 3 way splice is a bridge tap. ( Inside wire ,Drops, cable pair and Your test equipment on a working Line)

Load Coils

A high reflection single they look like a open
Subtract 1% of the reading for a good reading

Water in air core cable

When testing a wet section to see water, a TDR will show the cable to be dry if the section under test has 100% water from end to end.
Once an air core cable has water in it, there is nothing that can be done to Repair the cable. It has to be replaced.
When testing for water, make a test from each end of the section under test. Mark the distance to the first high single reflection on the ground then go to the other end and repeat the test, and mark the ground from that end. The distance between the 2 marks is the water.

Contact Jake if you have any questions.

“There is no problem in any twisted copper cable that cannot be located in a timely manner and a solution found for the repair of such problem” - John “Jake “Jacobsen

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