Intermittent faults ('floating' defects) are damages that manifest themselves periodically and are caused by poor-quality core connections or reduced insulation resistance. Customer complaints about short-term connection losses are evidence of defects of this kind. Such defects may appear due to mechanical damage to the cable (for example, in the event of vibration from heavy vehicles, rotary equipment, etc., nearby).

Typically, when a technician encounters this type of damage, he has to wait patiently for it to manifest itself, hoping the effect will last long enough to determine its location. There is no guarantee that the damage will reveal itself while the technician is on duty. The use of reflectometers allows one to automate this process and maximize productivity.

Some reflectometers have a special function for detecting intermittent faults. The device connected to the line accumulates all reflectograms over a certain period and displays them superimposed on each other. Where the reflectogram differs, the intermittent fault is located.

Finding intermittent faults

For example, consider the following situation: a particular pair of cables works fine for the better part of the day, but there is a momentary failure out of the blue.

We get two reflectograms for the same pair (with different gain settings) when checked. In the first one, with a gain of 12 dB, a surge of positive polarity is observed on the reflectogram of a working pair at a distance of 6760 feet, corresponding to the end of the cable. In the second one, when the gain increases by 14 dB, an additional spike appears on the reflectogram, the nature of which indicates the presence of a coupling in the cable at a distance of 3280 feet. By further increasing the vertical gain level, the reflectogram will not reveal the slightest sign of damage along the entire length of the cable being tested.

We will need the 'intermittent fault detection' function mentioned above. By continuously monitoring the pair's condition, the OTDR shows any deviations from the cable's rated impedance, allowing the location of intermittent faults to be pinpointed.

The reflectometer display will show the current reflectograms obtained during testing. Periodic inspections allow one to determine whether signs of malfunction have appeared. Once the non-persistent damage has been captured, the result should look approximately as shown in the figure.

The differences will be evident if one compares it with the previous one. A noticeable drop appears where there was nothing before. The location of the fault can be determined by simply moving the cursor to the front of the pulse reflected from the break and reading the distance from the display.

Random vibrations or other irregular events cause the connections to loosen and electrical contact to be temporarily lost, resulting in a fault similar to a partial break. Note that at the moment this fault occurs, the pulse reflected from the far open end of the line decreases because, due to a poor connection in the cable coupling, the magnitude of the electrical signal reaching the end of the cable is reduced.

What conclusions can be drawn? Almost every type of cable system is susceptible to intermittent faults. Such damage creates severe problems for users and technicians. The intermittent fault detection mode of reflectometers allows one to continuously monitor the cable over a long period, so the technician does not have to waste working hours waiting for the damage to manifest itself.

Pupin coils

Pupin coils can still be found on an analog telephone line. Pupin coils disrupt the homogeneity of the copper pair, turning it into an ideal low-pass filter with more substantial high-frequency attenuation.

Therefore, a prerequisite for using any xDSL technologies on existing phone lines is the removal of Pupin coils, which have been found to have extensive applications in US telephone networks. Servicing xDSL systems can always result in such a problem. In this case, one will need a reflectometer with a function for searching and counting Pupin coils.

Searching for installation locations of Pupin coils

A reflectometer is the only device that allows one to simply and accurately determine the location of Pupin coils. Since the pulses sent by the reflectometer are high-frequency, they are reflected from the Pupin coil, a low-pass filter. The coil on the reflectogram looks like a significant increase in the cable impedance, i.e., similar to the reflectogram of a line break.

As you can see, the outline of the pulse reflected from the Pupin coil is more rounded than the pulse reflected from the cable brake, and the coil itself is located at a distance of about 5600 feet. In Eastern Europe, there are several Pupinization systems: medium, light, extra light, and broadcasting light. All systems have the same pitch of 1.7 km and differ in the inductance of the coils, the bandwidth of the transmitted frequencies, and the distance between the amplifiers. Unfortunately, the reflectogram shows only the first coil.

Counting the number of Pupin coils

The reflectometer can also be used as an auxiliary device when taking resistive bridge measurements. The presence of Pupin coils introduces inaccuracy into the readings since each of them adds about 4 ohms to the resistance value obtained during the measurement. Using a Pupin coil counter allows one to approximately estimate the number of coils installed on the line and determine the error of measurements made using a resistive bridge.

For example, 4 ohms corresponds to approximately 500 feet of 20 AWG cable. This means that with each installed Pupin coil, the measurement values obtained using a resistive bridge will be almost 500 feet longer. If one suspects the results may contain an error, use a Pupin coil counter.