|Buried Street Light Faults:
Situation: In a residential subdivision the street lights are out due to a fault in the cable. From the riser pole, the cable extends about 1200 feet. The faulted cable is triplex secondary service cable consisting of two black insulated conductors and one yellow insulated neutral conductor.
The lamps are fed by one black and one yellow insulated conductor tapped into the faulted street light service cables extending from the riser.
The conductors are disconnected from the ballast at each lamp so that they will have no influence on the fault locating operations.
Fault Locating Operations: The conductors were isolated from the feeder at the riser pole.
Before installing the SPITFIRE cable fault locator to the faulted cable, since there were other instruments on hand, it was decided that we try to establish what type of fault we should expect to find.
Using the Impulse Phaser, the Yellow lead was connected to the driven ground at the riser pole. Holding the RED lead clear of ground, the instrument was turned "ON". The RED lamp, located on the panel of the Impulse Phaser Transmitter, lit indicating an open circuit. After switching "OFF" the Impulse Phaser Transmitter, the RED lead was connected to one of the phase conductors. Impulse Phaser Transmitter was turned "ON" again and the RED lamp went out but the AMBER lamp, located on the panel of the Impulse Phaser Transmitter, did not begin to pulse, indicating a high resistance fault in this cable. The RED lead was then connected to the other phase conductor. Again, the AMBER lamp did not begin to pulse, indicating a high resistance fault in this cable.
Leaving the RED lead connected to one of the conductors, the YELLOW lead was connected to the other conductor. The AMBER lamp began to blink indicating that a short circuit existed between these two conductors.
Since a short circuit existed, it was decided that we could possibly identify the shorted cable location if we installed the WAYFIND'R Impulse Cable Tracer. Using a portable generator to obtain a 120 volt source, we installed a pigtail socket into the generator's receptacle. The ground side of the pigtail was connected to a driven ground rod. The energized side of the pigtail was connected to one lead of the WAYFIND'R signal generator. The other lead of the signal generator was connected to one of the shorted phase conductors. The lamp in the end of the signal generator lit brightly, indicating that a good strong signal was being impressed on the cable conductor. A signal which cannot get off that conductor, nor can it be induced into other cables lying near it.
Traveling the route of the cable, about 900' from the riser pole, the signal stopped, indicating the location of the short circuit. The WAYFIND'R Cable Tracer was then removed from the cable.
To pinpoint the fault location, the SPITFIRE was connected to the same faulted conductor as the WAYFIND'R was. Once switched on, the Transmitter began pulsing the faulted conductor and the Detector indicated that the cable was indeed faulted.
Traveling the cable route toward where the short circuit was previously indicated by the WAYFIND'R, we also identified other cable faults at several lamp locations. The construction people indicated that each lamp had two conductors that were spliced into the main street light cables. We had pinpointed additional failures in these splices. (Another good thing about the SPITFIRE, you identify all of the failures before beginning to dig.) At the location where the short circuit was previously indicated by the WAYFIND'R, the fault was pinpointed.
It was decided that the short circuit problem should be corrected first, so excavation began. During the excavation, from time to time, the SPITFIRE was turned on to assist in pinpointing just where to dig to prevent accidental damage to the cable. Soon, an abandoned primary concentric neutral cable was exposed along with the secondary cables. More readings with the SPITFIRE indicated that the fault was still in this area, only deeper. The fault locating crew could not believe that this was possible.
To assist in double checking the fault location, the WAYFIND'R was again connected to the phase conductor. The signal was identified, not only on the secondary cable, but also on the concentric neutral of the abandoned primary cable. This indicated that the secondary cables were shorted to the concentric neutral somewhere close at hand. The crew still could not believe that the fault could be at this location.
It was decided to dig two more holes along the cable route, expose the secondary cables, and see if the WAYFIND'R signal could be found on either the secondary or primary cables. They exposed the cables at these two locations. Meanwhile, we had a different idea.
Since the cables were shorted somewhere in this 1200 foot run, if we connected the IMPULSE PHASER to these two shorted phase conductors, the RED signal would go up one conductor, and the return or YELLOW signal would come back on the other. At the first hole dug, closest to the IMPULSE PHASER Transmitter, if we obtained good signal indications on both cables, the fault location would be ahead of us toward the original short location indicated. Good signals were obtained!
At the next dug hole, the same results were obtained. This still indicated that the short was ahead of us toward where we originally indicated.
At a lamp close to the second hole, the cables appeared to be burned at the base of the globe. This was possibly the shorted location. If signals were obtained on these cables, we would know that the short was at the top of the lamp. We got no signal. This still indicated that the short was ahead of us toward where we originally indicated.
Using a backhoe the next day, the fault was found about 8 feet deep.
This is a good example of how, with the proper equipment, difficult faults, which are sometimes encountered in the field, can be successfully located.
Primary Fault Locating Opportunity:
Situation: We were called by a local state university and asked if we could, on an emergency basis, locate a primary cable fault. There were 15 buildings out of service including several laboratories that were conducting important experiments.
We loaded our high voltage fault locating equipment along with all of our other test equipment and headed to the job site.
It was 1500 foot circuit of 5kv cable. It contained many switches with taps. The cable run included paper/lead, shielded extruded dielectric, and unshielded extruded dielectric cables. All of the switches had been opened to make sure that the fault was not in a tap.
Fault Locating Operations: We carried our test equipment into the basement of the substation and removed the faulted circuits from the cutouts. Before we connected the high voltage test equipment to the cable, we decided to connect the IMPULSE PHASER to the cables. We knew that if the instrument began functioning, we could use the IMPULSE PHASER Detector and clamp-on to locate the fault.
We connected the YELLOW lead of the IMPULSE PHASER Transmitter to ground and connected the RED lead to each conductor to test for a fault. There was no indication of a fault to ground. We connected the RED and YELLOW lead to both of the conductors of the faulted circuit and discovered that the conductors were shorted together. This instrument was put away after identifying the type of fault we had.
The SPITFIRE Transmitter was now connected to the shorted circuit to obtain a higher energy pulse to the fault location. After checking for the signal in the basement on the shorted cables, we headed for a manhole that had been pumped.
The cable went in three directions. Using a SPITFIRE type clamp-on, we obtained a signal on the incoming cable and in one other direction out of the manhole. We moved to another branching manhole four manholes away. As we entered the manhole, we noticed some soot at one of the conduit entrances. We obtained an indication that the fault was beyond that manhole. We could even hear a "tick" when the SPITFIRE discharged. We went to the next manhole and could not obtain a signal, indicating the fault was between these two manholes.
Because of the circuit arrangement, we were able to cut the cable in these two holes, live cap the cable ends, and re-energize the circuit. We arrived at the job site at 11:00PM. The circuit was re-energized at 1:30AM. We were able to isolate the shorted section using the SPITFIRE secondary cable fault locator!
False Failure Location Problems:
Situation: This was a residential secondary cable failure, from the meter at the rear of the house to the transformer about 125 feet away. The customer on the lot where the transformer was set had planted shrubs which had grown quite tall. In the middle of the shrub was a telephone pedestal which was quite captive.
Fault Locating Operations: We isolated the customer's service by removing the meter. Using the WAYFIND'R Impulse Cable Tracer signal generator, we discovered that one cable was open and that the other cable was still energized. Using a temporary hook-up cord to obtain 120 volts to operate the SPITFIRE Transmitter in the meter box, we attached the RED Transmitter lead to the bad cable and attached the BLACK lead to a large screwdriver which had been pushed into the earth a couple of feet from the metal riser.
We began to impulse the bad conductor and obtained a good signal on our Detector meter. But, what about the other cable to which the hook-up cable was connected? We would like to be sure that this cable is all right. We just connected the RED Transmitter lead to the apparently good conductor which we were using for our power source, and discovered that this cable was also in a faulted condition. So, the SPITFIRE was capable of obtaining power from the apparently good cable to charge the Transmitter and then use that charge to impulse the same cable.
We pinpointed the fault under the large shrub at the transformer location. We then fault located the apparently good conductor and found that it was also faulted at the large shrub.
We then connected the WAYFIND'R to see exactly from which direction the cable entered the shrub and began to expose the cables.
To our surprise, we discovered that four conduit bends had been installed into the transformer for the four services. (The only way we could get the good signal that was observed was if the conduit had been crushed by the root system.) We had to determine which conduit housed our cables. We removed the probes from the SPITFIRE Detector and connected the optional clamp-on to the Detector. We quickly determined which conduit housed the cable in trouble.
While continuing to excavate, we reinstalled the probes into the Detector and continued to pinpoint the fault location. With one probe in the earth, we touched the telephone pedestal with the other probe. We obtained a much stronger signal. Could it be that the telephone pedestal had damaged the cable?
We soon were able to disconnect the cables at the transformer and pull them out of the conduit only to discover that there was nothing wrong with the cables. Further fault locating indicated the fault at the bottom of the metal pipe riser to the meter box. The cables had been damaged by the edge of the pipe.
Two things led us astray:
When all else fails, follow the instructions! The instructions clearly tell us to install the earth ground, to which the SPITFIRE BLACK lead is connected, as far off the cable route as possible. We settled for a couple of feet.
Attached to the riser pipe, a ground wire had been attached for the telephone service. The current path of the impulse was from the SPITFIRE transmitter to the pipe damaged cable, through the pipe to the telephone ground wire, out to the telephone pedestal ground at the padmounted transformer location.
Since the remote driven ground for the SPITFIRE had been placed so closely to the faulted location, the Detector was able to see the signal emanating from the telephone pedestal ground.
Lesson: Be sure to place the SPITFIRE grounding conductor as far from the cable as possible. Be sure that there are no other conductor connections to a metallic riser housing the cable you are trying to fault locate.