Appendix A: Notable Responses from the Survey Study

Question 4

A respondent wrote this note:

• “The majority of the transit systems have constructional issues that are unchangeable. For instance, the cable feeds from the DC switchgear line breakers to the track are always outdoors. Thus, they need to be able to withstand standing water, harsh chemicals, UV light, physical damage, and temperature variations that are normal conditions.”

Some answers remarked the environmental factors affecting the useful life of transit power cables as follows:

• “Most cables are embedded, and within duct banks, so the environmental temperature is not a significant factor. We have seen no problem with birds or rodents with TE cables (traction electric).”
• “Water that can get to the copper of the cable is a definite problem. Occasional water to the cable jacket is not an issue. TE cable connections exposed to moisture/water are the most frequent failure point.”
• “For some agencies, the temperature plays a more significant factor in our bare copper conductors more than the feeder cable. Besides, salt in the winter can play a factor with the equipment for the bridges over the water.”
• “Deterioration of the bridges and components plays a role.”
• “Rodents would also be a problem for the fiber optic cables where the cable is running in an overhead raceway.”
• “For light rail systems, for manholes in which the feeder cables exit the substation into the manholes and run through the conduit to the overhead system. There is a typical issue with rainwater filling these manholes and have been known to cause failures.”

Question 5

The following are critical notes to classify the physical parameters altering the useful life of transit power cables:

• “Cable insulation/jacket designed for wet locations.”
• “Insulation rated for well above system voltage to handle transient surges.”
• “Properly designed duct bank (drainage, depth) to maintain dry and cool cables.”
• “Proper separation and support of cables in manholes.”
• “Design duct bank for minimal cable-pulling stress (broad sweeps, frequent pull boxes).”

Question 7

Some responses to the lifetime guarantee by the suppliers:

• “There are no lifetime guarantees. No cable supplier would guarantee a lifetime, because there are so many variables affecting the life that is dependent on the installation.”
• “Lifetime guaranteed can be proposed, but the installation also determines the quality of the conductors and cables on how it would stand.”
• “Generally, 20 years lifetime guaranteed by the suppliers. Nothing is exclusive, aside from preventing additional friction and rubbing of the cables.”

Question 8

Several responses to the diagnostic tests:

• “The most common test is insulation resistance (meg-ohm) testing.”
• “Power factor/Doble testing provides better data on insulation condition.”
• “The frequency of testing should increase as cables age. Perhaps every 5 years with new cable, but annually for older cables. The frequency of cable failure should be factored into testing frequency schedule (as failures on the system increase, frequency of testing should increase).”
• “As for the electrical tests, the first task listed is to test for high resistance connections by either the resistance test across the bolted connections or by verifying “nearly equal currents” in multiple cables with a clamp-on ammeter.”

• “The benefits of performing a resistance test on non-exposed bolted connections are outweighed by the costs associated with taking apart and restoring sealed connections. Moreover, the use of a clamp-on ammeter to verify “nearly equal currents” is nearly impossible due to the highly variable nature of the DC loads.”

Question 9

Some responses are as follows:

• “To megger a cable (take resistance to ground) generally requires a cable crew to isolate the cable and typically has time of day restrictions when cable can be taken out of service. To test three cables (one set of feeders), figure a crew of three linemen and one technician plus travel time. Test four sets of feeders at one substation, one work shift.”
• “The actual megger/insulation test wouldn’t take more than 10 minutes, but the preparation takes more time than the actual test and it depends on each system.”
• “To perform any test, the operation has to be stopped. Power outage and service interruption are the main limitations to any test due to safety precautions. It is not recommended to perform unnecessary insulation testing, because it will be over stressing the cable insulation.”

Question 10

Some responses for the cable inspection and maintenance procedures

• “Yearly visual inspections are performed on cables and regular track structure to earth resistance testing is performed biannually.”
• “We would typically do a hard visual inspection on cables during the megger process. Now we do a visual inspection on feeder cables inside the substation during our annual track feeder breaker inspections.”

Question 11

Some remarkable responses to question 11 are the following:

• “Repairs to DC traction power cables are challenging. Best solutions are welded splices but are limited to locations where the welding process can best be applied. Post weldment treatment of the splice is important. Reinsulating of the splice is possible but not inexpensive. The use of crimp type connections in DC applications is ineffective in the long run.”
• “Cable repairs are dependent upon the type and extent of damage plus the type of cable used i.e., shield or non-shielded A simple cut in a cable that doesn’t involve electrical arcing damage can usually be spiced. There are splice kits available that work very well. In most cases it takes an experience electrical technician or engineer to evaluate the damage on a case by case basis.”
• “Cable repairs are limited to replacement or splicing the cable. I would choose repairing (splicing cable) over replacing due to the low cost. Short to ground is number one diagnostic for insulated cable failure, and if you can’t see a visual damage, it means the damage is somewhere along the duct bank and it means cable replacement. Cost for replacement depends on the length of the circuit and access plus labor and equipment. Repairing (Splicing) is the most cost effective, if it is visual. The splice kit costs about $500.00.”
• “As a cost-effective application, if splicing cable is allowed, start cut the cable in sections at each pull box/manhole and test each section individually till you find the failed cable and replace that bad section and splice.”

Question 12

The responses of those who apply smart replacement strategy are the following:

• “Monitor substation feeder breaker tripping & perform thermal infrared camera inspection on cables and connectors to determine heat signature on cable current flow.”
• “Electrical DC taps, we are trying to bring taps above ground, the outside contractor will install new insulated cables.”

Question 13

Some responses to stopping the operations:

• “A minor repair could be done with single tracking, but major repair shall stop rail operation on both tracks.”
• “Sometimes we need to stop operations service for repair and replacement.”
• “Only if it’s a single track area, we stop operations.”
• “Not entirely because traction substations are designed for contingency.”
• “Not always and mostly not for the whole replacement process.”

Question 14

A remarkable note from a respondent is the following:

“Metros seem to have a problem with cable damage in particular track-work sections. This damage is physical to the cables by rolling stock. Actual cable failures would usually be related to increased amperage, which is usually caused by poor connection procedures/practices. Other common causes for cable failures are by track gage regulators and other equipment used to maintain the track structure. In a perfect world, cables attached to the running rail would be best protected by coming from beneath the rail base and left protected by the rail profile. This is similar to the methodology employed in Europe when using precast concrete panels for the track structure on AC traction power systems.”