Chapter 3. Survey/Interviews

3.1. Survey Questions and Responses

1. The data collection process was carried out with transit agencies and practitioners in the industry with survey and case studies, research results were collected and analyzed.

The survey was carried out to understand the current practices at various locations in the United States. In the process, old, new, big, and small systems were covered. The survey was sent to define the properties of the participants. Some notable responses are presented in Appendix A. Table 3.1 show respondents’ place of employment and Table 3.2. presents information about the transit systems of the respondents. In total, 60 individuals submitted responses, though not all responded to every question. Responses were quantified with graphs for each question.

Table 3.1. Survey Respondent Place of Employment

Table 3.2. Survey Details for Data Collection Process

1. Please describe your network and type of network, length of network, age of the network, electrification system (voltage, AC or DC, current type, contact system).

2. What type and size of transit power cables are used? How many taps per hundred feet are in your cables, maximum number of taps allowed per circuit?

Responses to this question describe the type and size of transit cables used by the respondents. The common sizes range from 250 MCM to 2000 MCM, with the most common sizes being 500 MCM, 750 MCM, and 1000 MCM, respectively. According to respondents, MCM cables are far more widespread than DLO cables as most of the responses indicating MCM cable usage. Common practice is using MCM cables in various agencies. The higher rate of use of MCM cables would make MCM cables more common practices for the problems faced in agencies.

3. What is the expected useful lifetime of cables that you use? Do you follow any practices to increase the useful life of the cables? Please explain.

Most of the respondents expected the useful lifetime of cables to be 30 to 40 years. One response indicated ten years, and one response indicated 70 years. It is possible these outliers use specific, special-purpose cables that are not frequently used by others, giving them the vastly different life expectancy from the norm. With the exception of special-case scenarios, it can be assumed that cable lifespan is 30-to-40-year.

4. What are the environmental parameters such as temperature, animals (birds, rodents), water, above or below the water level, etc. affecting the useful life of transit power cables?

Environmental parameters play an essential role in the useful life of cables. Responses varied widely to this question and selected multiple parameters in their responses. However, the two stand-out factors affecting the useful life of transit power cables were water and temperature. According to the respondents, these factors are the most damaging environmental factors, and thus the most important to consider. Participants have also specified other environmental determinants such as UV light, rodents, corrosion, chemicals, vibration, salt, and dust.

5. What are the physical parameters including jacketing, insulation, duct bank affecting the useful life of transit power cables?

The most common physical parameters affecting the useful life of cables are jacketing, conduit and insulation. In the responses, the jacket is at the highest, conduit and insulation are at the same rate. The other physical parameter is cable structure. As such, jacket, conduit, and insulation are the most significant physical parameters for cable design, and all design recommendations should focus on these physical parameters.

6. How do you determine the degree of degradation of insulated cables? Do you have cable monitoring systems?

In this question, the most commonly employed methods were detected to define the degradation of insulated cables. These methods include the megger test, visual inspection, and ‘Run to Fail’ methods. The wide variety in cable monitoring methods indicates the lack of a universally agreed-upon best practice in the industry.

7. Is there a lifetime guarantee by the suppliers? Are there any maintenance, testing, replacement schedules suggested by the suppliers?

There is no lifetime guarantee by suppliers. Respondents indicated that suppliers do not offer lifetime guarantee. Respondents indicated that this is likely due to the supplier being unable to control if the cable was correctly installed or how carefully designed the system was to ensure the longevity of the cables. Survey responders have mentioned that it is challenging for the suppliers to control the construction environments and detect parameters affecting the system.

8. What are the diagnostic tests carried out on cables? How often do you need to run these tests?

Respondents responded with multipole parameters if they are using multiple tests in their agencies. Megger, hi-pot, and IR Testing are the most regularly implemented diagnosis tests. However, a large proportion of respondents are not using any diagnosis testing on transit cables. This lack of testing represents a considerable unknown infrastructure cost that could be incurred at any time. The status is unknown for these portions of the cables, which can lead to budget shortfalls and failures at inopportune times.

9. What are the costs to run each type of test including the number of operators and time length? Do you have to stop operations and what are the other limitations of the testing process? Does any of the tests have the potential to damage?

While most participants stated that the cost to run tests varies, they did emphasize that the usual crew comprises three to five people. Some of the respondents mentioned that the testing safety procedures require stopping operations completely, with additional costs. These costs are in the form of power outages, service interruptions, and crews kept from performing other vital tasks. Even though the most common test, the megger test, only takes ten minutes, the preparation and return to normal operation typically takes longer than the test itself.

10. Do you practice any cable inspection and maintenance procedures? If so, please describe your procedures including intervals and types.

Of all 60 respondents, 40% of the respondents indicated not having established inspection and maintenance procedures. This apparent gap further illustrates the need for an established guide for cable inspection and maintenance procedures. Without widespread adherence to universal standards across the industry, it may not be easy to plan appropriately for future maintenance and repair or replacement projects, especially across separate entities.

11. What types of repairs are made? Under what circumstances do you choose repairing the cable in comparison to replacing? What are the diagnostics indicators for replacing insulated cables? What are the general costs and durations of repair and replacement? Any cost-effective application you practice for repair and replacement?

Respondents chose to repair rather than replace a damaged cable if the cable is accessible, not in duct banks. If the cable is accessible, the best practice was to repair the damage. The type and size of cable was also indicated as having an impact on the decision to repair or replacement of a cable. Not only is this a more cost-efficient solution, but it provides a safety net: if the repair was to fail, it could be easily repaired again. Repairing inaccessible cables incurs significantly more costs, and thus it is more feasible to replace the cable rather than risking future damage or failure of the repair. While the quality of a replaced cable is more desirable than one which has been repaired, budgetary considerations necessitate the more economical solution.

12. Is there any replacement strategy, including the smart replacement strategy, that you are implementing for insulated cables?

Of all 60 respondents, 25% of the respondents applies a replacement strategy for their systems. The remainder of the respondents, 75%, do not apply a replacement strategy or said “Not available” for the replacement strategy application. 20% of the strategies is “smart”, 20% is classified as “as needed”. 60% is not in the smart replacement category. If further clarification on the benefits of smart replacement strategies were distributed, more implementations may be seen in the future.

13. Do you need to stop operations completely during repair and replacement?

One of the drawbacks to repairing and replacing cables is the inherent downtime in regular operations. Of all 60 respondents, 65% of respondents reported that operation is completely or partially stopped during repairs and replacement. Only 15% of them have stated that they do not need to stop operations. 20 respondents did clarify that the stoppage in operation can depend upon many interconnected factors, such as the location, cable type, track type, switch type, and time of day the repair or replacement is to occur. The location of the cable and non-revenue hours are determined accordingly.

14. Were there any failures being experienced by your agency? Are the reasons behind the failures known? How do they diagnose failures? Did you have any experience that you can share with us?

32 respondents reported experiencing failure. The most common failures occur at maintenance holes and joints due to water intrusion. More durable sealants around maintenance holes and at joints would help reduce the occurrence of failure in these areas. Better designed or redesigned systems could minimize the number of manholes and joints, reducing the potential for failure along the cable. Additionally, some respondents have hypothesized that some failures arise from insulation wearing, improper taps, and damage during installation, mechanical issues, and the aging of cables.

3.2. Statistical Analysis

To evaluate the survey results, a statistical analysis was carried out to understand the correlation between the answers from the responders. In the answers given by the participants, qualitative relationships were examined in the statistical analysis using Pearson Chi-Square and Fisher Exact methods.

3.3. Model Analysis

A model analysis is developed using content analysis of the survey results to determine the presence of certain words, themes, or concepts within some given qualitative data. Since the survey study was formed with open-ended questions, determining the most commonly used words is important to evaluate the survey results. The outcome of the modeling is grouped into two groups as (a) Tree Map Analysis Results, and (b) Word Cloud Analysis Results. The content analysis is based on the words used in the survey responses. The analysis was carried out with the technical terms used in the individual responses. The model analysis brings a compressive understanding of common practices in different states in the United States, including identifying differences between the practices. Based on this analysis, a helpful insight for strategies and solutions can be determined for defining best common practices regarding harmonization and coordination across the states.

3.4. Conclusions

For this research, a survey study of transit agencies, practitioners, and suppliers was carried out to determine current practices that helped to define the process. IEEE meetings were beneficial for the team to meet the practitioners and run the survey with the attendees. Collected responses were used to create an effective data gathering process. In the process, 14 questions were created based on the research objectives. The survey questions were prepared to determine the practices accurately for the cables. The questions were shared with the participants and their responses were requested. With the responses, existing problems and solution practices were determined. Summaries for survey responses are given in Table 3.3.

While transit power cables have many types and applications, the survey results showed that a few cable sizes and usage situations are more commonly used. These include the 500 MCM cable, the 750 MCM cable, and the 1000 MCM cable, with either no tap usage or when taps are used; they are frequently placed every 100 ft to 500 ft. Therefore, the main focus should be on these types of cables.

The survey results have shown that the provider average lifespan of 30 years for the cable is in line with the expected/observed lifespan of the cables in service. This confirmation helps establish a timeline in the guidelines for repair and replacement. It could be used to help weigh the costs and benefits of replacing a section of cable instead of repairing it. For example, it may be more economical to replace an older damaged cable instead of repairing it. Constructional difficulties also play an important role in decision-making. It is difficult to reach the cables for the repair in some cases. In such cases, replacing a cable is the only viable option.

Some of the environmental and physical parameters commonly reported to be negatively affecting the life of the cables include water intrusion, poor jacket insulation, and wide temperature fluctuations, the most detrimental of these being water. Water is the single largest cause of the failure of power cables. When located overhead, water can combine with other elements, causing cable failure. When located underground, water can make its way down manholes and into duct banks, causing unseen damage. A majority of survey respondents indicated that they had no cable monitoring system in place to detect this damage and operated in a ‘Run to Fail’ manner that increased the likelihood of unplanned service interruptions.

When the cable fails, there are different diagnostic tests available. Some of them are common, such as the megger test and the hi-pot test, while others see only occasional use, such as the Doble test and Loop Resistance test. With such a wide variety of tests employed, a standardized approach would be helpful. More accurate recommendations on repair or replacement can be made when more cables are tested. Typically, agencies do not have established replacement strategies, and the few do not utilize a Smart replacement strategy.

Table 3.3. Summary of Survey Responses