Forensics and Failure Assessments

The highest priority during recovery from a cable failure is to identify and repair or replace the damaged cable. However, care must be taken to preserve the failed cable section or component for forensic assessment. Gathering available forensic data and performing a thorough failure assessment will provide valuable information in determining whether the failure was from long-term aging, a manufacturing flaw, or installation damage. The results of the forensic assessment will provide a basis for the extent of condition considerations for the remaining cable population.

For these reasons, care should be taken during the troubleshooting and repair to preserve as much information possible so that a proper failure assessment can be made. The single most important piece of information to preserve is the failed cable section or sections. As a minimum, analysis of the failed section of cable and adjacent sections of cable is recommended. A reasonable length of cable on either side of the fault, at least 3–6 ft (1–2 m), is recommended. Longer sections allow additional electrical and physical tests to be performed. Nonfaulted, adjacent cable phases should be preserved and assessed, as well. In addition, a dry section some distance from the fault or a section of cable from the warehouse should be obtained for determining baseline conditions.

The sections of cable saved for analysis should not be handled more than necessary or cleaned.

Pointing to the fault with a pencil can leave carbon that has nothing to do with the failure. Over-handing can lead to physical disruption of the internal configuration that could mislead the forensics team. Cleaning with solvents could alter the chemistry of the materials and lead to corrosion of conductors that, again, could mislead the forensics team.

Digital pictures should be taken during removal of relevant areas of the failure. The segments saved to be analyzed should be wrapped, sealed in plastic, and quarantined with minimal handling to preserve their condition. Any damage that occurs as a result of the removal process should be documented and forwarded to the forensics team for consideration.

Response to Cable Failures

The forensics laboratory will use the dry or unaged cable to determine the characteristics of a good section of cable for comparison to the damaged section. The section adjacent to the fault will allow the laboratory to determine whether the adjacent phases had similar deterioration or whether the faulted phase had a unique problem. Removal and handling of the specimens should be supervised to ensure that the specimens are properly handled and preserved. Electricians are experienced in installing cables but are often not trained to preserve cables or terminations that have failed.

In addition to the cable specimens, the laboratory should be provided with the physical and operational conditions that the cable or cables experienced in service. The following information should be provided to the failure analyst, if possible:

 Cable manufacturer

 Cable ratings

 Circuit length

 Cable routing details (penetrations, bends, cable tray, conduit, direct buried, ducted, and so on)

 Number of conductors per phase

 Accessories and splices

 Time in service

 Operating voltage

 Operating current

 Operating duty cycle (continuous or standby)

 Environmental conditions (wetted, flooded, near heat source, and so on)

 Operating parameters at the time of failure (plant or system transient, equipment startup, shutdown, change in demand, and so on)

 Related operating experience

Forensic analysis of the cable should be performed by a laboratory that specializes in cable analysis. Although many sites have access to corporate laboratories that provide failure analysis, these labs might not have the knowledge or capability to evaluate the physical and electrical properties of the cable specimens. For example, corporate laboratories might have experience only with XLPE insulation used in distribution systems rather than EPR with helically wrapped tape shields used in power plants.

Response to Cable Failures

The properties of the cable that should be evaluated by the laboratory will vary by the cable type, the manufacturer’s design, and the nature of the failure. The following physical attributes should be measured, if applicable, depending on the design of the failed cable:

 Visual condition of the cable and the failure location

 Jacket type, thickness, and condition

 Metallic shield type, if applicable (copper tape, corrugated tape, drain wires, and so on) and condition

 Insulation shield type and condition

 Conductor shield type and condition

 Insulation type, thickness, and condition

 Conductor type and condition

 Tensile strength of jacket and insulation

 Elongation of jacket and insulation

 Moisture content of jacket, insulation, and conductors

 Microscopic examination of insulation for imperfections

In addition to the physical properties, the following electrical properties of the cable should be gathered, as applicable:

 Jacket electrical resistance (ohms/cm)

 Semiconducting shield resistance (if shield is different from jacket)

 Dissipation factor at 60 Hz or VLF

 PD

 Insulation resistance (normalized to 1000 foot of cable) at 5000 Vdc (see Appendix F, Insulation Resistance Test Measurements: Their Value and Limitations)

 AC voltage breakdown test or dissection of failure areas

Evaluation of local insulation resistance of the insulation between the conductor and the surface of the insulation can be useful in assessing water-related deterioration of rubber insulation. The insulation resistance between the conductor and a probe run over the surface of the insulation is used to detect areas of low resistance through the insulation. Such testing has identified areas of lower insulation resistance that were several orders of magnitude lower than that of the

surrounding insulation. This activity showed that several pockets of degradation were distributed along and around the insulation. The insulation consistently failed in these low-resistance

pockets when breakdown tests were subsequently performed. Even though the insulation was not uniformly degraded, the identification of several low-resistance pockets indicated that distributed general degradation existed, rather than a unique degradation site. See Section A.3.1, Failure of a 38-Year-Old Butyl Rubber Cable Due to Water-Induced Degradation, for an example of this technique.

Response to Cable Failures