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Root Cause Failure Analysis of Termination and Cable

Root cause analysis of a termination and cable failure, which had caused an outage and pole fire.

Kinectrics performed a root cause failure analysis on a termination and cable for a hydro company located in Ontario. The outage created a pole fire in the process.

The cable termination was accompanied by the associated bracket and lightning arrester, both of which were damaged. The objective of the work was to determine and understand the root cause to prevent future failures.

Kinectrics was asked to analyze a termination and cable sample, rated 28 kV 1000 MCM , which had been installed in 2006 and failed at a riser termination one year later.  The outage created a pole fire in the process.

The cable termination was accompanied by the associated bracket and lightning arrester, both of which were damaged. The cable termination, the lightning arrester, and the bracket were visually examined for general condition, and the termination was examined for any construction anomalies.   The fault was located right in the boundary of the stress grading of the termination and the semicon layer of the cable.

A 2 mm gap was found between the stress grading of termination and cable semicon, which left an area of uncontrolled stress concentration. This error in termination construction resulted in an increase in electric stress on the insulation surface, leaving a potential spot for cable failure.   There were clear signs of overheating of the stress grading material near the gap.

Weather history indicated there had been a record 92% average humidity during the week the failure had happened, which increased the chance of surface partial arcing and eventually flashover, over the housing of lightning arrester or the cable termination.

Evidence of dry band arcing along the outer surface of the lightning arrester and the termination housing was noticed. The thermal degradation of silicon rubber of the termination was very clear for the last three sheds (close to the cable), which proves long-term partial arcing activities along the termination.

There was also erosion and melting of the cable bracket metal due to heavy arcing activity, probably prior to cable failure. Corrosion of the bracket was observed in both sides, close to the ground wire of the arrester and ground wire of the termination.

Based on the physical evidence and the weather records, it is likely that there was an overvoltage on the lightning arrester, resulting in a flashover on the silicon rubber housing. 

The partial arcing might have started several days before the fault day, even before the four-day rain event, and continuing during the high humidity.  Due to improper installation of the cable termination, it was very vulnerable to these conditions and initiation of a failure was likely. 

The resulting arcing caused a large amount of cable melting and could have had sufficient energy to start the pole fire.  However, there was also a lot of evidence of arrester and termination flashover.  These factors could also have played a role in the resulting pole fire.

The following items were discounted as contributing to the failure:  cable defects, termination kit defect, stainless steel hose clamp support, or installation of the grounding braid.

About This Case Study

Industry Sector

  • Transmission & Distribution

Nature of Service

  • Inspection
  • |
  • Testing - Lab Services

Client Assets

  • Cables

Kinetrics Facilities

  • Electrical Testing Facilities

Tags

  • Failure Analysis
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  • Lightning Arrestors
  • |
  • Cable Termination
  • |
  • Flashover
  • |
  • Electrical Testing
  • |
  • Cables
  • |
  • Cable Testing
  • |
  • Pole Fire