Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analaysis


Wire Rope Failure


Undercarriage Leg Failure


Aircraft Towbar Failure


Hail Damage


Insulator Caps
- Part 1
- Part 2
- Part 3
- Activity 1


Fractography Resource

Dr David J Grieve ( developed the interactive Java pages for this website.


A number of uncoupling failures of insulator strings occurred on high voltage transmission lines crossing bushveldt in Namibia. These uncoupling failures were triggered by rifle fire damage to several of the toughened glass insulators, which then allowed flashover to occur across insulators in a string (a typical string is illustrated in Figure 2). The insulators in question are U70BL type and are shown in Figure 1. These are ball and socket 11 kV disc insulators with galvanised cast iron caps. The glass insulator is glued into the cap using Portland cement. The mode of failure is illustrated in Figure 3 and involves fracture of the galvanised cast iron cap during a arc flashover, allowing the connecting pin to pull out of the cement and cracked glass in the insulator cap. This results in dropping live high tension power cables on the ground with associated danger to life.

The electrical energy available in an arc flashover can be estimated, and is much less than the energy required to burst undamaged cast iron caps. The question posed by the power supply company related to the reasons for this failure, i.e. what had happened to the caps to allow them to suffer fracture in this manner under relatively low electrical energies.

This case study answers this question by linking metallography with fracture toughness knowledge, supported by fractography of failed caps.
U70BL2.gif (7261 bytes)
Figure 1 U70BL Insulator
Insulator string.jpg (12498 bytes)
Figure 2 Typical insulator string
Fractured_cap.JPG (56332 bytes)
Figure 3 Fractured cap
Proceed to second part of case study.


Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion