1. Summary
After a bus insulator failed in field, it and the insulators in the immediate area where sent to the
Technical Services and Research department for testing. It was concluded that the failure was a
result of cement growth in the grout bonding the porcelain. Testing also demonstrated a
method of ensuring the integrity of a bus insulator by striking the porcelain and comparing the
length of the tone produced to a known standard.
Background
While performing switching operations in a transmission substation a stack of bus insulators,
see fig.1, broke and fell dangerously close to the operator. That stack and several other stacks
were removed and sent to the Technical Services and Research (TS&R) department at
SaskPower to be tested for cantilever strength.
Figure 1: Bus insulator stack and cross section
Testing
At TS&R the insulators were marked according to their location in the yard and position in the
stack. Before performing the cantilever test the top and bottom skirts of the insulating sections
were stuck with a metal and wood mallet and the resulting tone emitted was recorded for
analysis.
The purpose of recording the tone produced was to determine if a correlation existed between
the pitch of the tone and the cantilever strength. It was also to compare the tones of a intact
insulators and those that were cracked or chipped.

2. Once all insulators were tested for cantilever strength, a sample of the grout that bonds the
ceramic to the metal post and cap from the failed insulators was sent to the chemistry
department for analysis.
Conclusion
The final conclusion of the testing found no relation between the pitch of the tone produced
and the cantilever strength. However, of the several samples given to chemistry all were found
to have a composition that was prone to grout expansion, the most likely cause of the cracked
insulators. As well, when the tone of a cracked insulator was compared to an intact one it was
found to be half the duration. It was therefore advised that to test the integrity of an insulator a
worker could compare the duration of the tone with a new or known to be intact insulator.
All cantilever tests and tone analysis were performed by Jahrus, and the report was written by
him as well.