THE NEW ZEALAND 
JOURNAL OF SCIENCE 
Vol. III. Wellington, November, 1920. No. 4. 
NOTES ON HIGH-TENSION INSULATORS. 
By L. Birks, B.Sc., M.Inst.C.E., and G. F. Ferguson, A.M.I.E.E. 
It is well recognized that the insulator is the chief weakness in the 
transmission of electrical energy at high voltages. In many of the insu¬ 
lators that have been available in the past the porcelain deteriorates, and 
in course of time becomes so weak dielectrically that breakdown occurs, 
resulting in interruptions to the power-supply, provided, of course, that 
measures are not adopted to weed out weak units before they reach the 
critical stage. The period of deterioration varies from one year upwards, 
depending on the inherent quality of the porcelain. 
The insulators on the 66,000-volt transmission-lines from Lake Coleridge 
to Christchurch, New Zealand, gave indications of distress early in the 
life of the plant,* and it is the purpose of the following notes to describe 
some of the investigations carried out by the authors on behalf of the 
Public Works Department to locate the cause and devise means for reducing 
breakdowns and the consequent interruptions to the supply. 
While insulator-failures on the lines were frequent, the time lost through 
interruption was not very serious, because the lines were in duplicate. 
However, the position was serious enough to merit a thorough investigation, 
on account of the annoyance and inconvenience of even momentary inter¬ 
ruptions to the supply, together with the cost of replacements, and the 
probability that the situation would become much more acute. The 
following table shows the number of line-breakdowns from 1915 to 1918, 
inclusive, due to insulator-failures, and the number of insulators replaced 
as a result of these breakdowns, as well as the time lost. A further 
twenty-eight breakdowns and stoppages were caused by short circuits on 
the line, due either to the bark of gum-trees blowing on to the wires, or to 
lightning, or to other obscure causes not directly resulting from insulator- 
failures. 
* E. Parry, Notes on Failures of High-tension Insulators, this Journal, vol. 1, 
pp. 286-92, 1918. 
12—Science. 
