286 
L 
The N.Z. Journal of Science and Technology. [Sept. 
NOTES ON FAILURES OF HIGH-TENSION 
INSULATORS.* 
By E. Parry, Chief Electrical Engineer, Public Works Department, New 
Zealand. 
There has recently been an excessive number of failures of high-tension 
insulators wherever such are in use, and in some instances these have 
been so serious as to necessitate the reconstruction of a number of trans¬ 
mission - lines. To this phenomenon a general term has been applied, 
indicating that the insulators have been undergoing deterioration with 
time, and that the five or six years during which they have been in service 
have been sufficient in some cases to render the insulators absolutely 
unserviceable. 
Various opinions have been put forward as to the cause of the alleged 
deterioration, as a rule differing considerably. The difference in opinion 
is due to the fact that differences in climatic conditions result in giving 
prominence to one or other of the numerous causes of failure. Where, 
moreover, the climatic conditions are extreme, failure of insulators results 
from causes which tend to obscure a fundamental defect that is only 
revealed under less severe or more equable climatic conditions, such as 
obtain in New Zealand : in consequence several factors which cause 
deterioration are eliminated, leaving only one agent operative. 
A number of insulator-failures have occurred on the Lake Coleridge- 
Christchurch transmission-lines, the number during the last financial year 
being fifteen insulators out of a total of 5,500 ; and, whilst this is remarkably 
low as compared with other installations, the failures are serious enough, 
entailing as they do a considerable addition to the cost of maintenance 
and some inconvenience to power-users. The cause of failure was for a 
long time obscure, and, for the reasons stated above, experience in other 
countries was of no assistance ; but ultimately a solution of the problem 
was reached, which it is the purpose of this paper to disclose. 
Before discussing cause and effect a description of insulators in use on 
high-tension lines is presented, as a knowledge of the design and properties 
of these insulators is necessary in order to understand the reasons for 
their failure. 
Eig. I is a drawing of the insulator used on the Lake Coleridge lines. 
It is what is called the “pin" type, and consists of four separate shells 
of porcelain about \ in. thick, cemented together with Portland cement, 
which fills the joint between the shells. The inner of the four shells is 
fitted with a thimble secured to the shell by means of Portland cement. 
The insulator is fastened to the bracket arm bv means of a bolt, one end 
of which is screwed into the thimble previously mentioned, the other end 
being fitted with a nut and washer. The top shell is grooved for receiv¬ 
ing the transmission-wire, and is also provided with a grooved neck for 
receiving a clamp, by means of which the conductor is held in place. The 
whole of the exposed surface of the shells is glazed, but as regards the 
inner surface in contact with the cement the usual practice is to leave it 
unglazed. This insulator carries three-phase lines at a pressure of 66,000 
volts between phases, which voltage is approaching the limit for the type 
* Lecture delivered before the Technological Section of the Wellington Philo 
sophical Society, 10th October, 1917. 
