182 The N.Z. Journal of Science and Technology. [Nov. 
after which they were transferred to a bath of cold water at about 60° F., 
the cycle being repeated three times. The top shell of one of the insulators 
cracked, the others remaining unaffected as far as visual inspection could 
detect. Two insulators of another make but somewhat different design, 
in that the top and second shells were integral, were subjected to the same 
heat-cycle treatment. Both of these insulators cracked badly in the top 
shell on the first transfer to cold water. It is worthy of note, however, 
that the latter type of insulator has been in service on a 50,000-volt line in 
New Zealand for about five years, and only two insulators have broken 
down in service in that period. Suspension insulators are used on the 
66,000-volt lines and 11,000-volt distribution-lines as strain insulators, 
and during the past year about six of those on the 11,000-volt lines have 
broken down, punctured badly from pin to cap. On testing, the porcelain 
was found to be of fair quality and only slightly porous. The insulators 
were cracked badly, but whether any of the cracking was done before 
breakdown could not be decided, though from tests made subsequent to 
failure it is believed that breakdown was caused by initial cracking of the 
porcelain, due to temperature effects. In this connection it may be noted 
that on breaking up insulators for examination small hair cracks about 
\ in. in length were observed on occasions. These cracks were found on the 
unglazed, cemented surface ; and, while it is possible that such cracks may 
have been caused by the breaking-up of the insulator, it is believed that 
they existed before this operation. From this it would appear possible 
that temperature and mechanical stresses may strain the porcelain suffi¬ 
ciently to cause one or more of these hair cracks, but not be severe 
enough to crack the shell entirely. However, repeated strains would 
develop the crack until the shell either cracked right through, or was 
punctured electrically through the weak path provided by the crack.* 
Absorption of moisture by the porcelain was early suspected as being 
the primary cause of the deterioration of the insulators, and a good deal 
of investigation was carried out to determine this and ascertain to what 
extent the insulators as a whole were affected in this way. Ordinary red 
ink applied to the broken surfaces of insulators showed that the porcelain 
would often readily absorb the ink in places, while in other places the 
porcelain appeared to be more vitrified and less inclined to take up the 
ink. 
For the purpose of investigating the general condition of the insulators 
in service, batches, each of about one dozen, were removed from various 
portions of each line, and, after wiping and numbering for identification, 
were subjected to the following tests. First megger readings were taken 
of resistance from shell to shell and from head to pin. Very useful indica¬ 
tions of the condition of an insulator were obtained from these megger tests, 
as are shown in Table 2 by comparison with the subsequent voltage test. 
A fair amount of experience is necessary for successful working with the 
megger to avoid inconsistent results, particularly when tests are conducted 
in the open, where even slight condensation may give very erroneous results. 
After the megger test, the insulators, with the exception of a few reserved 
for special treatment, were tested first from head to pin at 100,000 volts, and 
then one shell at a time up to 50,000 volts. It was found that by increasing 
the time element the test could often be relied upon to weed out a 
moisture-laden or cracked shell with a maximum of 30,000 volts. Table 1 
* J. C. Clark, Trans. Am. Inst. Elec. Eng., p. 1453, 1916, 
