186 The N.Z. Journal oe Science and Technology. [Nov. 
A comparison of the texture of the different samples showed that the 
porcelain already in service presented a dull, almost chalky appearance, 
while the English make of porcelain had a much closer grain and a harder- 
looking, silky fracture. The stock porcelain was between the two, but more 
resembled the porcelain in service. The English porcelain was also whiter 
in colour, possibly on account of being more thoroughly burned. 
The puncture of insulators apparently by high-voltage surges was noted 
in seveial insulators removed from the lines after breakdown early in 1916 
and subsequently. Fine hair punctures, ranging from lines scarcely visible 
to the eye to small channels about 1 mm. in diameter, were found in the 
porcelain. These punctures were not confined to the porcelain adjacent 
to the main - energy puncture, but on cracking up the insulator were 
found in various places, usually in the line of greatest electrostatic flux 
concentration, while some of these punctures would possibly result from 
surges set up by the final breakdown : i.e., final failure is promoted by a 
succession of such punctures which develop finally into a puncture through 
which sufficient energy may flow to break down the shells and destroy 
the insulator. 
On a recent inspection of the lines an insulator was removed punctured 
only in the two top shells. There was no sign of the insulator having 
flashed over. Both punctured shells were cracked ; the punctures were 
about ^ in. in diameter, and a small bead of glass was formed on one edge 
of each hole. The possibility arises of the cracking of the insulator being 
the cause of puncture, but, on the other hand, the puncture may have 
caused, and probably did cause, the crack. 
With a view to making the tests on any new insulators approximate 
somewhat to the conditions which may arise in actual operation on the lines, 
a high-frequency 125,000-volt oscillator testing-set was used (fig. 4). This 
oscillator, which is fully described by Creighton,* consists of an ordinary 
230-volt to 13,000-volt transformer of 3 K.V.A. capacity, in the primary 
of which is a current-limiting reactance and a quick-break control switch 
A condenser is connected across the secondary leads, which also connect 
to the oscillator transformer through an adjustable rotary spark-gap by 
means of which the voltage of the high-frequency transformer is regulated. 
The insulator to be tested is placed across the secondary terminals of the 
high-frequency transformer, the quick-break primary switch closed, and 
voltages regulated to the desired value by means of the rotary spark-gap. 
The high-frequency voltage is measured by means of a sphere gap provided 
with a graduated scale. One side of the condenser and oscillator trans¬ 
former is grounded. This oscillator is large enough to produce intense 
arc-over over a single shell of the 66,000-volt pin-type insulators and 
suspension insulators. As there is a possibility of damaging even good 
porcelain by testing at excessive voltages, it was decided to make the test 
of half-minute duration at a voltage such that an occasional arc-over took 
place. The five hundred stock insulators were tested in this way, and 
resulted in a loss of 8J per cent. The majority of the bad shells contained 
flaws which would easily pass a close inspection, but wdiich were developed by 
the high-frequency discharge. It should be noted that the high-frequency 
arc-over envelops the entire surface of the insulators and attacks the weak 
points even though the flow may be well out on the skirt of the insulator 
Tests with ordinary 50-cycle voltage are not so effective in finding weak- 
* E. E. F. Creighton, Trans. Am. Inst. Elec. Eng., p. 465, 1915. 
