Publication: American Electrician
New York, NY, United States
INSULATOR AND CABLE TESTING, WITH
SUGGESTIONS FOR NEW
BY N. MONROE HOPKINS, A. I. E. E.
The testing of insulation may be conveniently divided into two branches or systems; the break-down or puncture test and the actual measurement of electrical resistance in ohms.
In the testing of porcelain and glass insulators designed for long-distance power transmission, certain classes of railway work and all lines for high electrical tension, the break-down test prevails, and in many instances is preferable to careful galvanometer measurements in the testing of insulation on wires and cables. The puncture test may again be divided into the dry and the wet test, the latter being far more severe for very obvious reasons.
The difference of electrical potential brought to bear on insulators and cables varies, of course, with the thickness of the walls, in the case of porcelain insulators, and with the thickness and composition of the covering on wires and cables. In testing porcelain insulators a voltage of 40,000 for a period of forty minutes would be a good test for insulators whose walls are at least 1 in. in thickness. For insulators with walls 1/2 in. thick, 30,000 volts for forty minutes would be a good and severe test.
When an insulator remains intact when subjected to a test of approximately this severity, it is usually pronounced perfect. This, however, should not be the case, for many insulators undergo this tension without cracking or breaking in any way, for the presence of a conducting mineral in the clay carries the current. These faulty insulators can at once be detected, as they become intensely heated in the neighborhood of the conducting mineral. They are, however, in many cases, unjustly shipped with those that remain unaltered and cold, partially because they are not discovered, as they make no buzzing sound like the cracked ones, and partially because of the ignorance of the man in charge.
Again, a test of this character is not a fair one if several heated insulators are allowed to remain on the testing table with the others, for the conducting minerals in the faulty ones only furnish electrical paths, or equalizing paths, across the high-voltage feed wires, with a result at once apparent.
The method of holding the insulators on the testing racks or tables is frequently also very faulty, as the electrical stress is frequently not applied where it will eventually come in practice when the insulators are called upon to support a high-tension line wire. High-potential, long-distance lines are developing quite rapidly, and leaks due to faulty insulators only mean the combustion of additional pounds of coal in the generating station, or a smaller output in the case of turbine units.
It is the wish of the writer to describe here some methods of insulator and cable testing which may be made as severe as may be necessary for all purposes, bringing the electrical stress right when it will come in practice. The writer also offers one or two suggestions which may prove of assistance to those who wish to experiment along lines of this character. This puncture testing requires great carefulness, as carelessness in handling a testing laboratory of this class would obviously result in fatal accidents.
Fig. 1, represents an alternator designed for 1000 volts, with diagram of connections. A "step-down" transformer is introduced, lowering the voltage to 100, giving a convenient line current for the measuring instruments. A controlling or "choking" coil is included next, whereby the voltage is slowly raised. An oil immersion transformer, or group of ordinary transformers designed to give the proper raise in voltage