coal trade bulletin - Clpdigital.org

48 THE COAL TRADE BULLETIN.
taken from this circuit showed the potential wave were made of the insulation resistance of short
to be almost a sine curve. lengths of tape from each layer of the covering
After the insulation had broken down as a re- ° f the cambric and special samples both before
suit of the application of high potential, sections and after treatment. The purpose of this test
of the samples were inspected, the tensile strength waK to determine whether the solutions had acof
the cambric tapes and rubber compound was tually changed the conductivity of the insulating
measured by the Bureau of Standards, and some material or had merely
of the cambric tapes were examined with the aid ,.„,.„.,. ,„,.,.,„,„.. ,„... , „.,.„„
l l.l'.I 1 1,1'. 1 \\ I'.l'.-N IIS I.A1 Eii.S
of a microscope. Stretch tests, as prescribed by
the National Rubber Covered Wire Engineers' and thus provided a leakage path for the cur-
Association, were also made of the rubber com- rent. This test is herein termed the "tape test."
pound. In the opinion of the bureau's chemists, Finally a fresh sample o feaeh kind of insula-
it appeared that chemical examination was not tion was treated with plain water for 30 days in
capable of affording any definite information as order to compare the action of plain water with
to the extent of the acid penetration, although it the action of acidulated water. This test is
was possible to determine qualitatively the acid termed the "control test."
reaction of the various layers of the samples that Table A gives the ,-eSults obtained from
were insulated with cambric tape. In addition the periodic measurements of insulation reto
the foregoing, the appearance of the samples sistance. The values given are the average of
under test was examined within a few hours after samples under test in each tray. There were five
the completion of the high-potential tests. At samples in every instance unless the contrary is
the end of the two-year treatment measurements stated.
TABLE A.
Decrease of the insulation resistance of the samples as treatment progressed:
Date of Tempermeasure-
ature of Insulation resistance in megohms.
ment.* water. Rubber. Standard cambric. Special cambric. Lead-sheathed
paper, t
1910. C. Tankl. Tank 2. Tank 1.Tank 2 Tank 1. Tank 2. Tank 1. Tank 2.
Dec. 30 24 46,300 49,100 2,400 2,085 1,340 1,490 ....
1911.
Feb. 9 24 33,400 32.800 1,217 915 405 393
Mar. 11 $23 30,740 32,040 802 692 383 436 750,000 74,300
Apr. IS §24 26,940 29,000 446 3S7 282 310 1,215,000 63,000
June 6 24 21,840 182 ... 153 ... 9SO.0OO 59,700
June 7 24 22,830 ... 143 ... 166
July IS 24 20,540 22,310 93.3 77.9 106.5 112 1,390,000 66,500
Sept. 7 24 21,660 23,510 58.1 47.5 89.9 88.1 545,000 74 400
Oct. 25 24 21,600 23,320 38.5 29.1 72.0 69.4 865,000 48 300
Dec. 12 24 20,440 20.3 17.1 78.5 58.5 690,000 70,000
Dec. 14 24 19,190
1912.
Jan. 26 fl24 19,050 23,780 13.2 11.9 58.5 52.2 870 000
Mar. 13 24 20,300 21,000 9.3 8.5 63.1 46.7 S64ioOO 77 300
May 28 24 21.380 19,660 II6.0 4.6 45.7 29.5 536,000 67 000
July 24 24 21,780 20,540 °4.5 i'3.5 36.2 24.1 752,200 75 300
Sept. 17 24 19,680 18,960 °3.0 °2.7 25.5 18.0 1,386,000 70 600
Nov. 20 24 22,380 21,840 x3.0 **2.0 25.9 17.4 795 200 69 270
1913.
Jan. 14 24 22,800 22,400 **3.6 **1.9 42.0 16.5 795,200 67,200
* Treatment with acid water was begun on January 7, 1911.
t Only one sample in each tank. This material was not received until the tests of the other
samples had been started. The seeming discrepancy in the insulation resistance from month to
month is explained by the fact that the resistances were usually too great to be measured with
much accuracy without the exercise of more care than the purposes of this particular part of the
test demanded. The sample in tank 1 was seemingly more carefully prepared by the manufacturers
than the sample in tank 2. This accounts for the difference in insulation resistance.
t This figure refers only to tank 1. The temperature of tank 2 was 22.6.
§ This figure refers only to standard and special cambric samples. Temperature of rubber samples
in tank 1 was 23.75 and in tank 2, 23.3.
If This figure refers to tank 1. The temperature of tank 2 was 24.2.
|j Average of measurements made on 4 samples.
° Average of measurements made on 3 samples.
x Average of measurements made on 2 samples.
** Reading taken on the only sample left.