ASTM D273 (1955) Methods of Test for Conducting Paths in Electrical Slate

Standard Methods of Test for
CONDUCTING PATHS IN ELECTRICAL SLATE!

A.S.T.M. Designation: D 273 — 40

Apoptep, 1940.2

This Standard of the American Society for Testing Materials 3 is issued under —
the fixed designation D 273; the final number indicates the year of original
adoption as standard or, in the case of revision, the year of last revision.

Scope marily as a routine factory or quarry
1. These methods include two pro- test, and since the apparatus required
is very simple and inexpensive, it is
cedures for testing electrical slate for applicable even where a comparatively —
conducting paths or veins. The general small quantity of.slate is to be tested.
fields of application and limitations of If the spark coil employed is of ordinary
the two procedures are described in the size, this method is much less hazardous
sections on application which precede to the operator than any method (such |
each of the methods. Both methods aS Method B) which employs high vol-
are nondestructive and may thus be tage at commercial frequencies.
used for routine factory or quarry in- | Apparatus
spection work.

Metuop A 3. The apparatus shall consist of the
following:
Application of Method A (a) Spark Coil ®*—A spark coil of the

2. This method of test employs high vibrator type capable of giving a spark
voltage at very high frequencies ob- from§ to 1 in. in length between needle
tained from a simple spark coil. Nu- points.

merical readings are not obtainable (6) Current Source-—Six volts direct
with this test procedure, but it can be current. |
depended on to locate conducting paths
Note 1.—An automobile-type storage bat-
or veins with considerable certainty tery of about 100-amp-hr. capacity is suggested.
and will, furthermore, give a rough,
comparative indication of the impedance (c) Hlectrodes—One electrode con-
of the slate. It is recommended pri- sisting of a sharp-pointed No.8 B. &S. |
gage bare copper wire, about 4 in. in
- IUnđer the standardization procedure of the Society,
these methods are under the jurisdiction of the A.S.T.M. length, mounted on the end of a hard
Committee D-9 on Electrical Insulating Materials. _
- .2 Prior to adoption as standard, these methods were
published 2 as tentative from 1927 to 1940, being. revised iin 2A Model T Ford ignition-type spark coil has been
found satisfactory.

132 Trusts ror CONDUCTING PATHS IN State (D 273-40)

rubber rod about 1 in. in diameter and Conditioning
1 ft. in length, or a multiple-point comb
6. No conditioning of the reference
type electrode mounted on a similar standard or the test specimen is ordi-
hard rubber rod as shown in Fig. 1. narily required, if the surfaces appear
The comb type electrode permits the dry. As surface moisture tends to
exploration of large areas more rapidly increase the brush discharge, wet speci-
than the single-point electrode. The mens shall be dried in an oven at 150 F.
electrode used shall be connected to the (65 C.) for at least 16 hr. before testing,
high-tension side of the spark coil
with high-grade automobile ignition - Procedure
wire, or equivalent, not more than. 7. (a) With both electrodes connected

10 ft. in length. The other electrode to the spark coil and while supporting
shall be a sheet of metal of suitable size the pointed ungrounded electrode by the
on which the slate under test can be insulating handle, the spark coil vibrator
laid. This shall be connected to the shall be adjusted to give as smooth and

Brass Rod ~~ |
⁄⁄ “hyo Screw and Nut

Lge oe4 yt
“4. /é arn Brass or Copper Rods ©
TT ~

_Solaered + NU oy “Hardt Rub bee $20 } Threaded
Feet * and lapped

te. { —Suggested Multiple-Point Electrode for Exploring Slate for Conducting Paths and Veins
by the Spark Coll Method.

other terminal of the spark coil which steady a tone as possible. The pointed
shall be grounded. oe . ungrounded electrode shall then be
brought within about % to 1 in. (de-
Test Specimen pending upon the capacity of the coil
used) of the plate electrode, and a con-
4, The test specimens may. be of any tinuous spark discharge should take
size and shall have all surfaces, including place.
, edges, sand-rubbed . or hone-finished.. (b) The reference standard and the

Reference Standard test specimen shall be laid side by side on
the grounded electrode and the pointed
`. A reference standard consisting electrode brought to within about7 in.

of a sheet of electrical slate of a quality of the face of the standard. ‘Under this
to be agreed upon between the manu- condition a small amount of typical
__ facturer and the purchaser shall be used. bluish brush discharge will be noted ©
- - This standard may be of any convenient at the point. The test specimen shall
size > such as Ša by 12 by 12 in. | then be© carefully explored 0over its entire

— 'TESTS FOR CONDUCTING PATEHS IN SLArE (D273- 40) 133

face and edges with the pointed elec- thus primarily a laboratory method but
trode, holding it about + in. from the is applicable to routine factory or quarry
surfaces of the specimen. In the case inspection service where the volume of
of the comb type electrode, the correct
spacing between the points and the slate to be tested is sufficiently great to
surface is definitely fixed. When a justify the expense of the apparatus
conducting vein or path is located, and of providing suitable safeguards.
sparks will jump and scatter to it in
lightning-like forks over the surface Apparatus |
of the specimen (Note 2). Slate that
shows conducting paths or veins under 9. The apparatus shall consist of the |
this test is unsatisfactory for electrical following:
purposes and should be rejected. (a) Transformer—A testing trans-
Nore 2.—The intensity of the brush dis- former rated at 1 kva. or larger, high- —
voltage winding for 10,000 v. ) provided
charge depends in general on the impedance of with a middle tap. |
the slate, that is, the lower the impedance the (b) Ammeters—Two similar -alter- "
greater the brush discharge. Unless the slate nating-current ammeters having a range _
is to be used in apparatus where practically of 200 or 250 ma.
any leakage is objectionable, slight increase in (c) Current Source.—A source of alter-
the brush discharge noted on the specimen as nating current having a frequency of 60
compared with that on the standard is permis- cycles per second, of approximately _

sible, provided, however, that it is essentially sine-wave shape, and of a voltage suited ©
uniform over the entire area of the specimen. to the primary winding of the testing
Such an increase in the brush discharge must transformer. |
not be confused with the sparks which occur (d) Electrodes.—Electrodes consisting
when an actual conducting vein is approached. of two metallic disks each having a con-
Once observed, this latter sparking is unmis- tact area of not less than 2 sq. in,
takable as it will be of marked intensity both mounted in insulating handles for the
visibly and audibly and will tend to branch out effective protection of the operator,
- in many directions. The veins are sometimes and a plate electrode as wide and of the |
confined to narrow ribbon-like boundaries that same length as the specimen to be tested.
“run the full length of the specimen. Others If the plate electrode cannot be con-
are more generally distributed and in these veniently made as long as the specimen,
cases the spark discharge sometimes will enter then the specimen shall be tested in
- the surface at one point, emerge at another and sections until its entire area has been
‘re-enter again at still another indicating the covered (see Note 4, Section 13).
presence of substrata conducting paths.
Test Specimen
MeTrHOD B
Application of Method B 10. The test specimens may be of any -
size, and shall have all surfaces, in-
8. This method of test is primarily cluding edges, sand-rubbed or hone- —
recommended for use when numerical finished. |
readings are considered necessary.
Fundamentally, this procedure meas- Conditioning
ures the leakage current when the slate 11. No conditioning of the test speci-.
is subjected to a high voltage of com-
mercial frequency. It requires a mod- men is ordinarily required, if the surfaces
erate outlay for apparatus and unless appear dry. As surface moisture tends
surrounded by proper safeguards should to increase the leakage current, wet
be used only by an operator experienced specimens shall be dried in an oven at

in working with high voltages. It is

- 134 | | Tang FoR CONDUCTING PATHS IN SLATE (D 273 — 40)

150 F. (65 C.) for at least 16 hr. before (a) With the specimen lying on the
testing. — ~ plate electrode, both faces of the speci-
men shall be gone over with the disk
Procedure - electrodes held approximately, but not
12. (ø) The electrodes shall be con- less than, 2 in. apart. Observation
7 nected with the ammeters, and these in shall be made of the deflection of both
turn with the high-voltage terminals ammeters. (Notes3and4.) |
of the testing transformer, by flexible (b) The specimen shall be stood on
cables provided with insulation suitable edge on the plate electrode and the
for continuous use at 10,000 v. The ' two movable electrodes shall be moved
ammeters likewise shall be suitably over the edges of the specimen with
insulated from ground for operation at — attendant observation of both ammeters
10,000 v. and shall be screened or (Note 3).
guarded ‘against accidental contact by
the operator. Note 3.—It is necessary that observation
be made of both ammeters since the presence
(b) The plate electrode shall be con- of a conducting path may result in the deflec- ˆ
nected to the midpoint tap of the test tion of either or both instruments, depending |
transformer. on the course which the path takes.
| Nore 4.—Where a considerable number of
_(c) The primary of the transformer specimens are to be examined for conducting
shall be connected to the current source paths and the factory arrangements are such
through a series resistor or reactor that these can be moved forward on some type
which will limit the high-tension current of conveyor, it may be possibleto examine the
specimens in transit. To this end it will be
under short-circuit to not more than necessary to provide two sliding or rolling upper

0.25 amp. The applied voltage shall be electrodes. The lower metal plate electrode,
adjusted until the open-circuit voltage 6 in. broad or more, shall be so placed that the
of the high-tension winding is 10,000 v. specimen to be: examined will pass over it.
The two upper electrodes, of the sliding or
rolling type, shall be spaced approximately,
Leakage Currents. but not less than, 2 in. apart, and extend in

13. The leakage currents of the test parallel lines across the specimen at a suitable
height and so placed with respect to the lower
specimen as shown by the ammeters electrode as to make contact with the upper
shall be noted for the following: two surface of the specimen in the region which is
tests: | passing over the lower electrode.