1326

THE COLLIERY GUARDIAN.

June 25, 1915.

The Influence of Atmospheric Electricity in Underground Workings.

In a paper contributed to the Soci6te de 1’Industrie
Min6rale, M. Ferey records some experiences encoun-
tered in the Rochebelle and Fontanes mines in connec-
tion with shot-firing operations from the surface and
the means taken to safeguard the system.

At these mines special precautions are taken to meet
the ocurrence of sudden outbursts of gas. In all
headings the use of picks is forbidden, the drifting
being carried out exclusively by means of explosives
fired by electricity. This work has always been done
only when all workmen have been ’withdrawn, the shots
being fired from places removed as far as possible from
the face, and, if the circumstances permit, from a
higher level. All shots are fired in accordance with
definite instructions after the workings have been
thoroughly inspected. The workmen are also enjoined
to report immediately any outburst of carbonic acid gas,
even where no personal injury has resulted.

Latterly, these outbursts became so frequent that,
from 1903 onwards, arrangements were made to fire all
shots from the surface. This enabled the so-called
safety doors to be dispensed with, these doors being
employed to divide the mine into independent districts,
by which the ventilation could be modified or suppressed
during shot-firing and the subsequent examination.
There still exist, however, beside the. ventilation doors,
certain safety doors, which are closed on the shots being
fired, and assure .the complete isolation of the different
districts, but the ventilation is unchanged during the
shot-firing and the examination. In this way all
stoppage of the ventilation is obviated.

Shot-Firing from the Surface.

The most powerful exploders having been found
inadequate to assure the firing of 30 to 50 shots distri-
buted over groups of workings situated at a distance of
2 kiloms. or more, it was decided to use continuous
current at 120 volts derived from the dynamo gene-
rating current for lighting purposes; this current is

80 i

4-
!

4-

Fig. 1.—Details of Contacts.

supplied to the firing station by means of a bipolar
switch, and is ctit off immediately after blasting has
taken place.

The original shot-firing equipment, situated at the
mouth of the shaft, consisted of two boards, the first
carrying a glow-lamp indicator, a bipolar switch, and an
ammeter; the second, a series of contact studs, enabling
a pole to be attached to each line by means of bronze
plugs, the other pole being connected to earth. Each
board is under lock and key. The second board also
carried the conductors, which were furnished with lead
fuses; the conductors in the shafts consisted of wires of
galvanised iron fixed on porcelain insulators; in the
roadways they consisted of disused cables or, preferably,
of bell wires of 8 mm. diameter, and in the vicinity of the
workings of galvanised iron wires of 4| mm. diameter,
on porcelain insulators, which were placed up to within
6m. or 7 m. from the workings. The detonators in each
working place were grouped in series, branches being
led to the various working places from the line. Each
working place, or group of working places, was provided
with an earth.

Atmospheric Influences.

During a storm which took place on June 9, 1905,
about 4 p.m., whilst the workmen were leaving the
mine, shots suddenly went off in two places from which
the workmen had withdrawn themselves, after having
connected the detonators to the conductors. These
places were situated at distances of 1,490 m. and
1,410 m. respectively from the.firing station. Ignition
took place before the latter had been placed in circuit
with the, dynamo, and could only be attributed to
atmospheric discharges.

In order to provide against the recurrence of such
incidents, it was decided to cut the lines at the point
where they passed from the shafts into the roadways,
connection being made by the onsetters some moments
before firing took place. They were disconnected by
the workmen deputed to carry out the examination after
the firing of the shots.

Despite these precautions, a further incident occurred
on August 25, 1905. A shot placed at a distance of
1,360 m. from the firing station had missfired. About
10 p.m., whilst a storm was in progress, six shots
exploded in this place, whilst the workmen were still in
a neighbouring working place. These men found that
the wires had been duly disconnected near the shaft
bottom. An order was now issued to the examiners to
disconnect the circuit in any working place where shots
had missfired, and the following measures were taken
in addition. In the first place, when the wire was dis-
connected at the pit-eye, the shaft conductors were at
the same time connected with earth, a copper wire
serving, before and after shot-firing, to connect each
shaft conductor to the firing circuit or to earth respec-
tively. This was replaced by a two-way switch, and by
a flexible cable terminated by a brass contact, so
adapted as to be connected up with a contact soldered
or bolted on to the extremity of the wire extending into
the mine, or to a contact with earth. A gap was thus
secured of about 1 m. (fig. 1).

The Use of Commutators.

At the end of 1906, it was decided to replace the bare
conductors in the shafts by multiple cables, carefully
insulated with lead and armoured. The firing station
was also transformed on the lines shown in fig. 2. It
consists essentially of a. board, to which are led on
porcelain insulators the two wires from the generating
station. When current is switched on from the station
this is indicated by the lamp, but no connection with the
mine cables is made. An ammeter is provided, and the
current is broken by a bipolar switch, communicating
on the one hand with earth, and on the other with six
Morse-type commutators connected with the six wires
traversing the mine. These lines are provided with
fuses.

After current has been sent to the switchboard from
the generating station, the official in charge closes the
switch, and by examination of the ammeter ascertains
that no current is passing, the wires still being isolated
by means of the Morse commutators. He then presses
each of the commutators according to a pre-arranged
order, at the same time observing the ammeter. If the
needle oscillates slightly and is arrested at less than
2 amperes, he knows that shots have been properly
fired. If, on the contrary, the needle moves violently,
he knows that a short-circuit exists, and probably the
shot has missfired. ' The examining shift does not
descend until 10 minutes after the current has been
switched off and nothing abnormal has been observed.
•Two men are engaged to examine each quarter allotted
to them, and are equipped with three lamps, one of
which is an electric lamp.

This arrangement appeared to work satisfactorily, and
the management secured authority to dispense with a
return wire. On October 2, 1913, however, at 2 p.m.,
three shots went off without human agency during a
storm. This took place in a rise working of low section
under a nearly horizontal roof, in which large quan-
tities of water were present. About five minutes after
the workmen had connected up the detonators and left
the workings, the shots went off, the rush of air extin-
guishing the acetylene lamps in the level. In two
neighbouring headings the men had not yet charged
their shots. An examination showed that the shaft
conductors had been properly connected to earth by
the means described. The management accordingly
decided to establish a second gap, similar to that at the
pit eye, at a distance of about 100 m. from each of the
workings. An examination was further made of the
working place in which the shots had fired; it was found
to be the firing point situated nearest to the station on a
horizontal projection, nearly 1,254 m., at an average
depth of 200 m. below the surface. The actual length
of cable was 2,540 m. The circuit serves these
headings only, in a very damp quarter of the mine. No
sparking was noticed at the time. The two cables that
serve this portion of the mine follow the line of an iron
compressed air main, up to a point 1,500 m. from the
shaft; their distance from this main, 20 cm., is reduced
at certain points, and it cannot positively be stated that
sparking did not occur between the main and the cables.
Such sparking, it is remarked, has been observed during
the progress of storms on several occasions, not only
in the roadways, but in the shafts also.

It is difficult to attribute these incidents to a dis-
charge or to induction. The lightning seems to have
been directly responsible for the occurrence on June 9,
1905, having passed vid the iron shaft guides to the bare
cables, and the same may have been true of the other
occurrences, if a spark or slow discharge was produced
in a damp atmosphere.

Against this supposition, there is the fact that no
deterioration of the cables or shot-firing appliances has
been observed, and these incidents have always occurred
in the workings most remotely situated from the firing
station, whilst a direct discharge at the pithead would
have found an excellent earth. The true'cause, therefore,
was probably induction provoked by atmospheric dis-
charges. In this connection, it is. important to recognise
that the conductivity of the earth is not perfect,
especially at the moment of an external discharge.

The capacity of the line is more effective if its
extremity is connected to earth. Further, only a very
feeble current is necessary to cause the explosion of a
series of detonators. Each line having a resistance of

0-62 ohm, and requiring a current of 0-7 ampere, with
a pressure of 1-5 volts, is capable of firing three deto-
nators in series.

The last occurrence noted was particularly disquieting
to the management, as it showed that in stormy
weather not only are the workmen in a working place
exposed to danger through the possibility of charges
going off without warning at the moment when they are
being connected up, but if this should bring about an
outburst of gas it is to be feared that a number of men
would be overcome. Immediately after the occurrence
it was decided to sectionalise each branch line by means
of a gap placed at the entry of the quarter served by it,
and placed as near to the workings as possible. The
disconnection is made, and the wire earthed by the men
carrying out the examination, and the connection is
effected as nearly as possible at the end of the shift by
the last workmen to go out. The effect is to diminish
the capacity of the wires and the inductive influence of
the atmosphere. In accordance with the mining regula-
tions, a return wire is necessitated, and the wires will be
completely insulated. With this end in view, the
management are providing lead-insulated and armoured
shot-firing cables in the main roadways, similar to tho«e
installed in the shafts, but it has been decided to retain
the employment of gaps both at the pit-eye and near
the faces, since these are regarded as conducing greatly

Fig. 2.—Diagram of Arrangements
at Shot-firing Station at Pithead.

to the security of the system. The cables will each con-
tain two conductors, buried in the haulage-way at a
mean depth of 10 cm., so as to be protected from falls
or overturned wagons, etc.; they will be designed to
allow the firing of from 20 to 30 shots in series, in three
or four equal groups, as required, by means of branches,
at a distance from the firing station of up to 3 or
4 kiloms. The gaps near the shaft bottom will be about
1 m. in length, as at present, to prevent the direct
effect of lightning, and those in the vicinity of the
workings will be equipped with bipolar commutators to
obviate the phenomena of induction as well as the
direct passage of current. Finally, from these com-
mutators bare wires in pairs will be led to the workings,
as there is a lessened possibility of induction in these
short spans.

Grimsby Coal Exports. — Returns for the week ending
June 18 show that the coal exported from Grimsby was as
follow :—Foreign : To Esbjerg, 908; Dieppe, 555; Kallund-
borg, 649; Odense, 2,249; Randers, 585; Rotterdam, 1,085;
and Trelleborg, 551 tons. Coastwise : To London, 150
tons. Total, 6,582 tons foreign, and 150 ’ tons coastwise;
totals for the corresponding week last year were : Foreign,
15,283 tons, and 917 tons coastwise.

Approved Safety Lamps. — The Home Secretary gives
notice that on June 15, 1915, he made an Order under
section 33 of the Coal Mines Act, 1911, entitled “ The
Safety Lamps Order of June 15, 1915,” approving the
following safety lamps, until further order, for use in all
mines to which the Act applies, subject in each case to the
conditions specified in the schedule to the Order :—Messrs.
Patterson and Company’s safety lamp type B.8, the
“ Rutter ” miners’ electric safety lamp, and the Oldham
“ Emergency ” electric lamp type No. 2 (bullseye). A com-
plete list of the safety lamps, which have been approved by
Order under section 33 of the Coal Mines Act is printed as
an appendix to the Order.

Hull Coal Exports.—The official return of the exports of
coal from Hull to foreign countries for the week ending
Tuesday, June 15, is as follows:—Amsterdam, 949 tons;
Bergen, 4; Christiania, 775; Dunkirk, 268; Flekkefjord,
86; Fecamp, 1,643; Gothenburg, 1,457; Harlingen, 1,409;
Halmstadt, 467; Honfleur, 700; Havre, 643; Landscrona,
1,812; Norrkoping, 784; Rouen, 21,945; Rotterdam, 1,651;
Treport, 1,385—total, 35,978 tons. The above figures do
not include bunker coal, shipments for the British Admiralty,
nor the Allies’ Governments. Corresponding period, June
1914—total, 99,258 tons.