November 27, 1914.

THE COLLIERY GUARDIAN.

1123

since ventilating doors are invariably destroyed in all
explosions of any extent; and further, most of them
being near the mouths of side entries, it would be of the
greatest value to have an explosion wave arrested there
(see fig. 7). The principle underlying the door is to have
the enclosing framework put together without nails, and
keyed in position by the door fianie, so that when the
door frame is blown out the mass of dust held behind
the side and top boards is dislodged, and foims a dust
cloud. Again, it was found advisable to sustain a
certain amount of dust close to the roof, and this is
accomplished by attaching to the boards over the door
frame loose chains, hung by bolts into the roof or to
higher timbers These sustain some of the dust after
operation of the barrier and allow it to sift down in
case there is a considerable time interval between the
arrival of the pressure wave and the advance of the
flame. It has already been found that the door frame
must not be made too light or it is likely to be blown
down by the shock w«ve from the cannon. It can
therefore be a substantial frame, well braced to resist
the ordinary shock wave from blown-out shots.

Another device with a similar object is a rock-dust
protected overcast mad* of wood, or, if preferable, of
light concrete or brick constructor], not reinforced,
making the walls double, and filling same with rock dust,
nr flue dust, holes being provided for occasional inspec-
tion, and, if necessary, renewal of the dust.

To complete the devices for the protection of vulnerable
points a rock dust st< -pping was designed, which presents
some unique features. In the design, which was tested
with successful results, and which is illustrated in fig. 8,
the ease with which stoppings are blown out and their
vulnerability are taken into account. The dust pro-

Fig. 8.—Rock-dust Protected Stopping.

tection is practically in two halves, one independent of
the other, and advantage is taken of the angle of repose
of the rock dust. Practically, the device in each half
consists of a series of shelves one over the other, with a
space between the two halves in which there may be
any kind of stopping material—either rock dust,
ordinary masonry, concrete or wood. When the pressure
is first thrown on the stopping it will blow out as a
mass the in bye side, but if the explosion is not a heavy
one the outbye or fir*t set of shelves will remain
standing with a certain amount of rock dust on same,
to provide material for extinguishing the flame. To
protect the rock dust from getting wet, and yet at the
same time to permit it to be inspected, separate shallow
curtains are hung from shelf to shelf. It is not desired
to make these in one large curtain, since when the
whole stopping is under pressure there is a liability of
its going. out in a mass, but if the curtains are in
narrow strips they will flap up when the pressure wave
strikes the stopping barrier.*

Future Investigations.

Besides the investigation into protective devices, it
has become apparent that there is need for testing
dusts from coal beds of different districts in order to
determine the degree of inflammability or explosibility
of the pure coal dust, and then to determine what
admixture of roof material, or where artificially ground
rock dust is introduced, the amount of the foreign inert
material which will make the coal dust harmless; in
other words, to determine the explosive limit of the
particular dust (a) to prevent ignition, (b) to prevent
propagation. For example, for Pittsburg dust it
requires 70 per cent, of the roof shale in the mixture to
prevent ignition, but to prevent propagation it requir s
about 80 per cent. This is when the local shale is used;
but when limestone is employed in the mixture, owing
to there being no combustible matter in it, 60 per cent,
of the mixture will prevent ignition, and 75 per cent,
propagation. One low-volatile high-carbon coal which
was tested would not propagate an explosion with only
20 per cent, admixture of shale, so that a mine pro-
ducing coal of this character would apparently be
reasonably safe from dust explosions by the introduc-
tion of a relatively small quantity of rock dust. On
the other hand, this coal in a mixture with 40 per cent,
shale propagated an explosion when II per cent,
methane was present.

* Patents have been applied for by Mr. Rice for the fore-
going described devices, prospective rights being assigned
to the Bureau of Mines for the benefit of the public.

In tests of dusts from other districts at the experi-
mental mine, owing to the possible admixture of
particles of coal abraded from the ribs, which would
alter the results, it seemed necessary to line with
concrete a portion of the passage ways where such
tests are conducted, and this is now being arranged.

One of the most important series that will have to be
undertaken is that of determining how much the limit
of explosibility of each dust is changed by the introduc-
tion of small quantities of firedamp. Some tests have
been made, but not enough from which to draw con-
clusions. Special arrangements are being made for the
introduction of gas at the face of one of the entries
so that it may be put in behind a diaphragm, then
mixed with the air by a fan, in any desired proportions.
At the pre*ent time it is only feasible to use 2 per cent,
or less throughout the whole mine, whereas it is often
desirable to make a test with an explosive mixture of
gas at the face.	e

THE USE OF ELECTRICITY IN THE LIEGE
COALFIELD.

To a recent bulletin of the Society of Engineer
Graduates from the Montefiore Institute, M. J. Libert,
Inspector-General of Mines in Belgium, contributes an
interesting article on the present state of the applica-
tion of electricity in coal mines in the Liege
basin. It is necessary to mention that it was
written prior to outbreak of hostilities. M. Liebert
says it is. only within the last 12. years that Belgian
collieries have employed electric current on a large
scale for purposes such as winding, pumping, and

ventilation, and the change has been gradual, and
attended by considerable caution. The factors have
been mainly economic, a desire to render more efficient
the mechanical equipment, in view of increasing depth
of mining, th eextension of workings, the increased cost
of fuel, the reduction in hours of labour, and so forth.

In new collieries, such as those in the Campine dis-
trict, in which active operations on a huge scale have
been commenced to sink through tertiary and secondary
formations of considerable thickness, electricity will be
employed in unprecedented proportions to reduce to a
minimum the consumption of coal necessary for oper-
ating the mechanical equipment both below ground and
on the surface. Already for the sinking of the shafts
large central power stations have been erected to produce
the current necessary for the driving of the freezing
appliances used in the solidification of the strata and the
operation of the winding and fan engines and pumps
to deal with large heads of water.

Turning to particular applications of electricity, M.
Libert observes that electric signalling will soon be
obligatory in all Belgian mines. A complement is the
universal adoption of mine telephones, which are an
indispensable accessory of hydraulic stowage. The
latter within recent years has been extensively adopted
at collieries where injuries to surface buildings owing
to the consecutive subsidences have acquired a consider-
able importance.

The use of portable electric lamps has not grown so
rapidly as was anticipated. In the Liege coalfield they
are employed only for penetrating irrespirable atmo-
spheres, for examining shafts, and the lighting of under-
ground stations. At some pits in other basins, however,
they are used for general purposes. Their relative failure
has been due to several causes. In the first place, there
has been the difficulty of finding an ample source of
energy at low pressure, and notably the considerable
improvements in flame lamps in the direction of safety
as wrell as of lighting power, by the employment of
benzine lamps which form extremely sensitive registers
of firedamp.

M. Libert, to show the progress that electricity has
made in the Liege coalfield, recalls that in 1912 the
output of the basin amounted to 6,184,830 tons, the
number of persons employed being 37,878 and the
number of mines 74, each comprising one or more wind-
ing shafts. Thus the average production per mine w*as
83,572 tons, this low figure being due to the depth of
working, the dislocation of the measures, and to the
inclusion of many old pits. In consequence, there is
a lack of concentration in the supply of energy, which

is unfavourable from the standpoint of economy, and
the average production of the Campine mines will be at
least five or six times as great as in the Liege basin.
In the latter are installed for the production of elec-
tricity 883 steam engines, of a total capacity of 64,443-
horse power, or 45,992 kw. In addition, internal-com-
bustion engines, utilising coke oven gas, are installed
in five instances, the total nominal capacity of which is
4,200-horse power, or 3,090 kw. At the collieries them-
selves about 50,000 kw. are consumed, independent of
the current supplied from central stations working in
association with metallurgical works, and those which
are on an independent basis. The central stations con-
nected with collieries and coke oven establishments
associated with them number 54, in 116 separate
generating groups, with a total capacity of 29,959 kw.,
comprising 60 direct current dynamos, of a total capa-
city of 3,075 kw., and 56 alternators, of a total capacity
of 26,884 kw. The latter generates current at pressures
of 220, 500, 1,050, 2,100, and 3,000 volts or thereabouts.
The transformer stations number 52, comprising 85
static transformers and 46 rotary .converters.

The number of motors installed on the surface at
collieries, coke ovens, and briquette works number 680,
of 16,729 kw. capacity, divided into 184 direct current
motors (5,338 kw.), and 426 three-phase motors
(11,391 kw.). The three-phase system is chiefly in
favour, the continuous current installations having gener-
ally been in existence for some time. Below ground
there are at work 137 motors (11,398 kw.), only one of
which is of the direct current type. The following
classifies the three-phase motors, according to the
pressure used :—

Voltage	Number of	Capacity.
(volts).	motors.	Kw.
3,000	43	7,298
2,000	6	757
1,000	15	1,397
500 to 550 ..	32	1,412
Below 500	40	509

According to the different branches of service, the
number of motors employed was as follows :—

Number of	Capacity,

motors.	Kw.

Pumping ..............  108	  11,033

Underground conveyors 13	  225

Ditto fans .......... 8	  27

Air-compressors ......... 3	  66

Coal-cutting............. 4	  36

There is no instance of electric haulage to be found in
the Liege basin. On the other hand, the use of benzine
haulage locomotives has made rapid strides in the
Belgian collieries during the last few years, and is a
direct consequence of the laws limiting the hours of
labour below ground. Their first cost is lower than that
of electric locomotives, and they have been found more
suitable, especially in comparison with a trolley system,
in winding roadways.

The incandescent lamps installed below ground only
number 630, and are used mainly to light engine rooms,
landings, and main roadways. The pressure adopted is
generally 110 volts, or even less, but in exceptional cases
as high a current as 500 volts is employed. The latter
installations were mostly in existence when the regu-
tions were issued in 1909.

Heating and Insulation Deterioration of Current Carrying
Conductors.—A voluminous report dealing with theoretical
and experimental investigations on this subject was published
some time ago by the Electrotechnical Laboratory of Tokio,
and is quoted by the Electrical Review. The main object
of the research was to establish general formulae for the
temperature at any point in the covering of insulated con-
ductors, and for the insulation resistance and breakdown
voltage of the covering as a function of the temperature of
the conductor, and thus of the current carried. The
theoretical formulae established include :—(1) Formulae for
temperature rise as function of current carried; (2) general
and approximate formulae for insulation resistance as func-
tion of current carried and room temperature; (3) formulae
for breakdown voltage. The thermal and electrical con-
stants involved by these equations being determined by
experimental investigation of the thermal and electrical
characteristics of various covering materials, “ practical ”
formulae have been derived by substitution in, and simpli-
fication of, the general theoretical formulae. The conductors
investigated were covered with : (1) High-grade insulation
(pure, separator and jacket rubber, cotton tape, and impreg-
nated cotton braid); (2) low-grade insulation (vulcanised
rubber and cotton braid); (3) Tokio-Sen (impregnated
cotton). Very fair agreement is shown between data calcu-
lated from the formulae and data determined by experiment,
and from the new formulae it follows that a No. 20 S.W.G.
wire will carry 10 or 11 amperes; a No. 8 S.W.G. wire
about 75 amperes; and a No. 0 S.W.G. wire 195-200
amperes, if insulated with Tokio-Sen or the high- or low-
grade insulations mentioned above. These ratings assume
20 degs. Cent, temperature rise (15 degs. to 35 degs. Cent.),
and at the higher temperature the insulation resistance is in
each case approximately one-third or one-fourth as great as
at the lower temperature. The author suggests that the
current carrying capacity of insulated conductors be deter-
mined by the deterioration in their insulation due to heating,
and in this connection his formulae are readily applicable to
determination of the current corresponding to any specified
decrement in insulation resistance. Actually, however,
there would be excessive pressure drop in conductors rated
simply by the 20 degs. Cent, heating limitation. Only short
circuits could be thus operated, and in few cases would
mechanical considerations and the actual current to be
transmitted enable full advantage to be taken of the thermal