May 31, 1918.

THE COLLIERY GUARDIAN.

1095

cables successfully. It is also claimed that paper
cable can be worked at higher temperature, and there-
fore at a greater current density. Paper cables can
certainly be worked at a higher temperature than
vulcanised bitumen. This, however, does not neces-
sarily imply that a greater current density is possible,
since the construction of the latter type provides better
means of dissipating heat generated in the cable by
C'R losses, owing to the increased thickness of
dielectric. The over-all diameter, and consequently
the area of radiating surface, of a vulcanised bitumen
cable is considerably greater than that of a paper
cable with a similar size of conductor. In practice,
the question of temperature limitations is rarely a
deciding factor, as the sectional area of the conductors
is usually determined by the permissible loss of volts
in transmission, which keeps the current density well
within these limits. For portable machines, such as
coal cutters, etc., the cab tyre sheath cable excels
any other type in all respects, but the writer thinks
that cable makers would greatly add to the safety
of the working of portable apparatus if an earth
shield was made to surround the cores of such cables.
The writer was called to inspect and report upon an
electrical installation, and when visiting a portable
machine, the size of the trailing cable immediately
caused doubt as to the earth core being that called
for in the Home Office Rules. Upon thorough investi-
gation of cable ends these doubts were confirmed,
the earth core being about 15 per cent, of the section
called for in the Rules. The cable in question was
supplied through agents who supplied the machine,
and who evidently were not aware of the clause regard-
ing minimum size of earth conductors, viz., 0*022,
although the specification read that all was in con-
formity with the Home Office Rules. Such cases as
these are liable to get colliery owners and officials in
unpleasant quarters.

The writer has a large area underground and over-
head covered by high and low tension, and can say
that for the last six years not a single fault has
developed due to faulty manufacture or due to any-
thing other than accidental mechanical damage.
Some makers seem to think that to deliver cables with
space of | in. and f in. between armour wires is
not detrimental to the cable; but it is certainly so,
as it means that when the jute serving is cut off, which
is a common occurrence due to falls of roof, the layer
of armoured becomes quite loose. It also makes it
easier for sharp tools making contacts with cores
making it alive, and is liable to allow moisture to get
in and corrode the armoured wires in course of time,
as some pit waters attack cables badly.

It is essential to give serious consideration to area
of feeders when cutting developments are in pro-
gress in view of the advancing of coal faces, and the
tendency is to overlook the voltage drop question,
which is the cause of a great deal of motor trouble.
There is also the question of the conductivity of the
armour of high tension systems when transformers
are used. This should be equal to 50 per cent, of low
tension feeder, unless an efficient earth plate can be
placed inbye, which is not often obtainable, or unless
an earth core forming an integral part of the cable
is used, which is the best. The installation of dupli-
cate cables in upcast and downcast shafts is worthy
of consideration in view of accidents, so as to enable
the most important part of the colliery to be worked,
such as fans and pumps. The recording of areas,
length of individual cables, is essential, in order that
such may be available for fault-finding purposes. The
cables should be periodically painted to preserve the
jute serving. When enquiring for cables the fullest
information should be given to the makers in order
that they may offer the best cable for the conditions.
The question of overheating of cables is often over-
looked.

The ordering of cables should not be left to stores
clerks, etc., who simply look at cable makers’ lists
for a cable to carry so many amperes. Two or three
recent cases may be cited where cables have been
ordered in this manner. The section that will carry
the numbers of amperes required is purchased. This
policy has been the cause of a great many electrical
breakdowns. The question of the testing of trailing
cables deserves special attention. The greatest care
should be taken to maintain the conductivity of the
earth conductor equal to that required by rules. The
practice of testing such cables with galvanometer is
useless, as, for instance, an earth core of 91/0 018
complies with the requirements, but even one strand
missing will make such uncompliable, and even if
there is only one strand continuous, a deflection could
be obtained. The best practice is to test for ohmic
resistances, or pass full load current through earth
core at least once per week. Also, double working
pressure should be applied to such cables weekly, and
a continuity test applied with low resistance bell and
battery daily is useful. Several accidents have
occurred in the past due to defective earth conductors
in trailing cables, and it is imperative, therefore, for
the strictest attention to be given to such cables. The
use of armoured cables of single and multi-core types
is to be highly recommended for signalling, tele-
phoning, and lighting circuits underground, and for
inter-connecting motors and switchgear.

Transformers.

These will be more called for in future than
generating plants. It is therefore essential to give
consideration to some points in connection with their
use, etc., as the cheapest transformer offered is not
always the best and cheapest in the end. When
sending out enquiries for transformers, it is import-
ant that the time be specified during which the
transformer has to stand full load. If it has to stand
twenty-four hours’ full load it should have a low
copper loss, but if the load be intermittent, a low iron
loss is advisable. The conditions under which the
transformer will have to operate are best when—(a)
Well ventilated, (b) Not too confined. If more than
one transformer is in the chamber, plenty of room
should be left between the transformers, so as to give
sufficient cooling, (c) Air temperature, considering

such special positions as man-holes, or transformers,
for below ground work, (d) Facilities for handling,
bearing in mind periodical inspections, including lift-
ing the transformer out of its tank, (e) Hollers.
State if these are required. In some cases skids are
fitted to base to facilitate transport. (/) Tappings.
It is very often advisable to have two or three adjust-
able tappings on the high tension side to allow for
alteration of primary volts due to different lengths
of feeders, (p) If neutral point is required to be
available for earthing purposes on star side.

When considering the type of transformer, it is
often overlooked that for large size the forced cooled
type is cheaper than oil-immersed self-cooled type,
and that the oil-immersed self-cooled type is cheaper
than the natural air-cooled type. The question of
single and three phase is worth consideration. The
advantage of the three phase is that the floor space
is smaller, the cost lower, weight less, better over-all
efficiency, and being cheaper to instal, and the ad-
vantage of single phase being less individual weight,
easier to transport to distant points in mines with
small roads, a spare unit is comparatively cheap. The
possibility of complete stoppage from breakdown of
three single phase transformers is less than with a
three phase type, as two of the single phase trans-
formers can be connected. It is essential that the
transformer be of sound mechanical construction. It
is not sufficient to have lengths of wire wound round
an iron core: the end turns should be heavily insu-
lated to withstand surges; tapping terminals should
be fitted, and not ends left loose in the transformer
as in sotne makes, although one does not anticipate
repairs when purchasing a transformer (when of
reputable makes). It is necessary to bear this item
in mind. Replacing burnt-out coils is a rather diffi-
cult matter on shell type transformer, whereas it is
a fairly simple matter on core type transformer, and
generally a good working electrician can fit such coils
quite satisfactorily on site. The question of the use
of earth shields is of interest, as it is not generally
considered that these are more a source of danger
than of advantage, but they place an earth path be-
tween the primary and secondary coil, and so put a
greater strain upon the windings; therefore they are
not to be recommended, as it is considered better
practice to insert a stout insulating tube between the
two windings and add an earthing device to protect
the low tension windings in order to comply with the
rules. When considering the oil-immersed and air-
cooled transformer, it must be borne in mind that
the oil-immersed type has a much longer life, as oil
acts as a preservative on the insulating material.
When considering offers of transformers with a
number of tappings, care should be taken to see that
the k.v.a. output is the same for the various ratios.
This is apt to be overlooked. It is essential that
good quality oil should be supplied, even if it costs
a little more, as it would easily pay for itself, if only
for the smaller quantity of sludge that will be formed
in the transformer. The writer has found the Fer-
ranti float device a very efficient method of preventing
sludging of the oil. It is quite common to see trans-
formers supplied with no provision made for finding
temperature; and, after all, the life of a transformer
depends upon this factor. Cases are known to the
w’riter where low load conditions give greater heat
than full load.

Motors.

The two chief kinds of motors adopted in the past
have been the squirrel cage and slip-ring induction
motors; the former in view of high efficiency, sim-
plicity, mechanical strength, and absence of sparking
(the three first items keep the maintenance costs
down), and the latter makes it particularly suitable
for installation in coal mines and other dangerous
places. It has the disadvantage of having a low
starting torque, and will not start against heavy loads,
and causes disturbances in the lines when started up,
but the latter is not serious. The writer has a large
number of coal cutters with this class of motor, and
can thoroughly recommend it for such purposes, all
bearings being of ball and roller type. There are
a great many other drives in colliery work where it
can be used with advantage. The slip-ring type motor
is chiefly used for starting up against heavy loads
and where speed variation is required, such as for
main and tail haulages. The writer has a large
number of these doing excellent work, the chief cause
of a few troubles having been due to defective and
weak bolting down girders and cast wheels on haulage
gears causing excessive vibration. With substantial
foundations and machine-cut wheels such troubles are
greatly minimised. When ordering duplicate motors
it is essential to state requirements definitely, as
manufacturers sometimes change their designers, and
with different makers rotor volts and amperes vary,
with the result that control and starting gear is not
suitable. Such points as centres of holding bolts,
size of bearings, type windings, phase resistance,
radial air gap, power factor, speed efficiency and
temperature rise. It is also good practice to specify
for the phase connections to be brought out to a*
terminal box, as this greatly assists fault finding. The
proper rating of motors is a question that is well worth
ventilating. This has to some extent been referred
to under heading “ Power Factor.”

In connection with A.C. motors and turbine pumps
it is necessary to specify that the pump shall do its
full load duty at a reduced frequency, for the duty
of the pump is dependent upon speed, which in turn
is dependent upon the frequency. Manufacturers
would greatly assist users by standardising cable tri-
furcating box for stator frequency. These are
required easy of detachment, so that the cables and
motor windings may be tested independently. In
some designs this is not possible and is the cause of
great delays. Motors are supplied at collieries at the
present time with plain terminal boxes suitable for
rubber tails. Some makers have gone so far as to
make them so large that diagram of connections is
cast on the front cover, and they are to be recom-
mended. In one case recently inspected the bare

terminal was such that could be easily touched by the
attendant.

Switchgear.

The present switchgear has greatly improved from
that manufactured twelve years ago. The tendency
to-day is to instal totally enclosed oil, iron or steel
clad immersed circuit breakers for high and low ten-
sion, even for power house and surface and under-
ground sub-stations, etc. This is to be highly recom-
mended in view of eliminating the possibility of
contact with live conductors, the whole being sur-
rounded by efficiently earthed metal cases. Oil
switches should be large enough to open the circuit
in safety under the worst conditions, which will
probably be a short circuit on the cables just beyond
the switch. It is good practice to standardise oil
switches for certain amperes and volts capacity, the
rates being the same for 600 and 3,000 volts. Of
course the current transformers, trip coils and meters
are made to suit each individual feeder. For safety,
the ideal gear is a draw-out panel, as this is specially
recommended as affording the maximum protection to
life. A clear understanding of the apparatus by those
who are permitted to operate the gear is obviously
necessary. Conspicuous labels should be placed on all
switches indicating the districts, motors and trans-
formers controlled by each. There is in use a great
deal of gear which is not what may be termed fool-
proof, and with strict supervision this is suitable.
Labels should also be placed on oil tank switch and
joint box covers, saying it is dangerous to interfere
with them unless certain switches and links are in
the '“off” position. It may appear absurd to say
this, but experience has proved its necessity. With
the best of men there is the possibility of forgetting
at times. All electrical apparatus should be treated
as if it were alive until positive proof is obtained
that it is all dead. The writer has known cases of
one phase contact of oil switch remaining in contact
after the switch has operated. It is therefore best
to instal isolating links between line switches and
apparatus in order to ensure the maximum of safety.
Of course, with draw-out type this is not required,
although switchgear design has improved as pre-
viously indicated. There is, however, room for
improvement still in some designs. Some makers
adopted alternative methods for cable leading in
and out.

The subject of standardisation of switchgear for
mining work is one that deserves special mention.

The use of liquid controllers has become greater of
late, while they have, no doubt, their advantage as
regards flexibility of control. They have their dis-
advantages for use while on auxiliary haulages in-bye
in mines, owing to the amount of water required. In
some cases they have been introduced in order to
reduce the number of cable leads that require to
be mechanically protected, but the writer’s practice
is to instal metallic resistances, oil and air cooled
with oil immersed controllers.

Earthing.

Earthing is now recognised as a necessary and
important factor in connection with colliery electrical
installation, and recognition of its importance has
been effected not only by the Regulations enforced
by the Home Office, but also by the commendable
desire of those responsible to eliminate as far as
possible all danger. A good method of earthing is
to use. three plates connected together to common
bus-bar to many circuits; by disconnecting from the
bus-bar it is easy to make tests. The wires and
connections to the earth plates should be insulated
to obviate corrosion and to have ammeter and volt-
meter w’ith multiple way switch, and so arranged that
tests between plates may be easily effected. A good
motto is to consider a broken earth wire as serious
as a bare conductor. It is very necessary that all
persons in charge of electrical apparatus should be
trained, and all men operating switch-gear and other
electrical apparatus should be properly instructed by
an electrician as to the proper handling. It is a great
mistake to think that anyone from an engine road is
qualified to operate electrical machinery at a few
minutes’ notice, but unfortunately this has often been
the case.

Premiums for Austrian Miners.—The Zeit learns that
a new system of premiums has been introduced in the
Ostrau-Karwin mines in order to promote production to
the fullest possible extent. The average output of the
miners in the last half of 1917 and the first quarter of
1918 is calculated, and the earnings of the workmen are
increased by a system of premiums up to 100 per cent,
in proportion as their output exceeds that of the last half
of 1917. A similar system is to be introduced in Hungarian
mines.

Central Association of Miners’ Permanent Relief
Societies.—The report of the consultative committee states
that the returns from the societies on the work for 1917
compare favourably with the previous year, when it is
taken into account that a large number of members have
been called upon to join his Majesty’s Forces. The follow-
ing increases have taken place : Membership increased by
7,293 members, contributions by £35,433, revenue by
£39,710, accumulated funds £32,613, and benefits paid by
£9,160. On December 31 societies had 2,594 permanently
disabled members on their funds, and the recipients in-
cluded 3,757 widows and 3,321 children. A comparison
showing the number of lives lost in the years 1915 and
1916, as compared with the claims dealt with by the
miners’ permanent relief societies in the same period proved
the need for the extension of these institutions. The
lives lost in the United Kingdom numbered 1,313 in 1916,
and 1,297 in the previous year. Fatalities to members
of the miners’ permanent relief societies numbered 403 in
1915, ditto in 1916, and 453 in 1917. In the Minnie Pit
disaster, North Staffordshire, only 39 of the killed were
members of the North Staffordshire Miners’ Permanent
Relief Society, leaving chargeable to that society 16 widows
and 31 children. Provision had to be made, and various
public funds were opened, and are still open, for the
benefit of the widows and dependants. According to public
report approximately <£8,000 to <£9,000 has been raised.