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THE COLLIERY (iLiARDLAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.	FRIDAY, JULY 5, 1918.	No. 3001.

The Safe Operation of Power= Plant Switchboards.

By L. FOKES.

One of the chief factors determining the more
extensive use or otherwise of electrical energy in and
about collieries is the degree of safety with which all
operations incidental to the generation, distribution and
utilisation of the current can be carried out.

A great deal of prejudice was created in the minds of
colliery workers at one time by the inability of designers
to appreciate the conditions under which electrical
apparatus had to work, and in consequence numerous
accidents resulted therefrom.

Vast improvements have since been effected and the
dangers which once existed are entirely removed where
modern apparatus has been substituted for that which,
in many cases, should never have been introduced into
a mine.

Whilst it is now almost impossible for the colliery
worker, however careless, to get into danger, those
engaged in the electrical staffs of large undertakings
have to exercise considerable care in order to avoid
accidents. Fool-proof arrangements are being con-
tinually introduced in one direction or another, but
even with the many valuable contrivances, the human
element has still to be reckoned with. Nothing, however,
should be left undone which would assist men to carry
out the various duties without danger to themselves or
to others.

Many colliery installations to-day are assuming very
large proportions, often involving large generating and
sub-stations, with some miles of transmission lines,
requiring large switchboards in the main generating
stations and considerable switchgear throughout the
various sub-stations.

Again, in connection with colliery plant continuity of
service is an all-important factor, and the isolation,repair
and re-connection of apparatus or cables in many cases
must be carried out with the utmost despatch. In such
circumstances it is easy to make a mistake in connection
with switching operations, and any methods or apparatus
which tend to minimise such possibilities should always
receive the consideration of those in responsible
positions.

The methods outlined below are intended to enable
greater safety to be obtained in the various switching
and isolating operations common to all large installa-
tions; and apply equally to underground apparatus and
that employed on the surface. One of the safest ways
of indicating when apparatus is alive or dead, or even
not sufficiently isolated, is by means of pilot lamps,
suitably placed and operated by the movements of switch
gear, or by the presence of current.

It is obvious that, in any case, to interrupt a high-
pressure circuit, carrying perhaps some thousands of
kilowatts, by opening it with a slow-break switch, the
result must be disastrous to the switch and possibly to
the operator as well. This can, however, be safeguarded
by the use of interlocking gear; but npt every design

Fig 1.

Fig. 2.

otherwise, the secondary should be opened first and the
pri mary afterwards.

(b) Ring Mains.—In some cases where a wide distri-
bution of energy is necessary, and a number of pits are
suitably placed, use is often made of what are known
as “ ring mains ” (fig. 2). The scheme simply consists
of running a main transmission line from the power
station, following a route which embraces in turn each
of the collieries to be supplied, the last one being as
near as possible the same distance from the generating
station as the first on the route, so that the main,
having described a more or less circular path, again
reaches the power station.

In such a case, each pit can receive a supply of energy •
from either direction, or from both at the same time.
Also, any section may be isolated for overhauling or
repairs without interrupting the supply, the pit at each
end of such an isolated section being fed from the
opposite side.

When switchgear, on one of the mains at the power
station, requires overhauling, it is obviously insufficient
to isolate the switchgear from the bus bars, as the oil
switch is still “ live,” being fed back from the first sub-
station on that main (the supply coming, of course,
from the other end of the ring main at the power
station). It is necessary, therefore, when examining
an oil circuit breaker, not only to have its isolator open,
but also the oil switch and isolator at the other end of
the main at the first sub-station. The same precautions
are necessary for making a section of line dead, i.e., the
circuit breaker and isolator must be opened at the sub-
stations at each end of the section.

Cables or Transmission Lines Working in Parallel.—
A reference to figs. 16 or 17, which will serve for illus-
trating cables in parallel, as well as in the form of a
ring, shows that the same precautions are necessary in
both cases when isolating switchgear or sections of
mains.

Many transmission lines are run in two parts working
in parallel, and the same method is often adopted in
connection with pit shaft cables.

There are many other conditions which call for special
care in operation; but those already outlined are of a
general character and have to be dealt with every day,
whereas the others are mostly special cases which must
be dealt with as they arise, and such precautions taken
as will meet the local requirements.

Pilot Lamp Supply.

Various methods may be used in order to supply the
required current for the pilot lamps. This may not, at
first sight, appear a serious matter: but, considering
that a pilot may be burning continuously, the cost of
energy consumed by each lamp becomes a considera-

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Fig. 6.

r

Fig. 3.

Fig. 5.

Fig. 7.





Fig. 4.

Fig. 8.

A brief outline will first be given of the direction in
which danger is likely to arise in the ordinary course of
working and maintenance of power stations and sub-
station switchboards.

Possibility of Opening an Isolating Switch when
the Circuit Breaker is “ In.”

This is a danger which is always present on remote,
mechanically controlled switchboards where the switch
gear is below the operating platform. In many such
cases, in order to open an isolating switch it is necessary
to descend into the basement, and unless some means
be provided for indicating which panels have the oil
switches closed and which have them open, a mistake
can very easily be made.

Isolating switches are not built for quickly breaking
the circuit. Sometimes they are operated by a lever,
controlling the isolating links which all move together,
whilst in others, where the pressure is very high,
separate single-pole isolating switches are mounted in
compartments by themselves and are operated by
insulated rods and gloves.

of board lends itself to these methods, and in these
cases pilot lights are essential.

Circuits Ped from Two Directions.

(a) Transformers Working in Parallel.—Great care is
needed, where a number of transformers are operating
in parallel, to make certain that the switchgear on
both the high and low tension sides is open, when a
unit is being isolated for repairs or inspection.

The primary circuit breaker and isolator of a trans-
former working in parallel may be opened, but the
transformer still remains live though supplying
nothing to the circuit.

If the secondary or low tension switches have not
been opened, the transformer is acting as a step-up
transformer with its secondary on open circuit, the
secondary really being in this case what is normally
the primary. Fig. 1 should make this clear, where two
transformers are shown connected in parallel to their
respective high and low tension bus bars.

It may be said that as a rule when switching a trans-
former out of circuit, whether working in parallel or

tion, and when a large number are in use and energy is
produced from outside, the annual charge may become
rather high.

Single Pilots.—These consist of single lamps, which
may be connected direct to the lighting supply, as
shown in fig. 3, or fed from accumulators, as in fig. 4.
Each lamp is operated by the switch in its circuit being
closed when some specific operation is carried out, the
result of .which the lamp is designed to indicate or
confirm.

Series Pilots.—These may consist of two or more
lamps in series and, possibly, situated at different
points, but all indicating the same fact, and all require d
to be lit or extinguished simultaneously. These, too,
may be fed from a lighting supply or from accumu-
lators, as shown in figs. 5 and 6.

Constant Current Series Pilots.—It is clear, from
inspection, that any of the foregoing methods are not
economical as regards the consumption of current,
especially where a considerable number is employed.

In this system, which is described as a constant
current system, the current is not exactly constant, but