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VIII. COMMENTS AND RECOMMENDATIONS
The Lower Three Quarters Drifts were practically level cross-measure roadways in which the strata were not abnormal and in which the presence of coal, either in regard to quantity or quality, was not unusual. Their drivage was a routine pit job requiring nothing more than a good average standard of pitmanship. Nevertheless, inflammable gas was allowed to accumulate to a dangerous extent and apparatus was misused in such a way that an explosion was occasioned of sufficient violence to cause the deaths of two persons.
It is with regret that I point out that there would have been no loss of life had there been compliance with two simple precautions attached to the Approval in regard to the use of the exploder, namely, "The shotfirer’s station shall be at least 150 yards from the shotholes", and "Before firing the shots the shotfirer shall see that all other persons are in proper places of safety not less than 200 yards from the shotholes."
Ventilation
The auxiliary fans ventilating the two drifts were prevented by power failure from running continuously throughout the shift in which the explosion took place. There was no evidence of frequent tripping of the transwitch unit supplying the fans, but the simultaneous starting or running of a considerable portion of the large connected load could result in an overload.
Stoppage of any ventilating current is bad at any time, but is particularly so in narrow drivages which are ventilated by auxiliary fans. It is well known that firedamp is lighter than air and accordingly, tends to gather in roof cavities and near the roof, but it would appear that the dangers and difficulties arising therefrom are not properly appreciated.
Useful and indeed remarkable, information regarding the variation in air velocity over the cross section of an airway, particularly one in which the air speed is low, is given in Safety Pamphlet No. 26 on "The Ventilation of Rippings" *. This publication is one which is worthy of close study by all officials
I am not satisfied with the standard of gas testing in the Main Drift, particularly at roof level and in cavities. Evidence has led me to believe that it was not given sufficient attention and that the dangers inherent in the presence of inflammable gas at roof level were ignored.
Facilities should be provided to enable all persons to reach easily and conveniently all points at which tests for inflammable gas should be made by them. In addition, these persons should ensure that their flame safety lamps are held, with the middle air feed closed, for a sufficiently long period of time at each point of examination to allow any firedamp present to be drawn into the lamp for detection. I suggest that a lamp should be so held for at least half a minute unless inflammable gas has been detected before the lapse of that time.
As demonstrated in the recovery operations in the Main Drift, firedamp which has accumulated near the roof is not easy to dislodge, and its dispersal may require an air current which is considerably larger than is normally necessary.
It is definitely laid down in Section 55 (1) of the Mines and Quarries Act, 1954, that adequate ventilation must be constantly produced in all parts of a mine belowground to dilute inflammable or noxious gases so as to render them harmless and to remove them, and it is incumbent upon the management to anticipate changes in the ventilation requirements in every part of the mine.
The provision of large roadways has much to commend it, but the driving of them may give rise to difficulties. The larger the cross-sectional area of a road, the greater is the tendency, particularly when auxiliary fans are used, for “layering” and for accumulations of firedamp to gather at the roof.
I suggest that the ventilation of every such narrow drivage should be a subject for special consideration to ensure that at all times the air quantity passing at roof level, as distinct from the middle of the roadway, say, is sufficient to dilute and remove any inflammable gas which may accumulate there.
Shotfiring
Amongst the difficulties found in driving large roadways is the fact that a considerable number of shots is required in each round or "pull", many more than can be fired in one round by a six-shot exploder which is approved for use in all mines.
The preparation and firing of several rounds, instead of one, at the face of a cross-measure drift entails certain risks of injury to the persons so engaged and, accordingly, in circumstances which are considered suitable a more powerful type of exploder is allowed to be used under approvals granted individually in accordance with the terms of the Explosives Regulations. An exploder of this type is capable of producing an incendive spark, and its use under the Approval is governed by certain conditions. Evidently the risks involved by misuse of this apparatus are not always appreciated and the greatest thought and care must be given to the whole of the shotfiring arrangements when large rounds of shots are being fired:
| (a) |
Testing of the circuit: In accordance with Regulation 32 (7) of the Explosives Regulations an approved testing device has to be used for testing the shotfiring circuit for continuity. I recommend that this instrument should be brought out of the mine at least once each week for examination, test and report by an electrician of the mine.
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| (b) |
Shotfiring cables: The essential requirements for shotfiring cables are (i) that the conductors should be of suitable cross-sectional area, (ii) the insulation should be sufficient in relation to the maximum voltage output of the exploder with which they are used, (iii) there should be adequate protection against abrasion, and (iv) each cable should, as far as practicable, be one continuous length without any joints or otherwise uninsulated places.
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Unavoidable joints in a cable and necessary joints between a cable and a detonator lead or between detonator leads should be (i) securely made and properly insulated with suitable material provided for the purpose, or (ii) formed by the use of a jointing and insulation device specifically provided for this particular purpose.
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Each cable, when in use, should be hung in a safe position on suitable insulators.
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It is absolutely essential that any shotfiring cable which is found to be damaged or defective is replaced at once by a cable in good condition, and is not used again until it has been taken to the surface and properly repaired by an electrician.
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A weekly examination and test of each cable should be made on the surface by an electrician.
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| (c) |
Cable specification: A substantial twin-core cable might have advantages, e.g., in installation, maintenance and smaller risk of damage, over two single-core cables which must be used at present. On the other hand, disadvantage arises in regard to the coupling of the twin wire to the detonator leads. The possible merits of twincore cables with a suitable connection for attachment to the detonator leads should, I consider, be investigated.
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| (d) |
Exploders: Unfortunately, the production of sufficient energy to fire a large round of shots has proved so far to be beyond the capacity of an exploder which cannot produce an incendive spark in any conditions of use. As a result of my investigation of the explosion, I have felt compelled to make the preceding suggestions and recommendations with a view to reducing risk of ignition of firedamp in the future. Several of them would be less critical, however, if a completely safe exploder were available.
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In this country the exploder in general use for the firing of large rounds of shots consists essentially of a hand operated generator which charges a 6 microfarad 1500 v. D.C. capacitor, the discharge of which through the firing circuit fires the round of shots. Reliance has been placed on the ability of the capacitor to give an immediate and rapid output without the introduction of a mechanical device.
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However, there is also in use another type in which a spring driven sell excited compound wound generator supplies current at 500 v. to the firing circuit. This type can be supplied with a time limit switch which reduces to a very small fraction of a second the time during which current flows.
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It appears to me that there is every advantage in having a shotfiring circuit alive for the shortest possible time and at as low a voltage as possible, provided the risk of missfire is not thereby increased.
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Accordingly, I recommend that as matter of some urgency critical examination be made of both types in order to decide which is, or can be made, the safer.
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