Records and plans show that four schemes were considered for 28's district and it is by no means clear why the scheme adopted came to be chosen, but the plan dated October, 1956, adopted for working and ventilating 28's district was signed by the —

This was the first armoured conveyor on a prop-free front face installed in this mine and the problems associated with advancing the conveyor by snaking were relatively new. The changes in planning, and particularly the change to full caving of the waste, accentuated to a large extent the roof difficulties experienced in the loader gate roadhead.

Falls of roof and the cavity at the loader gate roadhead. The statutory reports show that soon after 28's face started, roof trouble began on the loader gate in the vicinity of the roadhead. This is not surprising because the 12 feet by 10 feet steel arches supporting this road were subjected to severe pressure and it was necessary to remove the left side pack and the left half of each steel arch to allow the face conveyor and its driving unit to be advanced after each cut. As the left half of each steel arch was removed, a piece of straight girder of the same size was coupled up to extend over the place originally occupied by the left side pack.

There is no doubt that this process of removing and rebuilding the pack and resetting arches in this road as the face advanced had an important bearing on the damage done to this road and to the surrounding strata. As early as the night shift of 30th-31st January, 1957, the deputy reported slight falls in the loader gate and again on the following night his report refers to broken roof at the loader gate. On Sunday, 10th February, the statutory report recorded a fall of 50 tubs of dirt in the main gate and the following night's report indicated that it had been cleared up.

It was about this time that the intermediate packs were stopped and full caving started, after consultation between the Group Manager and the Acting Manager. This was a very bad move having regard to the possibility of the caving extending over the left side pack of the loader gate and thus on to the gate itself. It would have been better not to start caving until the coal face had advanced beyond the original development heading which had become the loader gate. It made what was already a difficult operation at the site of the face conveyor even more difficult and dangerous as regards the risk of accidents from falls of roof in this area.

Apparently the adverse effects of this action were not sufficiently considered by the officials, even though falls became more frequent and larger and a big cavity was being left over the newly erected arches on this road.

It was not really necessary, however, to cave the waste to the pack, only about 9 feet wide, on the left of the loader gate and risk the caving extending over this pack to the loader gate itself. An additional temporary road could have been ripped and packed to the left of the loader gate and this would have provided its own packing material and also protected the loader gate against excessive damage, which was almost inevitable when the caving was taken up to the narrow pack on the left of the loader gate. Alternatively, the pack on the left side of the loader gate could and should have been widened to about 20 feet to keep the caved waste far enough from the gate.

Statutory reports by the night shift deputy on 11th-12th February again refer to a fall of about 50 tubs of dirt at the loader gate having been cleared to allow the face conveyor to run. The afternoon shift deputy reported on 12th February that 15 tubs of dirt had fallen at the loader gate end. The night shift deputy on 19th-20th February referred to broken roof at the loader gate end. A cavity 36 feet long, 18 feet wide and about 12 feet high, had resulted from these falls and it extended to the coal face.

On 21st February, between 6 and 7 a.m., another fall of roof occurred and some of it fell on to the face conveyor motor and drive head. Men were withdrawn from the face and some of them put on to clear a passage through the debris. An electrician says that he examined the electrical gear to see if any damage had been caused by the fall. He was satisfied that no damage had been caused at that time, which was about 8 a.m. The conveyor was restarted and the men went back on to the face.

After this the acting under-manager spent some time at this roadhead and was seen shining his light into the cavity, and small stones kept falling. He was near this roadhead again when, at about 11.30 a.m., a stone, 3 feet by 3 feet by 1½ feet, fell from the same cavity and struck the cable gland on the conveyor motor. There was a flash as an electrical short circuit occurred owing to the smashing of the cast steel gland and the tearing away of the conductors. The automatic overload and leakage protection devices operated and cut off the current but not before firedamp, which had apparently been brought down from the cavity by the falling stone, had been ignited. The flames of burning firedamp spread back along the intake gate and also along the coal face and burned 25 persons, of whom 22 were at or near the coal face and one, the conveyor drive attendant, at the roadhead.

It has been established without any doubt that a large roof cavity developed in 28's intake loader gate as the longwall face advanced. Not only did it extend to a considerable height but, due to the weak, crumbling left side, it became necessary to insert 5-feet extension pieces at the crown of the arches, in order to make contact with the left side. Thus, the roadway became 17 feet wide by 9 feet high. The roof of the cavity above the steel arches was supported by two chocks only, each about 2 feet square, and these were built on top of wood lagging over arch extension pieces. The first of these chocks was set during the week prior to the explosion and was about 9 feet high over the arches. The second chock was built on the Tuesday night shift, 19th/20th February, 1957, and was said to be 6 to 7 feet high over the arches. Reliance was placed on these chocks and on boarding or lagging the arches all round to arrest any falls which might occur. The first chock was seen in its original position after the explosion, but the second chock was reeled out by the fall which occurred between 6 and 7 a.m. on 21st February before the explosion happened and it was not replaced. Plan No. 3 shows the position of the chocks; the position of the second chock has been located on the plan in accordance with the evidence of the men who built it. It would seem that little attention had been given to the cavity, to pack it, ventilate it, or to secure the top and sides after the fall occurred four to five hours before the explosion.

The evidence indicates failure to carry out the important requirements of Regulation 12 of the Coal and Other Mines (Support) Regulations, 1956, following the fall of roof and displacement of supports at 28's loader gate roadhead between 6 and 7 a.m. on 21st February.

Progressive enlargement of the cavity and the frequent falls which occurred should have caused steps to be taken by way of controlling movement of the strata and properly supporting the roof and sides as were necessary for keeping the working place secure as required by Section 48 of the Mines and Quarries Act, 1954.

Ventilation. The new Davidson double inlet Diesel driven fan was due to start up at the end of 1956, but as difficulties were experienced by the manufacturers during its final testing, the old fan was maintained in use longer than had been anticipated. The manager intimated that the new fan would be put into operation during the week-end, 19th/20th January, 1957. However, mechanical difficulties appear to have again arisen and made a further delay inevitable. Thus it was not possible to start up the new fan until the week-end following the explosion and this was then successfully achieved.

The quantity of air circulating 28's face according to the only record- dated 20th February-was 3,200 cubic feet per minute at about 10 yards from the face on 28's intake gate. The corresponding record for the return gate air measurement was 3,024 cubic feet per minute. There must have been difficulty in making these air velocity measurements because they would be of the order of 25 feet per minute and the anemometer used would scarcely turn at this velocity. Near the roof there would be scarcely any flow of air*. Mr. J. King, who recorded these air measurements, was questioned about them. He said that to get the anemometer to show a reading he had to erect a brattice sheet to reduce the cross-section of the road. He was unable to produce any written record of the actual measurements.

There were many places on the airways and travelling roads, not used for transport, in which the cross sections were too small and undoubtedly they restricted the flow of air. These should have been remedied before they had become so bad as to restrict ventilation and travelling by persons.

Little regard appears to have been paid to the probable adverse consequences. of the restriction to the ventilation arising from falls at the loader gate roadhead, although earlier in the shift it had been deemed necessary to withdraw all the men off the face due to this cause. To make matters worse, just before the ignition occurred, ripping shots were fired in the return gate roadhead and the debris from these shots further restricted the ventilation of 28's face.

There was considerable leakage taking place at 30's overcast. In the main intake trunk roadway in the one in six drift there was, according to the air measurements made after the event on 21st February, 13,160 cubic feet per minute passing, and this quantity had fallen to 7,590 cubic feet per minute at a point outbye 30's loader gate junction. It will be seen from the plan of the mine (No. 1) that the only possible source of leakage was at 30's overcast, and this represented some 5,570 cubic feet per minute or 42 per cent, of the available quantity. The management must have been fully aware of this high leakage taking place at 30's overcast, but no definite action was taken until after the explosion.

As already indicated, 28's and 30's faces were ventilated in series but the reliability of the ventilation of 28's sheet near the outbye end of 30's right sidegate, because air passing through this part of 30's right side gate would miss 28's face. Perhaps of greater importance was the dependence on two sheets of brattice near the outbye end of 30's middle gate, which was also the conveyor gate and therefore the conveyor and coal had to pass through the sheets. If the sheets became torn or displaced more air would pass into this gate and rob the ventilation of both 28's face and the right half of 30's face. Sheets are not dependable for such a purpose and there should have been at least two stoppings with hinged doors for the conveyor to pass through. The air measurements recorded, including those for 20th February show that a disproportionate amount of the ventilation was passing into 30's middle gate. It was more than twice the amount going into 28's face to ventilate also the right half of 30's face.

In recent years more long faces are being ventilated by two intake gates, one of them at one end of the face and the other near the middle, which is also. the loader gate. This arrangement is unsatisfactory and unreliable because there may be only a small, and fluctuating, ventilating pressure across that part of the face between the intakes. The fluctuating pressure may be responsible for movement to and fro of firedamp in wastes between the gates, and fluctuations in the quantity of air flowing may allow accumulations of firedamp to occur unexpectedly at the roadhead rippings and even along the roof of the roads themselves if the air velocity in them is small.

The restrictions, leakages and unreliable features about the ventilation were sufficient to account for the ventilation in 28's district being insufficient to prevent firedamp, which accumulated in the unventilated cavity above the arches at the loader gate roadhead, from spreading in a layer along the roof both along the coal face and also back against the air current along the loader gate.

Firedamp reports. At no time is there any reference in the statutory reports of the deputies to firedamp having been found in 28's district.

Falls of roof on the loader gate had left a large cavity above the steel arches supporting the road and it was necessary to question the deputies as to examinations they made for firedamp accumulations in this cavity, because any issue of firedamp in the vicinity of this roadhead was likely to accumulate in the cavity.

T. Buckley, day shift overman, said that he visited the loader gate on 20th February-the day before the explosion-but he made no tests for firedamp in the cavity because Mr. Bacon, under-manager, and Mr. Kirk, deputy, were in the district and he wanted to get away to 30's district.

W. Warren, afternoon shift overman, said that on the afternoon of 20th February he tested for firedamp in the cavity up to the roof using his safety lamp. He was not sure whether he used the top feed on his safety lamp, but he found no firedamp. W. Adams, contractor ripper, confirmed this.

J. Aldred, afternoon shift deputy, said that he tested for firedamp in the cavity, but he did not use the top feed on his safety lamp. He found no gas in the cavity. He said that two rippers, H. Camm and H. Coleman, saw him make the test.

A. Walker, night shift overman, said that he spent most of the shift preceding that on which the explosion occurred supervising the work in progress on 28's loader gate. A shot was fired in the floor dint on 28's loader gate to enable the face conveyor engine to be advanced on the next shift. He saw the t prepared and saw a test made for firedamp in the cavity. M. Cox night shift deputy, and Walker said they made tests for gas together up to the roof of the cavity, by standing on the covering boards on top of the arches. No firedamp was found. According to Cox the arches over the forward end of the cavity, above the face conveyor engine, had been covered with split bars and covering boards.

The next occasion on which an examination for firedamp should have been made in this cavity was before the charging and firing of four shots in the coal at the loader gate stable hole at about 6 a.m. on 21st February. S. Derbyshire, shot firer, who fired these shots said that he tested for firedamp before charging. He said that his tests extended from 10 to 15 yards along the coal face, at the waste edge, in the advance heading and back along the loader gate for 15 yards. He tested to the height of the horsehead girders in the loader gate. He could not get his lamp higher at that time because of covering timber, consisting of round props and splits set skin to skin over the arches. He did not test in the cavity. It was a high cavity and he considered it to be dangerous to enter the cavity as small pieces of dirt were falling from the sides. Shortly after these shots were fired in the stable hole another fall occurred and dislodged a chock, cover boards and girders, leaving the top and sides of the cavity exposed above the conveyor driving motor.

Here then, we have at the loader gate roadhead a large roof cavity which the shot firer considered too dangerous to enter, but nothing was done to make it safe and shot firing and normal coal production were continued

There is no doubt that the cavity contained firedamp at the time of the explosion and, indeed, the overflow may have tailed off as a thin layer near the roof along, at least, part of the face, as well as migrating against the ventilation back along the roof of the intake roadway, which was rising outbye at 1 in 8, to another cavity at the junction of 28's loader gate and 30's right hand gate where an overcast was being prepared. This might well be expected in view of the extremely low air velocity prevailing in this road at the time, the lightness or buoyancy of the firedamp and the inclination of the road.

The probable source of firedamp in the intake was the coal on the right side of the intake gate. It is likely that the increased abutment pressure on this coal was liberating firedamp, which would also pass along the roof bed partings into the unventilated cavity where it would accumulate until it spilled over to trail along the roof at the face and also back against the ventilation up the intake gate as it must have been doing at the time of the explosion.

This emphasises once again the importance of providing a brisk velocity in large roadways in order to prevent firedamp accumulating or layering near the roof or in cavities above the supports.

A significant statement indicating the presence of firedamp in 28's return gate on the morning of this event was made by G. I. Deakin, a 16-year old supplies hand engaged in carrying supplies to 28's return gate. He said that about 7.30 a.m. on 21st February he saw a lot of men in the return gate and later he saw that they had gone back to the face. Some time later, he cannot remember the time, two other men, Betts and Bailey, were carrying and dragging supplies down the return gate to the face, and Deakin went as far as the shot firing curtain about 20 yards from the face. There was a flame safety lamp hanging on the shot firing curtain on the left side looking inbye. The flame of the lamp was jumping past the top of the glass and there was a blue haze on it. J. W. Betts showed him the lamp and said "just look at that oil lamp". He did not know whose lamp it was but he remembered Betts saying to S. Derbyshire, the shot firer, "look at that lamp" and Derbyshire replied but he could not remember the exact words. Derbyshire had a lamp of his own which he was trying to relight.

W. Bailey, who worked with Betts, confirmed that between 10 and 10.30 a.m. on 21st February, Betts, who died as a result of the accident, drew Derbyshire's attention to the flame safety lamp which was hanging on the shot firing curtain in 28's return gate. Derbyshire said he would light his own lamp and see what was there, but he could not light his lamp. J. W. Kirk, deputy, came down the gate at that time. Bailey turned down the flame of the lamp, but it continued to rise up and down. A blue flame was rising into the gauze. Both Derbyshire and Kirk, when questioned about this incident, said they could not remember it.

Bailey, Betts and Deakin then went on with their work back to 30's right hand gate and they did not know what action was taken about the flame safety lamp. They saw J. W. K irk again about 11.20 a.m. going towards the loader gate but he did not mention the lamp.

If the flame of the safety lamp was behaving in that way, it indicated that there was at that time firedamp in dangerous quantity in the air current in 28's return gate and that it required immediate action by the deputy and the shot firer to ensure the safety of the men in the district. The Anderton shearer was taking its second cut at that time and this was likely to increase the rate of issue of firedamp.

Statutory air measurements and samples. The management failed to have air measurements and air samples taken as required by the Coal and Other Mines (Ventilation) Regulations, 1956. The only records of air measurements and samples for firedamp content in the general body of the air current in 28's district were for the day before the explosion, despite the fact that the development of this face had been completed by the week ended 29th December, 1956. These samples indicated a firedamp content of 0.22 per cent., making about 7 cubic feet per minute, in the general body of the air in the intake and 0.57 per cent., making about 17 cubic feet per minute, in the general body of the air in the return gate, at statutory points 10 yards outbye the face. Because statutory samples and air measurements had not been made in 28's district until the day before the event, and the results of those were not available to the management until after the event, essential information about the quantity of air flowing and the firedamp discharging in the district was not available as it should have been.

General conclusions on ventilation. The ventilation along 28's intake loader gate failed to dilute and render harmless the inflammable gas which eventually filled the cavity. This appears to have been a breach of Section 55 (1) (a) of Mines and Quarries Act, 1954. To what extent the falls across the entrance to the face at 28's intake loader gate roadhead contributed to the accumulation of gas in the cavity is not known. The deputies and other officials state that they made regular examinations for gas in this cavity and that they failed to detect any. If this is true, then the cavity must have filled up with gas during the day shift of the day on which the explosion occurred.

Regulation 22 (2) of the Coal and Other Mines (Ventilation) Regulations, 1956, is also relevant in that only two sheets were used to restrict the flow of air on 30's loader gate, although this Regulation requires that there shall be provided and properly maintained at least two doors, or if that is impracticable at least one door and one sheet or two sheets. It was quite practicable to have two doors on that road.

Ignition. Contraband was not found in the district and smoking or the illegal use of naked lights can be ruled out.

All the safety lamps in use in the district were impounded and later examined by H.M. Inspectors. All were found in good order with the exception of an electric cap lamp (Oldham G.W. No. 324) used by J. W. Kirk, deputy, which had a severed cable. It has been established that this cable was cut at Mansfield General Hospital.

Careful consideration has been given to other possible sources of ignition, including the firing of two shots in the left return gate ripping lip just before the accident. The face of this ripping lip was behind the waste edge line, and investigations by H.M. Inspectors some four hours after the explosion did not reveal any sockets, excessive bed separation or induced breaks. It would appear, therefore, that shot firing as a source of ignition can also be ruled out.

The igniting medium was undoubtedly an electrical flash and this was agreed upon by all concerned in the investigation. The armoured conveyor was running at the time and the cable terminal box of the driving motor was struck by a large stone falling from the cavity. The terminal box was smashed and one lead of the cable was pulled adrift. Part of the smashed terminal box was forced into contact with another part of the earthed casing and the live conductor, causing a flash which ignited the firedamp.

Electrical installation and apparatus involved. The electricity supply for the mine, obtained from the East Midlands Electricity Board, was alternating current at 11,000 volts, 3-phase, 50 cycles. This supply was transformed on the surface down to 3,300 volts for general distribution on the surface and underground. An electricity supply at this voltage was taken inbye to No. 28's substation some 350 yards from No. 28's coal face where the accident occurred.

There was a 250 KVA 3-phase transformer in this substation where the voltage was transformed down to 550 volts, 3-phase, for supplying the electrical apparatus on No. 28's face. The neutral point of the 550 volts supply was connected with earth.

Adjacent to this transformer was the section switch for controlling the 320 yards long 0.1 square inch P.I.L.C.D.W.A. feeder cable to No. 28's intake gate-end boxes.

This section switch, of flameproof design and covered by a Ministry of Power Flameproof Certificate, was of the airbreak type with integral and interlocked isolator, fitted with three over-current trips, set at 200 amperes, with time lags and also with core balance earth leakage protection device designed to trip when a fault current from phase to earth exceeded 5 amperes.

The feeder cable from the section switch in the intake gate road supplied a bank of four flameproof airbreak gate-end boxes and a drill transformer unit. Of the four gate-end boxes, each covered by a Ministry of Power Flameproof Certificate, one was a spare and the others supplied respectively (i) the face conveyor motor, (ii) the stage loader and (iii) the stable hole cutter or alternatively the Anderton shearer coal cutter. The four gate-end boxes, all with interlocked isolators, were fitted with over-current protection and core balance earth leakage protective device.

Between the gate-end box and the face conveyor motor there was a length of Type 22 pliable wire armoured 4-core flexible cable, the cable containing conductors for a pilot remote control circuit, which with a remote "START- STOP" push button unit situated near the stage loader motor enabled the operator to start and stop the face conveyor.

The face conveyor was driven by a 50-h.p. squirrel-cage motor of flameproof design covered by a Ministry of Power Flameproof Certificate. Attached to the terminal box already referred to was a 100-ampere restrained type plug and socket coupler and an interrupted pilot circuit gland to which was connected the 2-core Type 60 remote control cable, at the other end of which was connected the remote "START-STOP" push button unit. Mr. Burkie, H.M. Electrical Inspector, found this unit, which was covered by a Ministry of Power Flameproof Certificate, in the "RUN" position. There appeared to be no defect in the flameproof enclosure of this apparatus.

In addition to the face conveyor motor there was an Anderton shearer loader mounted on the face conveyor and situated adjacent to the conveyor motor. The flexible cable and plug attached to the Anderton shearer loader was supplied from a gate-end box some 150 yards away in the tail gate road. The controller of the machine was in the "OFF" position when found after the explosion. The electrical part of the machine was covered by a Ministry of Power Flameproof Certificate and no defect of the flameproof enclosure was seen during examination.

The motor of the stage loader some 30 yards outbye from the face conveyor was of a flameproof squirrel-cage type covered by a Ministry of Power Flameproof Certificate. The "START-STOP" push button unit controlling this motor was in the" RUN" position. This unit, which was of flameproof design, was also covered by a Ministry of Power Flameproof Certificate. An examination of the flameproof enclosure of the motor and the "START-STOP" unit revealed no defect of flameproof enclosure.

Two intrinsically safe bells with associated Approved Type batteries were impounded and tested at the Safety in Mines Research Establishment. The result of the tests indicated that they were generally in good condition and that the standard of intrinsic safety had not been impaired in any way.

The evidence of electrical fusion on one of the conductors in the terminal box where the ignition started and on part of the terminal box casing indicate that the motor was running at the time. This fusion was no doubt caused when a broken part of the smashed terminal box casing was forced into contact with another part of the earthed casing and the live conductor.

Plates I, II and III give some indication of the damage sustained by the terminal box. Three of the seven S.W.G. 17 copper wires of the conductor in the terminal box were fused asunder and another wire was on the point of being severed by fusion (see Plate IV). The damaged conductor had also been pulled away from the shell clamp washers on the middle terminal of the three motor terminals to which it had been attached. There was evidence of electrical fusion at two places on the broken part of the terminal box casing, indicating electrical contact simultaneously with the remaining earthed metal casing of the terminal box and the damaged live conductor.

As would be expected under the circumstances, the section switch previously referred to tripped out on earth leakage protection at the time and, as the result of the electrical phase to earth fault, the resetting device was automatically mechanically locked out to prevent unauthorised resetting. It is probable that the gate-end box controlling the flexible cable to the face conveyor also tripped out on earth leakage protection at the time, but there was no evidence that this device had mechanically locked out.

The section switch and the gate-end box concerned were removed to the surface for the purpose of testing the operation of the leakage trips. Power was applied to each switch in turn and an electrical phase to earth fault deliberately created. The tests, using an oscilloscope and ammeter, showed that the tripping current of 5 amperes and time of operation of one-fifth of a second of each switch was generally in line with that prescribed by the manufacturers and the condition of the switchgear was satisfactory.

The time of action of automatic devices at present available for safeguarding electrical circuits, whether operating on overload or earth leakage, is such that incendive sparking can occur. When, as in this case, certified flameproof gear is so damaged as to destroy the flameproof nature of the gear, the incendive sparking is quite capable of igniting firedamp external to the gear.

On the important matter of protection of electrical gear against damage by falls of ground, the requirements of Regulations 10 and 11(1) of the Coal and Other Mines (Electricity) Regulations, 1956, are as follows:—-

Precautions against the explosion risk from coal dust. Both stone dusting of the roads and stone dust barriers were used for this purpose. The quantity of stone dust sent into the mine was 26 tons per calendar month, which represents approximately 2¾ lb. per ton of coal raised, a figure which is somewhat lower than the average for collieries in the area. The standard of cleaning up and stone dusting appeared to be reasonable having regard to the conditions. Estimation of incombustible matter in the dust samples is by colour test and the statutory records show that all the samples taken in this district of the mine contained more than 75 per cent. of incombustible matter.

After the explosion, dust samples were taken by one of H.M. Inspectors in 30's district with the following results:—

A stone dust barrier, consisting of several shelves loaded with stone dust, was fixed on 30's loader gate about 100 yards outbye of 28's loader gate transfer point.