MISFIRE DEFINITION
An explosive charge in a drill hole that has partly or completely failed to explode as planned. Causes include unskilled charging; defective explosive, detonator, or fuse; broken electric circuit or--most dangerous--cutting off of part or all of the charge through lateral rock movement as other holes in the vicinity are fired. Stringent safety precautions cover procedure in minimizing these risks and in dealing with known or suspected misfires. A smoldering fuse may delay explosion, causing a hangfire, so return to workings after a suspected failure is necessary. Another main cause of accident is drilling into or dangerously near a socket--an apparently empty drill hole.
Rules for misfires ( MMMR-2002)
A.When a misfire occurs in a mine, indicated by the counting of exploded shots in case of fuse blasting, or observed by the blaster from other indications in case of electrical or detonating cord blasting, no person shall enter the place of blasting until 30 minutes in case of fuse blasting and 5 minutes in case of electrical or detonating cord blasting after the firing of shots. The place of the misfire shall be marked by a red flag to indicate it as a danger zone.
B. In the event of a misfire, the following procedure shall be followed:
1.The tamping may be sludged out with compressed air or water under pressure and the hole may be reprimed and fired.
2. When the misfire is due to a faulty cable, or is due to a faulty connection in case of electrical firing, the shot may be fired again after the defect has been rectified.
3. Where the steps in Sections 1 and 2 have not been successful, another shot may be fired in a relieving hole where practicable. The relieving hole shall be drilled at least 30 cm away from and parallel to the misfire hole.
4.After the relieving hole has been fired, a careful search shall be made for cartridges and detonators amongst the material brought down by the shot.
5.Before leaving the mine, the blaster shall inform the foreman and the manager of the misfire that has occurred; when a misfired shot has not been relieved or re-blasted, he shall also inform the officer of the next shift before he leaves the mine.
6. The blaster shall record every incident of misfire in a bound-paged book kept for that purpose.
C. The manager shall ensure that utmost care is exercised when working is carried out in the mine in an area where the presence of a misfired hole has been reported and the hole has not been relieved.
HOW TO AVOID MISFIRE
To avoid misfire, the condition of the hole in check for water before loading must be done, proper selection of Explosive and avoid using deteriorated Explosive, careful loading and tamping during stemming to avoid detonating cord damage, proper priming to ensure good placing of the cap into the prime Explosive cartridge.
Defining misfire as
In the case of accidental detonation of Explosive
Precautions include:
1. Proper handling of Explosive during storage and transportation
2. Avoid using low current exploder
3. Stop all Blasting operation under heavy thunderstorms
How to handle misfire when it happens
1. Detect misfire from blast sound and fume observation
2. Give clearance time 15-30mins before visiting the blast area
3. Carryout post Blasting inspection after blasting
4. Demarcate or barricade blast zone with signs after observing misfire
5. Never try to pull out the detonation cord with force from the charge hole
6. Check if the Explosive charge and cord can still be detonated, if yes reblast the charges. If no redrill and recharge the formation with all eye Care. Reblast drill holes should be drill vertically and Abit far from the undetonated hole.
7. Remove any budged or undetonated Explosive and accessories from rock socket to avoid damage to loading , transporting or processing equipments.
Note all undetonated Explosive left over in misfire are unstable, they must be handle with Care.
Life and property must be secured.
Misfire can be classified into three base on cause and source.
Misfire resulting from
1. Manufacturer factor
2. Design factor
3. Human factor.
HOW TO DEAL WITH MISFIRE
1. identify type and position of misfire
2. Take remedial action with misfire
3. Recover Explosive material or refire.
STEPS TO DEAL WITH MISFIRE
1.REMOVING STEMMING AND REPRIMING: In doing this, "No use of force". For electric detonator, do not use high velocity air as it can generate static charge sufficient to initiate the Explosive primer.
For top priming do not forcefully remove Stemming with metal or iron bar. In case of decking, remove inert material from each deck gradually and continuous with care.
For bulk/slurry Explosive, wash off with pressurized water, but for cartridge Explosive, care should be use to avoid unwanted initiation.
Use large quantity of water to desensitize Explosive.
2.DRILLING AND FIRING RELIEVING HOLES
in case of stocked in Explosive charge which I unremovable, instead of using force and create static charge, drilling relieve hole at a define distance from the load Explosive to release the holes is done with precaution.
The relieving hole distance from the undetonated hole depend on the following factors:
a. Diameter of the undetonated hole
b. Inclination of the undetonated hole and that of the Drilling machine
c. Sensitivity of the Explosive in the undetonated hole
All relieving hole must be;
a. Parallel to the undetonated (misfire) hole
b. Be of thesame length with the undetonated hole.
c. Must be lightly charged.
3.DISCOVERY AND RETRIEVAL OF EXPLOSIVE
Most misfire leave Explosive in Muckpile, boulder socket and on bench wall.
Unretrieve Explosive can led to unexpected detonation during loading, haulage or crushing. Therefore, to avoid any unwanted initiation, retrieving all undetonated Explosive cartridge from muckpile help to avoid accident in both mine and milling factory.
Above all steps, post Blasting inspection and blast risk assessment should be done and any evidence of misfire must be handled by expert and competent person.
Misfire zone should be barricade and *All clear* signal should not be given until misfire location is clear.
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