Ask any underground coal miner what worries them most, and gas will come up quickly. Methane is always somewhere in the background, and every blast creates heat. Putting those two things together safely is the whole reason permitted explosives exist.
In this post, we go through the basics step by step: what explosives are, how low and high explosives differ, what a gassy mine is, and how the P-1, P-3 and P-5 classes are used in Indian coal mines. We also cover the tests and the DGMS approval process, which come up often in exams.
What Is an Explosive?
An explosive is a substance, or a combination of substances, that changes almost instantly into a very large volume of gas at high temperature and high pressure when it receives a shock or heat. This is called detonation. When the explosive is packed into a hole in a coal seam or rock, those gases push hard against the surrounding strata and break it apart.
Low Explosives vs High Explosives
Explosives are broadly split into two groups, and the difference is both chemical and practical.
Low explosives
A low explosive is a mixture of two things: a substance that burns and a substance that supplies oxygen. It is fired with a fuse. When it goes off, it throws out a long-lasting, hot flame along with sparks and a lot of smoke. The pressure of the gases is what breaks the strata, so a low explosive works only through gas energy. Gunpowder is the usual example.
High explosives
A high explosive is itself an explosive chemical compound, or contains one. Given a violent shock, it reacts at very high speed and produces high detonation pressure and a shock wave. The shock wave first cracks the strata, and the gases that follow push the broken material out. A high explosive therefore gives both shock energy and gas energy.
High explosives that fire with the shock of a detonator are called cap sensitive. Those that need another explosive to set them off are non-cap sensitive. All explosives used in underground mines are cap sensitive.
In practice, low explosives are used only in very small stone quarries. All underground mines and opencast mines use high explosives.
Gassy Mines and Their Degrees
Why explosives are a risk in coal mines
Every blast releases hot gases. Almost every underground coal mine has, or may have, inflammable gas (methane). Once methane is mixed with air in the explosive range, which is roughly 5 to 14 percent, the hot gases from a blast can set it off. In some conditions, coal dust can explode as well. This is the danger permitted explosives are designed to remove.
What is a gassy mine?
A gassy mine is an underground mine where inflammable gas is found, or is likely to be found. In India, every underground coal mine is treated as gassy for this purpose. They are divided into three degrees:
To understand how gas is detected in the field, see our related post on the flame safety lamp, and browse our full collection on mine gases.
What Is a Permitted Explosive?
A permitted explosive is a type of explosive that DGMS (Directorate General of Mines Safety) has allowed for use in gassy underground coal mines of degree I, II or III. Before this happens, samples supplied by the manufacturer are tested successfully at CIMFR in Dhanbad, and the name is then added to the list of permitted explosives in the DGMS gazette. The wrapper on the cartridge carries the class, either P-1, P-3 or P-5.
What makes it "permitted" is a cooling ingredient mixed into the explosive. When the shot fires, the flame is lower in temperature and shorter in duration, so a methane-air mixture or a coal dust-air mixture is unlikely to ignite. The fumes are also limited, specifically carbon monoxide and nitrous fumes. In underground coal mines, only permitted explosives can be used for blasting in coal.
P-1, P-3 and P-5: Which One Goes Where?
There are five classes of permitted explosive: P-1, P-2, P-3, P-4 and P-5. Each has set properties and must pass set tests. In India, only P-1, P-3 and P-5 are made. P-2 is not in use anywhere any more, and P-3 has taken its place. P-4 is used in some countries for roof ripping in very thin seams.
| Class | Also known as | Where it is used |
|---|---|---|
| P-1 | Unsheathed explosive, now P(o), Permitted Ordinary | Cutting faces in degree I gassy coal mines |
| P-3 | Equivalent sheathed explosive, now P(eqs), Permitted Equivalent Sheathed | Cutting faces in degree II and III gassy coal mines |
| P-5 | Ultra safe explosive, now P(s), Permitted Solid Blasting | Solid blasting in degree I, II and III mines |
Among the three, P-1 is the strongest and P-5 is the weakest. P-5 is roughly 25 percent weaker than P-3.
Why is P-5 called "ultra safe"?
All three classes are safe in the event of a blown-out shot or a cut-off, and the maximum charge per hole is limited to reduce the chance of a blow-out. P-5 goes a step further. It stays safe even in deflagration, where the explosive burns very rapidly instead of detonating.
This matters in solid blasting. The explosive in one hole can be desensitised by the pressure of the blast in the previous delay hole, and then burn instead of detonating. The inflammable gas released from the earlier hole could ignite as a result. Solid blasting also has a higher chance of blow-outs. For these reasons, P-5 is the class chosen for solid blasting.
Product names you may come across
Different companies sell permitted explosives under different brand names. A few examples are:
- P-1: Powergel P-101, Indocoal-1, Godyne, Solarcoal-1
- P-3: Powergel P-301, Indocoal-3, Permadyne, Solarcoal-3
- P-5: Powergel P-501, Indocoal-5, Pentadyne
Where can non-permitted explosives be used?
Non-permitted explosives can be used in a stone drift if there is no coal dust. They cannot be used in a cross-measure drift, meaning a stone drift driven from one coal seam to another, because a thin coal seam may be met on the way. In a sinking shaft, non-permitted explosives can also be used, but within 10 m of a coal seam only permitted explosives are allowed.
Special case: BG panel ring holes
In blasting gallery (BG) panels, a special explosive called "power ring" is used for ring hole blasting. It falls in the P-3 class.
NG-Based, Slurry and Emulsion Explosives
Modern explosives used in mines are high explosives, and they come in three broad families.
- NG-based explosives: The main explosive ingredient is nitro-glycerine.
- Slurry explosives: They contain no nitro-glycerine. They are made as a soft, thick slurry, with ammonium nitrate as the main ingredient along with a fuel, gum, water and a sensitiser. They are safer than NG-based explosives.
- Emulsion explosives: A more advanced development of slurry explosives, and the most efficient of the three. Fine particles of ammonium nitrate are mixed closely in an oil and water medium.
Since 2004, the Government of India has banned the manufacture, sale and possession of NG-based explosives, because they were being misused for terrorist activity. Today, underground mines use only slurry and emulsion explosives. Both permitted and non-permitted explosives are one of these two types.
In underground mines, explosives must be supplied in cartridge form only. Opencast mines also use pumpable bulk explosives, which do not come in cartridges. To see how blasting problems show up on the ground, read our post on the causes of blasting accidents in opencast mines.
Tests for Permitted Explosives
A permitted explosive must pass a series of tests before approval. The main one is the gallery test, where the explosive is fired in a test gallery filled with a methane-air mixture (methane at 9.0 ± 0.25 percent) or a coal dust-air mixture to see if it causes ignition. The table below follows the figures given in our source text.
| Gallery test condition | Class | Charge and shots | Max. ignitions allowed |
|---|---|---|---|
| Methane-air, direct initiation (with stemming) | P-1 | 795 g, 5 shots | Nil |
| P-3 | 1000 g, 5 shots | Nil | |
| P-5 | 1030 g, 5 shots | Nil | |
| Methane-air, inverse initiation (without stemming) | P-1 | 142 g, 26 shots | 13 |
| P-3 | 397 g, 26 shots | 13 | |
| P-5 | 565 g, 20 shots | Nil | |
| Coal dust-air, direct initiation | P-1 | 795 g, 5 shots | Nil |
| Coal dust-air, inverse initiation | P-3 | 565 g, 5 shots | Nil |
| P-5 | 565 g, 5 shots | Nil |
Besides the gallery test, other tests include:
- Cap sensitivity: The explosive must fire with a No. 6 strength detonator.
- Air gap sensitivity: The minimum required gap is 2 cm.
- Post-detonation fumes: Not more than 40 litres of CO and 10 litres of nitrous fumes per kg of explosive, or 50 litres of both combined.
- Velocity of detonation (VOD): Within ±400 m/s of the figure declared by the manufacturer.
- Deflagration test: Applicable only to P-5.
- Strength test and water proofness test.
- Density test: Within ±0.05 g/cc of the declared figure.
- Shelf life test: At least 6 months. The shelf life is marked on every cartridge.
Reference: DGMS Circular Approval 6/2014.
New names for P-1, P-3 and P-5
In 2014, the classes were given new names through DGMS Circular Approval 6/2014 dated 17.11.2014. P-1 became P(o), P-3 became P(eqs), and P-5 became P(s). You will see both naming styles in books and in exam questions.
How DGMS Approves an Explosive
- The manufacturer applies to DGMS along with a certificate issued by CIMFR.
- DGMS allows field trials in selected coal mines.
- At least 200 trial shots are fired jointly by the manufacturer and the colliery management. In some cases a DGMS officer is also present.
- P-1: Provisional approval is given for one year. If performance is satisfactory, regular approval follows.
- P-3 and P-5: Approval is given in stages, first for degree I, then for degree II, and finally for degree III.
Quick Revision and FAQ
- Low explosive gives gas energy only; high explosive gives shock and gas energy.
- Methane is explosive between about 5 and 14 percent in air.
- P-1 is the strongest, P-5 the weakest. India makes only P-1, P-3 and P-5.
- P-1 for degree I cutting faces, P-3 for degree II and III cutting faces, P-5 for solid blasting.
- Cap sensitive means it fires with a No. 6 strength detonator.
- NG-based explosives have been banned in India since 2004.
- New names (2014): P(o), P(eqs) and P(s).
Can permitted explosives be used in opencast mines?
They are meant for underground coal mines. Opencast mining has its own explosive types, including pumpable bulk explosives.
What does "cap sensitive" mean?
An explosive that can be fired by a No. 6 strength detonator is called cap sensitive.
Why is P-5 used for solid blasting?
Because it stays safe even if it deflagrates, and solid blasting has a higher chance of blow-outs.
Which mine decides the class of explosive I must use?
The gas degree of the mine and the type of blasting, whether cutting face or solid blasting.
Conclusion
Permitted explosives are one of the quiet safeguards of underground coal mining. The idea is simple: keep the flame cool and short, limit the fumes, and match the class of explosive to the gas degree of the mine and the kind of blasting. If you remember that logic, the details of P-1, P-3 and P-5 become much easier to recall in an exam and to apply on the job.
Found this useful? Share it with a friend preparing for DGMS exams, and leave your questions in the comments.
Disclaimer: This post is for educational purposes and is based on standard mining textbook content and DGMS circulars. Always follow the latest DGMS circulars, the Coal Mines Regulations and your mine's own approved rules when handling or using explosives.
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