A cartridge of explosive does nothing on its own. It needs a push, and that push comes from a small metal tube about the width of a pencil lead. In the mining trade it is called a topi or detonator, and nothing in a blast happens without it.
In this post, we go through what a detonator is, how plain and electric detonators differ, what the delay numbers mean, and why coal mines use a special "carrick" type. We also cover firing currents and the tests that approved detonators must pass, which are favourite topics in DGMS exams.
What Is a Detonator?
A detonator is a thin tube, about 6.5 mm across, filled with fast-acting explosive. When it fires, it creates a violent shock, and that shock is what sets off a high explosive cartridge. The detonator is pushed into one cartridge of the charge in the hole. That cartridge goes off first, and it then sets off the other cartridges around it.
The cartridge that holds the detonator is called the priming cartridge.
Detonators come in two basic kinds: the plain (ordinary) detonator, fired with a safety fuse, and the electric detonator, fired with an electric current from an exploder.
Methods of Firing High Explosives
| Method | Where it is used |
|---|---|
| (a) Plain detonator with safety fuse. The safety fuse is lit directly. | Small stone quarries only |
| (b) Electric detonator. Current is sent through its wires from an exploder. | Underground mines, and small holes in small opencast mines |
| (c) Detonating fuse. An electric detonator is fixed to it and fired by the exploder. The blast wave travels along the fuse into the explosive. | Most opencast mines |
| (d) Non-electric detonator. A shock wave is sent in through detonators and detonating fuse. | Opencast mines |
Methods (c) and (d) are covered in the opencast part of the book this post is based on. You can read more in our post on blasting accidents in opencast mines.
Plain Detonator
A plain detonator is a thin aluminium tube, about 37 mm long and 6.5 mm across, with one end open. It holds two kinds of explosive: the main charge (PETN) at the bottom, and a primer charge on top of it. A safety fuse is pushed in and the open end is crimped tight with a special tool.
At least 1.2 m of the safety fuse must stay outside the hole. When it is lit, the flame reaches the primer charge, the detonator fires, and this sets off the main charge. The safety fuse burns at about 1 metre per 100 seconds. If different holes need different delay times, the fuse length is simply made longer or shorter.
Instantaneous Electric Detonator
An instantaneous electric detonator is a tube of copper, copper-alloy-coated steel or aluminium, about 37 mm long and 6.5 mm across. Inside is the same pair of explosive charges: PETN at the bottom and a primer charge above it. Two steel wires with PVC insulation come out of the open end, held by a PVC plug that keeps out water and dirt.
Inside the tube, the lower ends of the two wires are joined by a very thin high-resistance wire, called the bridge wire. A quick-burning composition coats it. This coated wire is the fuse head. When the exploder sends current, the bridge wire heats up, the fuse head burns, and the detonator fires at once. The wires are usually 2, 3 or 4 m long.
Delay Detonators
A delay detonator is built like an instantaneous one, but it has a delay element between the fuse head and the primer charge. When current arrives, the fuse head burns and lights the delay element. The element burns slowly and passes the spark on. Only when it has burnt through does the detonator fire.
The amount of delay material is set according to the delay time needed. More delay means more material, which is why the tube gets longer as the delay number goes up. The tube of a No. 0 detonator is about 37 mm long, and that of No. 10 is about 77 mm.
Types of delay detonator
- Short delay or millisecond (ms) delay detonators: Made in numbers 0 to 10. A millisecond is one thousandth of a second. Orica detonators have a 25 ms gap between numbers. GOCL detonators have a 25 ms gap for numbers 1 to 6, 50 ms for numbers 7 and 8, and 75 ms for numbers 9 and 10. They are also known as sahasransh vilambit topi.
- Long delay detonators (LDD): Gap of 300 ms between numbers, in numbers 0 to 10. Used in drifting and shaft sinking.
- Coal delay or carrick delay detonators: A special type of 25 ms detonator for solid blasting in coal mines, in numbers 0 to 6.
How to recognise a delay detonator
The wire carries a tag printed with the delay number, and the number is also punched into the base of the tube. The wire colour is also different from that of the same company's instantaneous detonators.
Why use delay detonators?
- Saves explosive: The hole that fires first creates a free face for the next one.
- Better fragmentation: Broken rock from different delays collides, and there is more time for the material to be thrown.
- More safety: The whole face is blasted in one go, so the shotfirer does not have to keep returning to connect and check.
- Saves time: All holes are fired together in one sequence.
- Less ground vibration: The charge firing at any one instant is smaller.
Delay detonators are used in solid blasting, shaft sinking, drifts and opencast blasting. In an underground coal mine, permission from DGMS is needed to use them.
When different delay numbers are joined in series across the same face and the exploder is operated, the holes fire one after another because of the difference in delay times.
Carrick (Coal Delay) Detonators
Carrick or coal delay detonators are made specially for use in gassy coal mines. They have a copper tube (or copper-coated steel tube), and the delay material inside is surrounded by lead. The lead melts and soaks up the heat, so there is no risk of the detonator igniting methane. For this reason they are called non-incendive detonators.
They are used in solid blasting in coal and come in numbers 0 to 6, with a 25 ms gap between numbers. The No. 0 detonator has no delay element.
| Delay no. | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| Delay (ms) | 0 | 25 | 50 | 75 | 100 | 125 | 150 |
Underground mines do not normally use detonating fuse; it is used in opencast. The exception is the blasting gallery (BG) panel, where holes up to 15 m long blast the full thickness of a coal seam together. There, a special detonating fuse (power cord) is used along with the detonator. We will look at it separately in the BG panel topic.
No. 6 and No. 8 Strength, and Firing Current
Detonators differ in strength according to how much PETN they contain.
| Strength | PETN in the tube | Note |
|---|---|---|
| No. 6 | 0.30 g | Until some time ago, only No. 6 was used in India |
| No. 8 | 0.60 g | Now commonly used to fire emulsion explosive cartridges |
Current needed to fire
- Single firing: at least 0.5 ampere.
- Series firing: at least 1.2 ampere (for 4 milliseconds).
No-fire current
This is the current a detonator should safely withstand without firing. For example, a GOCL detonator is rated at 0.18 A for 180 ms, and an Orica detonator at 0.30 A for 50 ms.
Which is more dangerous: the cartridge or the detonator?
The detonator. It contains a sensitive explosive that can go off from even a slight knock. Handle and store detonators with great care, and always separately from explosive cartridges.
Tests for Approved Detonators
Before approval, detonators are tested on the following points:
- Safety in a gassy (methane) atmosphere
- Safety in a coal dust atmosphere
- Electrical resistance, which must not exceed 0.5 ohm
- No-fire current of at least 180 mA for 300 seconds
- Minimum series firing current of 1.2 A for 4 milliseconds
- Water resistance test
- Strength test
- Vibration test, drop test and snatch test
- For delay detonators: delay time, and overlapping must not be more than 5 percent
Quick Revision and FAQ
- The cartridge that holds the detonator is the priming cartridge.
- Plain detonators are banned underground. Safety fuse burns at 1 m per 100 s, and 1.2 m must stay outside the hole.
- No. 6 holds 0.30 g PETN; No. 8 holds 0.60 g.
- Single firing needs at least 0.5 A; series firing at least 1.2 A.
- Carrick delay: numbers 0 to 6, 25 ms gap, lead-surrounded delay element, non-incendive.
- Maximum span between first and last detonator in a solid blasting round: 150 ms.
- Underground coal mines: copper or copper-coated steel tubes only. Opencast: aluminium.
What is the difference between a plain and an electric detonator?
A plain detonator is fired by lighting a safety fuse. An electric detonator is fired by current from an exploder.
Why are delay detonators used?
They give better fragmentation, save explosive, reduce ground vibration and make blasting safer and quicker.
What makes carrick detonators safe for gassy coal mines?
Their delay material is surrounded by lead, which melts and absorbs heat, so they do not ignite methane.
Is permission needed to use delay detonators underground?
Yes. In underground coal mines, permission from DGMS is needed.
Conclusion
The detonator is a small part with a big job. Once you understand how a plain detonator, an electric detonator and a delay detonator differ, the rest of blasting practice, from firing circuits to delay sequences, starts to make sense. And the reason for the carrick detonator is a good example of how mine safety rules are built around one hazard: methane.
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. Product details such as delay intervals and firing currents vary by manufacturer. Always follow the manufacturer's instructions, the latest DGMS circulars, the Coal Mines Regulations and your mine's approved blasting rules.
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