A shotfirer is only as safe as the equipment in their hands. A worn stemming rod, a damaged cable or an exploder that has not been tested can turn a routine blast into a misfire or an accident. That is why the law and good practice are so specific about what a shotfirer carries and how each item is checked.
In this post, we go through the shotfirer's tools and why each one is needed, then look at shotfiring cable, exploders, the circuit tester, how circuit resistance is worked out, and how to test a cable for continuity and insulation. If you are preparing for a DGMS exam, this is a topic where the numbers matter.
What a Shotfirer Carries, and Why
| Item | What it is for |
|---|---|
| Scraper (khapchuri) | Cleaning the hole |
| Break detector (pointed rod) | Finding cracks or breaks in the hole. Must be of an approved design |
| Pricker | Making a place for the detonator in the priming cartridge |
| Stemming rod (tamping rod) | Checking depth of cut, checking depth and direction of the hole, pushing explosive into the hole, and tamping the clay stemming. Wholly wooden |
| Circuit tester | Testing the blasting circuit and cable. Approved type |
| Shotfiring cable | Connecting the circuit to the exploder from a safe place. Approved type, never shorter than 50 m |
| Exploder and key | Firing the electric detonators. Approved type |
| Safety lamp | Gas checking. In solid blasting, a methanometer as well |
| Cap lamp | Light for working |
| Whistle | Warning before blasting |
| Stop watch | Timing the waiting period after the blast |
Other things to have
- Chalk (khadiya): to mark the direction of holes on the roof and sides.
- Note book and pen or pencil: to keep a record of how many holes were blasted at each face and how many cartridges and detonators were used.
- Methanometer: for gas testing when solid blasting is to be done.
The Hand Tools in Detail
Stemming rod
A stemming rod is made entirely of wood. Its thickness is about the same as the cartridge, around 32 mm, since the holes are about 37 mm across. Its ends are flat. It is used to check the depth of the cut, to check the depth and direction of the hole, to push the explosive in, and to tamp the stemming material.
Why must the ends of a stemming rod be flat?
If the end is pointed, it can push into the priming cartridge, strike the detonator and set it off. A flat end removes that risk. Always check the rod before each shift, and have a pointed end cut straight.
Break detector and scraper
A break detector is of a type approved by DGMS. It is made so that a crack as narrow as about 3 mm (one thread, or 1/8 inch) in the hole can be felt. It is a brass rod, 12 mm thick and about 1.5 m long, with a small pointed triangular blade of copper or brass at one end, about 2 mm thick. The other end carries the scraper, a spoon-shaped brass blade, which is also called a khapchuri.
Pricker
A pricker is made of wood, or of a metal other than iron, such as copper or brass. It looks like a pointed pen and is used to make a place for the detonator in the priming cartridge. Iron is avoided because of the risk of sparks.
Shotfiring Cable
A shotfiring cable is a pair of insulated copper conductors. It connects the detonator circuit to the exploder so that firing can be done from a safe distance.
- Twin core cable: both conductors are laid together in one cable.
- Single core cable: the two conductors are separate, and two single core cables are used together.
- Multi-shot firing: only single core cables are used.
- Standard: approved cables are made to Indian Standard IS 5950 of 1984. The conductors are made of many fine wires, with a total copper cross-section of 1 sq. mm, and are insulated for at least 250 V.
- Supply: cables come in reels of 100, 150 or 300 m.
How long should the cable be?
Long enough for the shotfirer to take two right-angle turns away from the blasting face and fire from a safe place, but in any case never less than 50 m. The textbook gives 150 m as the figure for a stone drift and 200 m for an opencast mine.
Exploders
What does an exploder do?
An exploder sends a controlled electric current to the electric detonators to fire them. There are two types: the magneto exploder and the battery condenser (capacitor discharge) exploder.
Magneto exploder
Turning the key drives a small generator (a DC generator) that produces electricity. The two terminal nuts on the exploder are connected to the cable, and the current flows through the cable to the detonators. Examples include the Shaffer and the IDL 350.
Battery condenser or capacitor discharge exploder
Today, most exploders are of this type and use electronics. A battery inside supplies the power. When the key is turned, the electric condenser charges. A bulb (a neon lamp) lights up to show that it is charged. When the key is then turned back to its old position, the condenser releases its charge through the terminals into the cable and on to the detonators. The Rhino exploder is an example.
Two advantages of electronic exploders
- If the circuit resistance is higher than the exploder's capacity, the condenser does not charge, so no blast takes place. If the shotfirer connects more detonators than the exploder can handle, the chance of a misfire is reduced.
- Current goes into the blasting circuit only when the required current and voltage are available to fire the specified number of detonators.
Approved exploders
An exploder used for underground work must be of a type approved by DGMS and carry the DGMS approval mark. Exploders used elsewhere must be made to Indian Standard BIS 9836 of 1981.
By capacity, exploders are called single-shot or multi-shot. The number of shots an exploder can fire at once is marked on it. Never try to fire more holes than the number marked, or a misfire will result. In India, exploders are made in 3, 25, 30, 100 and 200 shot capacities.
Features of an approved exploder
- It is of a type approved by DGMS and bears the approval mark.
- It delivers very little electricity, and only for 4 milliseconds, so that there is no risk of igniting a methane-air mixture. After 4 ms, the current is cut off automatically.
- Right after firing, any leftover electricity is automatically discharged. If the two lead ends of the cable, thrown aside at the time of blasting, were to touch suddenly, leftover electricity could cause a spark and ignite inflammable gas.
Examples of approved models
| Feature | 25-shot exploder (Electrical Equipment Corporation, Gurgaon) | 30-shot electronic exploder PB 30 | Rhino 25-shot battery condenser exploder (Kolkata) |
|---|---|---|---|
| Capacity | 25 shots in series | 30 shots in series | 25 shots in series |
| Firing current | 1.5 A | 1.2 A | 1.5 A |
| Current duration | 4 ms (max.) | 4 ms (max.) | 4 ms (max.) |
| Acceptable resistance with cable | 150 ohm (max.) | 150 ohm (max.) | 150 ohm (max.) |
| Battery | 10.5 V cell or 9.6 V nickel-cadmium | 6 AA cells (total 9 V) | 6 V P-276 Eveready |
| Output voltage | 500 to 600 V DC | Not stated | 600 V |
| Special feature | A resistance sensor will not allow firing if circuit resistance is more than 150 ohm or there is no continuity |
How to check an exploder
- The key should turn easilily.
- The terminal nuts should be clean and capable of being tightened properly.
- There should be no visible defect.
Maintenance requirements
- If an exploder fails to fire the specified number of holes, or shows any other defect, stop using it. It may be used again only after repair and testing.
- Every three months, a competent person tests the exploder to confirm it is in safe working condition.
- When needed, it is sent for repair or overhaul to the manufacturer or an authorised agency. After repair, the agency issues a usability certificate with the exploder, stating that all safety features are in order, and giving (a) current duration, (b) output voltage and (c) residual current.
- When the exploder returns from repair, the colliery engineer must be satisfied on these points.
- Repairs, servicing and overhauls are recorded in a bound register with numbered pages, and each entry is signed.
The Circuit Tester
What is it, and why is it needed?
When more than one detonator is fired, they are connected in series. The regulations require the continuity and resistance of the circuit to be tested before blasting. This is done with a circuit tester. It contains a battery and a special circuit, with a dial and pointer on the front, and two terminal nuts to connect the shotfiring cable. It sends such a tiny current through the circuit that the detonators cannot fire. Besides the blasting circuit, it can also be used to test the cable.
Why is it safe?
Only an approved circuit tester may be used. It is intrinsically safe, with a current of at most 50 mA. A detonator's no-fire current is 180 mA (for 300 milliseconds), so there is a wide safety margin. Even so, the rule is that testing is done from the safe place where blasting will be done, after all persons have been withdrawn.
Current figures worth remembering
| Item | Value |
|---|---|
| Minimum current to fire a detonator in series firing | 1.2 A |
| Gyro Ohm meter (approved circuit tester) | Testing current 10 mA; can measure up to 300 ohm |
| GOCL Blastometer | Current less than 8 mA; can measure up to 100 ohm |
| Orica Digital Circuit Tester | Only 4 mA |
| Maximum current of an approved circuit tester | 50 mA |
| Detonator no-fire current (typical) | 180 mA for 300 ms |
| Orica detonator no-fire current | 300 mA for 50 ms |
Safety when testing the circuit
- Keep the circuit tester at the safe place where the exploder will be kept for blasting.
- Test only when everyone has moved to a safe place and sentries have been posted on all approaches to the face.
If the tester gives so little current, why clear everyone first?
Because a faulty tester, or any other cause, could let more current flow, and then a blast could take place. The rule exists for the day something goes wrong.
How does the tester show continuity?
Connect the shotfiring cable, joined to the series circuit of detonators, to the circuit tester. If the needle turns and shows some electrical resistance, continuity is good. If the circuit is broken somewhere, the needle moves right out of the range and shows infinity.
Resistance of the Firing Circuit
The resistance of the shotfiring circuit must not be more than the exploder's capacity. Otherwise, the blast will not happen, or there will be a misfire.
The real resistance depends on the number of detonators and the length of the cable. Detonators from different companies have a resistance of about 3 to 5 ohm, depending also on whether the wires are 2 m, 3 m or 4 m long. For IDL detonators, the fuse head resistance is 1.6 to 2.4 ohm, and the lead wire adds about 0.7 ohm per metre as given in the textbook. A hundred metres of cable has a resistance of about 3 ohm.
In an underground mine, detonators are connected in series, so:
Illustrative example: 20 detonators of 4 ohm each, with 100 m of cable (about 3 ohm), give 20 × 4 + 3 = 83 ohm. That is within the 150 ohm limit quoted for the exploders above. The numbers are only for practice. For your mine, ask the Assistant Manager or Manager for the figures for the detonators and cable in use, and learn how the calculation is done.
Checking Equipment Before Going into the Mine
| Item | What to check |
|---|---|
| Cap lamp | Switch on and off. Light should be good and focus correct |
| Oil safety lamp | The flame forms properly and can be turned small and large. The lamp is clean and not leaking oil. Blow above and below the glass: if the flame does not flicker, the packing ring is good |
| Stemming rod | Ends not pointed. If pointed, get them cut straight |
| Exploder | Nuts clean and not jammed. Turn the key 3 to 4 times to see if it turns easily |
| Shotfiring cable | Right length, no broken wires, joints wrapped with insulating tape. Test with the circuit tester |
Testing a Shotfiring Cable with the Circuit Tester
The method is the same whether the cable is twin core or a pair of single cores. Two tests are carried out: a continuity test and an insulation test.
1. Continuity test
Join the ends of the two conductors together at one end of the cable, and connect the other two ends to the circuit tester. Press the switch. The tester shows the cable's resistance. If continuity is not good, which means there is a break somewhere, the needle goes right out of the range and shows infinity.
2. Insulation test
Disconnect (leave open) the ends of the two conductors at one end, and connect the other two ends to the circuit tester. Press the switch. If the needle stays at infinity, the insulation is good. If there is a short circuit somewhere in the cable, the needle shows some resistance, not infinity.
Quick Revision and FAQ
- Stemming rod: wooden, flat ends. A pointed end could pierce the priming cartridge.
- Pricker: wood or non-iron metal (copper or brass).
- Shotfiring cable: at least 50 m. Approved to IS 5950 (1984), 1 sq. mm copper, insulated for at least 250 V. Multi-shot firing uses single core cables.
- Two types of exploder: magneto and capacitor discharge. Approved exploders give current for only 4 ms and discharge residual electricity.
- Exploder capacity is marked on it. Do not exceed it. Competent person tests it every three months.
- Circuit tester: approved, intrinsically safe, 50 mA maximum. Detonator no-fire current is 180 mA for 300 ms. Series firing needs at least 1.2 A.
- Test the circuit only from a safe place, after withdrawing everyone and posting sentries.
- Total resistance = one detonator × number of detonators + cable resistance.
- Continuity test: reading expected. Insulation test: infinity expected.
Why does an exploder have a capacity marked on it?
Because it can only supply enough current for a certain number of detonators in series. Asking it to fire more may result in a misfire.
Why are exploders tested every three months?
To make sure they are still safe and working properly. A competent person does the test, and repairs are recorded in a bound register.
What is the difference between continuity and insulation tests?
The continuity test confirms that the path is unbroken. The insulation test confirms that the two conductors are not touching each other or leaking.
Why should the cable never be shorter than 50 m?
So that the shotfirer can fire from a safe distance. The cable must be long enough to turn two right-angle corners away from the face.
Conclusion
A shotfirer's kit is simple, but every item has a reason behind it. Flat stemming rods protect the detonator, approved exploders limit the current to a few milliseconds, and the circuit tester lets you check the circuit without risking a blast. If you understand why each rule exists, the numbers become easy to remember, and good habits become routine.
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 models, currents, resistances and distances vary by manufacturer and by the rules in force. Always follow the manufacturer's instructions, the Coal Mines Regulations, the latest DGMS circulars and your mine's approved blasting procedures.
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