Electric Initiation Systems

By:

Shitij Malhotra

10108EN009

Part IV

Mining Engineering

Indian Institute of Technology (Banaras Hindu University), Varanasi

What is an Explosive

An explosive is a substance which, when properly initiated, is very rapidly converted to gases at high temperature and pressure. This process is called detonation.

What are Initiation Explosives

Initiating explosives are designed to safely activate larger explosive charges at a controlled time and in a pre-determined sequence (‘delay blasting’).

Initiating explosives can be broadly classified into electric and non-electric types.

In electric systems, a device that can generate or store electrical energy transmits that energy to the initiating explosives via a circuit of insulated conductors.

Electric Detonator Description

The principle of operation of an electric detonator is that, when a high enough electric current is passed through the legwires, a bridge wire is heated in the fuse-head, which then deflagrates and initiates the delay element which in turn initiates the explosive in bottom of the detonator after a time determined by the length and content of the delay element.

There are in principal two types of detonator designs available;

Non Primary Explosive Detonator (NPED)

Lead azide based detonator.

The detonator shell, which is made of aluminium, contains a base charge of RDX in case of NPED detonators, and of PETN in case of the lead azide detonators. The total amount of explosives in the detonator is approx. 1 g. In the delay element the detonation is delayed pyrotechnically for a predetermined time after the firing impulse has reached the detonator.

Primer A primer is the explosive unit (cartridge, cast primer) which contains a detonator

(blasting cap, detonating cord).

Precautions to be taken with a primer

NEVER prepare more primers than immediately needed.

NEVER prepare primers in a magazine or near large quantities of explosive materials.

NEVER slit, drop, twist, or tamp a primer.

NEVER use a cast primer or booster if the hole for the detonator is too small.

NEVER enlarge a hole in a cast primer or booster to accept a detonator.

NEVER punch explosive material that is very hard or frozen.

NEVER force a detonator into explosive material.

Typ

es

of

deto

nato

rs o

n

basi

s of

dela

yInstantaneous – no delay element, thus the detonator

initiates directly after receiving the firing impulse

Millisecond delay (MS) – the delay time increases in increments of 25 ms between each period number

Half second delay (HS) – the delay time increases in increments of 500 ms between each period number

Delay Element

The delay times vary in steps between 25 millisecond and 5000 millisecond (0.025 – 5 seconds).

Enclosing the fuse-head is an electrostatic protective sleeve that decreases the risk of unintentional initiation due to static discharge.

Electric HS-detonators are designed for use in underground operations as longer delay times are needed in tunnel rounds to give enough time for the rock to break and be thrown from the tunnel face. HS detonators are prohibited for use in surface operations as the delay times are too long and can cause flyrock.

Electric MS-detonators are used in surface operations and mostly in smaller operations.

Safe and Successful Blasting

A firing pattern designed for the round to be blasted.

A blasting machine capable of firing the size of the round with the type of detonators being used.

Knowledge of electric hazards at the worksite and ensuring they are eliminated.

That each series in series/parallel blasts are of the same size. Careful connection of lead wires and scrupulous testing of the different parts of the round.

Types of Detonators

Electric detonators are classified in 4 classes depending on their electric properties.

European denomination Nordic Denomination Older Denomination

Class 1 Group 1 Type A/S/NT

Class 2 Group 1A Type U

Class 3 Group 2 Type VA

Class 4 Group 3 Type HU/XS

Mechanical Properties

Electrical Properties

Delay Time Selection

Accidental Fire HazardsThe various hazards are –

Lightning

A lightning strike close to a blasting circuit can initiate some of the detonators; a direct hit will cause the entire circuit to fire. If an electrical storm approaches a blast site, charging operations must stop and the site be evacuated.

Static electricity

The build-up of static charge on an object can be sufficient to initiate an electrical blasting circuit. The most likely cause is by blow-loading of ANFO in dry conditions. Charging equipment should be properly earthed and a semi-conductive charging hose should be used.

Stray currents

Stray currents from faulty electrical equipment can initiate electric detonators. The most serious hazard is faulty insulation of high voltage cables. All electrical equipment should be properly earthed and detonators leads and firing cables should be placed well clear of any power lines.

Electro-magnetic radiation

If a powerful radio frequency transmitter is close enough and the length and orientation of the lead blasting wires is correct, the radio waves may induce sufficient current in the blasting cables to initiate detonators. Mobile radio transmitters and telephones must therefore be kept away from electrically primed blast holes. Alternatively, signs should be posted instructing persons to switch off their communication devices when entering a blast site.

Shock Tubes The signal tube system consists of narrow plastic tubes coated on the inside

surface with a very thin layer of high explosive, typically HMX or PETN. One kilometer of tube will contain about a teaspoon of HMX.

Signal tube can be initiated by an electric detonator, detonator cord or mechanical shot shell starter device.

Advantages of Shock Tube

The main advantages of this initiation system are:

It is not susceptible to stray electric currents

Separate lengths of signal tube cannot initiate each other through direct contact, knots or other simple connections

The tube is robust, having a high tensile strength and abrasion resistance

It is very difficult to ‘kink’

The initiation is virtually non-violent compared to detonating cord and is hence much safer to use

Advantages of Electric Detonators

High initiation strength - 720 mg PETN or RDX base charge ensuring reliable initiation of all cap sensitive explosives.

Detonator shell design reduces the possibility of any mechanical damages as well as water hammer effect.

High level safety features due to an antistatic sleeve surrounding the fusehead with safeguarding spark gap.

Protected primary charge enclosed inside heavy duty steel delay element.

Superior fusehead design unmatched for reliability.

Multi-step quality control system tests components and finished products to ensure reliable performance.

High level accuracy and delay intervals permit greater flexibility in blast design to control vibration, frequencies and fragmentation, with no overlap.

Excellent water resistance is provided by triple crimping.

Reliability over wide temperature range.

High compatibility with all blasting machine types.

Marking by means of an identification tag fastened to the leg wire.

High quality packaging in a strict conformity with the International Agreement of Road, Train, Sea and Air Transport.

Excellent storage characteristics and easy handling.

Standard Sensitivity Groups

Standard Sensitivity

Groups

Group 1 Group 2 Group 3 Group 4

No fire Current 0.18A per 5 min 0.45 A per 5 min 1.2 A per 5 min 4 A per 5 min

No fire Impulse 0.8mJ per ohm 8 mJ per ohm 80 mJ per ohm 1.1 J per ohm

Fire Current >1 A per 4 ms >2.12 A per 4 ms >3.5 A > 30 A per 4 ms

Firing Impulse > 3 mJ per ohm > 18 mJ per ohm > 140 mJ per ohm > 3 mJ per ohm

Standard Wire Insulation Colour

White Yellow Pink Brown

Standard Delay Range

Delay Range No. of Delays Delay Interval

Insulation Colour

Shell Material

Instantaneous - 0 Red Aluminum

Instantaneous permissible

- 0 Red Copper

Millisecond 30 25 & 50 ms Yellow Aluminum

Millisecond permissible

16 30 ms Yellow Copper

80ms / 250 ms 30 80 & 50 ms Red Aluminum

Quartersecond 21 250 ms Green Aluminum

Half Second 12 500 ms Blue Aluminum

Different types of Electric Detonators

Mondial Defence No.8 Instant Electric Detonator

• 720 mg PETN base charge far exceeds the standard #8 strength detonator

• Provide precise control necessary to give sure fire initiation of all types of explosive, without need for additional boosterType Sensitive Instantaneous Electric

Detonator

Detonator Shell Material Aluminum

No. of delay intervals 1

Delay interval -

Primary charge Dextrinated Lead Azide

Secondary Charge 721 mg PETN

Insulation material PVC

Electric Super Long Period Delay Detonator

• High strength, long period delay electric detonator featuring 19 delay periods designed to provide precision and accuracy in all delay periods with no overlap.

• Specifically designed to provide the long delay times necessary for improved relief and fragmentation in underground mining (non-coal) and construction applications such as drift development, raise, shafts, slopes and tunnels.

Delay Period

Nominal Firing Time

Delay Period

Nominal Firing Time

Delay Period

Nominal Firing Time

1 25 7 1200 13 2900

2 200 8 1400 14 3300

3 400 9 1600 15 3800

4 600 10 1900 16 4400

5 800 11 2200 17 5100

6 1000 12 2500

Dyno Nobel Electric Detonator

• The detonator is of the NPED-type, (Non Primary Explosive Detonator), in which the primary explosive (lead azide) has been replaced with a secondary explosive. This gives increased safety in both manufacturing and handling.

• The electric detonator uses electrical energy to heat a bridge wire that ignites a burning mixture which in turns ignites a secondary explosive.

• For use in initiating blasts in underground and surface applications.

Class 1, Millisecond Electric Detonator

• The detonators are of NPED type, meaning that they do not contain primary explosives, making them considerably less sensitive to shocks and impacts than detonators containing primary explosives.

• The aluminium detonator shell contains a base charge of RDX/PETN.

• The total amount of explosives in the detonator is approx. 1 gram.

• In the delay element, the detonation is pyrotechnically delayed a predetermined time after the firing pulse is applied.

• A protective sheath around the fuse head reduces the risk of unintended ignition through electrostatic discharge.

Type Millisecond

Interval 1 – 20

Delay times 25 – 500 ms

Wire Length 4 to 25 m

Wire Colour Yellow and green

Wire Material Copper

Leg Wire Isolation Polyethylene

Water Resistance 3 bar at 7 days

References1. Electronic Initiation and Waveform Analysis Improve Off-Site Blasting Effects by

DynoNobel

2. Electronic Initiation System by DynoNobel

3. Electronic initiation sparks a surge in safety and efficiency at George Fisher Mine by DynoNobel

4. Electronic Initiation System by ‘Special Devices Inc.’

5. Evaluation of the Dyno-Nobel Hotshot Electronic Blast Initiation System- requirements for shunting and circuit Testing by Tom Lobb, Harry Verakis And Dean Nichols - United States Department Of Labor Mine Safety And Health Administration

6. Combining Paired Blastholes with Electronic Initiation Systems by DynoNobel

7. BlastPED Centralized Blast Initiation by Mine Site Technologies

8. Use of electronic initiation systems in mining industry by jože KortniK, Julijan Bratun

9. Electronic Initiation in Underground Development by D. Scott Scovira, NA Group Manager, Advanced Mining Solutions, Orica USA, Inc.

10. Unplanned initiation of an electric detonator by an electronic detonator firing unit by Department of Employment, Economic Development and Innovation, New Zealand

Thank You