Explosives TodaySeries 4 No 3

Safe use of detonating cord in Surface Mining applicationsKen Meiring, Senior Explosives Engineer

Detonating cord comprises of a continuous core of high explosive water resistant pentaerythritol tetranitrate (PETN), held in a plastic sheath and wrapped in yarns with a waterproof overcoat. It detonates at a very high velocity (>6000m/s) and will initiate any sufficiently sensitive explosives charge with which it is in contact.

The AEL detonating cord range is supplied under the trade name of Powercord®, with three standard products, namely Powercord 6, Powercord 8 and Powercord 10 and one premium product, Powercord 8 Premium. These cords are distinguished from each other by their colour and the mass of their core load, as follows:

Product Colour Core load Benefits Features

Powercord 6 Orange 6 g/m • Relatively insensitive to detonation by heat, electrostatic discharge or other forms of electricity

• Excellent resistance to side penetration by oil or water

• High tensile strength

• Easy to handle and tie into knots

• Flexible• High packing

density of PETN core (Premium only)

• VOD≥6000m/s• Reliably initiated

with 6D strength detonator

• Easy to connect branch lines

• Easy to initiate and can be extended with knots

• Can be used in very wet conditions (Premium only)

Powercord 8 Blue 8 g/m

Powercord 10 Yellow 10 g/m

Powercord 8 Premium

Red with black stripe

8 g/m

Detonating cord as a down line with Pentolite boosters

Detonating cord as a down line with Pentolite boosters

This method of priming a blast hole is used very effectively in hot hole conditions. Hot holes are normally encountered in the surface coal mining industry where previously mined coal seams (that are burning), are being mined from surface.

This prevents the use of conventional detonators, and a Pentolite booster and detonating cord provides the solution. Once the cord has been laced through the booster, the tail end should be knotted to prevent the cord pulling out of the booster when charging of the hole takes place.

The example shown on the far left is Powercord 8 in combination with a 150g Pentolite booster. The other sample shown is Powercord 8 Premium in combination with a 400g Pentolite booster shell.

ENVIRONMENTAL CONCERNS

It is important to remember that detonating cord can have negative impacts on the surrounding environment. Airblast is the most significant of these, especially when working with detonating cord in close proximity to built-up areas. Always ensure that when using detonating cord under these circumstances that measures are put in place to limit the impact on the environment. This could include covering of the detonating lines with mats and loose sand.

DESTRUCTION OF DETONATING CORDDetonating cord should only be destroyed by burning. Never attempt to destroy detonating cord while it is still wound onto its reel or spool. At the burning area, the detonating cord is first unreeled and inspected for any knots, splices and aluminium caps which must be removed and burned separately. It is then cut into 1 metre lengths, weighed and placed in bags of not more than 5kg. Place the individual 5kg batches in single piles 3m apart as per markers. Do not exceed the license.

This document replaces all previous Explosives Today on this subject including Series 2. No 29: September 1982 (J L Miller)

Disclaimer: Any advice and/or recommendations given by AEL Mining Services Limited (“AEL”) in this publication, is given by AEL in good faith in order to provide assistance to the reader. AEL does not however: 1.1 warrant the correctness of its advice and/or recommendations; 1.2 warrant that particular results or effects will be achieved if AEL’s advice and/or recommendations are implemented; 1.3 accept liability for any losses or damages that may be suffered, as a result of a party acting, or failing to act, on the advice and/or recommendations given by AEL; 1.4 accept liability for any acts or omissions of its employees. representatives and/or agents, whether negligent or otherwise.Copyright: All copyright that subsists in this publication together with any and all diagrams and annexures contained herein, which shall include all and/or any ideas, plans, models and/or intellectual property contained in this document vests in AEL. Any unauthorised reproduction, adaptation, alteration, translation, publication, distribution or dissemination (including, but not limited to, broadcasting and causing the work to be transmitted in a diffusion service) of the whole or any part of this document in any manner, form or medium (including, but not limited to, electronic, oral, aural, visual and tactile media) whatsoever, will constitute an act of copyright infringement in terms of the Copyright Act No.98 of 1978 and will render the transgressor liable to civil action and may in certain circumstances render the transgressor liable to criminal prosecution. This document remains the intellectual property of AEL.Intellectual Property: All ideas, concepts, know-how and designs forming part of this publication belong to AEL, save for where it is clearly indicated to the contrary.

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AEL Mining Services Limited (PTY) Ltd1 Platinum Drive, Longmeadow Business Estate NorthModderfontein, 1645Tel: +27 11 606 0000www.aelminingservices.com

When connecting detonating cord to a detonating relay, ensure that the cord is correctly laced through the eyelets of the relay, with sufficient excess to allow for moisture “creep” at the ends. Avoid making the ends too long as these could touch and cause “jumping” of the relay.

The ends of the incoming detonating cord lines should face away from each other once they have been laced into the relay.

During connection of a relay to a length of detonating cord, do not allow sand or mud to build up between the detonating cord and the relay’s internal detonators as this can interfere with energy transfer.

When initiating detonating cord using an instantaneous delay detonator (IED) or electronic detonator, always ensure that the detonator is pointing in the direction of initiation.

Always make sure the IED leads are twisted together, before the detonator is taped to the cord as shown in the photograph.

Other knots include the DOUBLE HALF HITCH and DOUBLE WRAP CLOVE HITCH.

When detonating cord is used to initiate shocktube, it is recommended that the connector clip provided with the shock tube be used, as this provides the right angled connection required. Alternatively, the cord can be taped at right angles to the shocktube if no clip is available.

When using a Cluster shock tube connector to initiate detonating cord, ensure that only DETONATING CORD is placed into the Cluster connecter and not shocktube or combinations of detonating cord and shocktube. The reason is that detonating cord propagates much faster (6000 m/s) than the shocktube (2000 m/s), thus outrunning the shocktube’ s propagating detonation front and almost certainly resulting in a cut off.

Also ensure that any connections to a branch line are spaced far enough apart to prevent shrapnel damage to adjoining connections.

The most common suitable branching knot is the CLOVE HITCH. This knot ensures a rapid, good surface area connection that will not pull loose, at right angles to the cord being initiated.

Detonating cords are used to detonate explosive charges, including detonating relays, in various blasting and mining applications. They can be used as downlines or trunklines and in shock tube blasts.

SAFETY

While detonating cord is very safe to use, as with any explosive product it is advisable to always treat it with caution. Detonating cord is classified as an explosive and must be transported and stored as such. It can be initiated by a sharp blow, such as from a rock falling onto it or by cutting it using the incorrect tool. Therefore when using detonating cord the correct approved cord cutter must be used and every precaution should be taken to prevent it being exposed to impact when being tied into a blast or as a lead in or trunkline.

Approved cord cutter

Designed with safety in mind – captured bladeDesigned so it can be used one handed

Always cut away from yourself; Disposable – little danger of injury as the blade is not replaceable.

Exposure to water or wet conditions

While the three standard Powercord products are waterproofed, with the Premium cord being specifically manufactured for use in very wet conditions; there will always be some penetration by water (through capillary action) into the PETN core at the cut ends. To prevent this affecting the initiation allow for at least a metre of “creep” when making connections to other cord and detonating

relays, or shocktube. It may also be also be wise to tape the cut end of the cord, if it is known that it will be submerged in water. Wet PETN is difficult to initiate, but once detonation takes place, it will propagate though any wet section if the cord is undamaged (PETN core still confined).

CONNECTIONS

Extensions

To extend a detonating cord line, the quickest and simplest joint is a REEF KNOT, alternatively a taped joint with an overlap of not less than 150mm can be made.

Joints should be avoided in down lines as they may:

a) Result in propagation failures through moisture penetration

b) Prevent the proper placement of the Pentolite boosterc) Reduce the physical strength of the line

The position of any factory made splices are written on the reel flanges

Branch lines, trunklines and lead in tie up

It is very important to remember that detonating cord in close proximity to another detonating cord line or a shock tube line can cause shrapnel damage and therefore a potential cut off or unplanned initiation of the other line. Make sure that when laying out detonating cord that it is only at right angles to any detonating cord or shock tube that is planned for initiation by the incoming detonating cord line.

Branch connections must be at right angles to avoid the possibility of the branch line being cut by the approaching main line detonation front. Connections using the Cluster connector block should point in the direction of required initiation.

Powercord 8 Premium

Potential cut-offs of shock tube by detonating cord

Too close

Too close

Potential cut-off of shock tube by detonating cord

Direction of initiation

Always use masking tape and not insulation tape to tape the detonator to the detonating cord, due to the potential for static electricity from the plastic insulation tape causing premature detonation.

Detonating cord as a down line with cartridged explosives

One of the most popular uses of detonating cord is in surface mines, where it is used in conjunction with cartridged explosives in pre-splitting operations. Pre-splitting is the creation of a split in the rock by drilling closely spaced holes and using decoupled explosives which are initiated simultaneously. Detonating cord is ideal for this application due to its high VOD.

To ensure the cord makes intimate contact with the cartridge, it should be knotted using a clove hitch or half hitch at the bottom of the cartridge and then hitched again at the top. Alternative methods include taping the cord to the cartridges or using cable ties to attach the cord to the cartridges.

This photograph shows the half hitch and clove hitch methods of connecting detonating cord to a cartridge for use in pre-splitting or when using decoupled explosives cartridges initiated by detonating cord. It is important to ensure that the detonating cord makes intimate contact with the cartridge, preferably at two points on the cartridge to prevent point or side initiation which could result in inadequate detonation pressure being applied to the cartridge to ensure it detonates at full VOD instantaneously.

In these examples, the cartridges have been taped to the Powercord 8 and the straining rope.

Powercord 8 laid out in a doubled up harness with an electronic detonator as the initiator, in a pre-splitting application. Note the connecting knot – double wrap clove hitch.