OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 36 Mining

Section 642 Circuit requirement

Subsection 642(1)

This section details specifications for the blasting machine, power circuits and lead wires. Any deviation will compromise efficiency and performance and may create safety problems. Most of these specifications are quite clear and self-explanatory. For ease of reference an excerpt from Clause 3.7 of CSA Standard CAN/CSA M421-00 (R2007), Use of Electricity in Mines, is provided below. Reader should note that the Standard deals only with the case where electricity is supplied from the power distribution system.

3.7 Electric Blasting

3.7.1 General
Clause 3.7 applies to the use of electricity supplied from the power-distribution system for blasting. Where the power-distribution system is not used for blasting, the mine shall establish alternative procedures.
Note: See Blaster’s Handbook
The mine shall have procedures to prevent inadvertent detonation of electric blasting caps in the presence of radio transmitters or other radio-frequency fields (cellular phones, GPS, portable hand-held radios, etc.).
Electric blasting circuits shall be tested for continuity before a blast is set off.

3.7.2 Supply Characteristics
An isolated, underground power source shall be used for electric blasting; it shall have adequate capacity for the number of caps involved, and it shall be used for blasting only.

3.7.3 Conductors
Acceptable blasting circuit conductors shall be
(a) not less than No. 12 AWG;
(b) without splices, as far as practical; and
(c) readily identifiable as being for blasting use, preferably red.
Where expendable connecting wires are used for the lead wires to the leg wires of the blasting caps, they shall be not less than No. 20 AWG.

3.7.4 Stray-Current Precautions
Blasting-circuit conductors shall be kept at least 150 mm away from power or lighting cables and, where possible, they shall be run on the side of the working opposite power and lighting circuits.
Where blasting lines are installed for a short time only, they may be fastened to sprags or sticks for adequate support during such temporary usage.
Blasting-circuit conductors shall not contact pipes, rails, or other electrically conductive materials that might be accidentally energized or be vulnerable to static charges.

3.7.5 Control
A fused service switch with provision for locking shall be installed between the source of power and the blasting switch.
In all cases where a blasting switch is used, a lightning gap of not less than 1.5 m shall be provided between the service switch and the blasting switch, and such gap shall be closed only at the time of blasting by means such as a twist-type locking device.

3.7.6 Blasting Switch
The blasting switch shall be constructed so that gravity tends to open the circuit and short the blasting leads. Where the power source exceeds 300 V, the blasting switch shall be electromagnetically operated.
The blasting switch shall be within a fixed, locked box and shall be accessible only to the authorized blaster.

3.7.7 Multiple Blasting
Where a single blasting switch is used for several blasting circuits, a three-way isolating switch that can be locked in either the shorted or closed position shall be installed in each circuit to provide for
(a) shorting the circuit;
(b) energizing the circuit; and
(c) testing the circuit.

The isolating switches shall be located in a safe place.

3.7.8 Maintenance
Permanent blasting lines, blasting switches, and service switches shall be maintained by a qualified person.

3.7.9 Programmable Logic Control (PLC)
In addition to the requirements of Clauses and, where a PLC or computer is used to control or initiate the blast, the system shall be approved by a professional engineer.

The detonation power source must provide sufficient current when the switch is closed. If the power circuit is also providing electricity to other equipment, the demand of this equipment may reduce the electricity available for blasting machines. This, in turn, may provide less than adequate power through the circuits resulting in misfires.

Subsection 642(2)

Lead wires for blasting are standard products supplied by an explosives accessories company and are readily identifiable. Despite the standardization, the blaster is ultimately responsible for ensuring that the correct products are being used at a mine blast site. This subsection lists minimum specifications that must be confirmed by the blaster prior to putting the products to use. Key specifications are listed and include “AWG wire size”, “waterproof”, “readily identifiable for blasting use” and “used only for blasting”.

Additional safety requirements are listed and are meant to minimize the potential for premature detonation as a result of extraneous electrical current. This subsection specifies minimum distances that must be maintained between the lead wires and power or lighting cables, pipes, rails or other electrically conductive materials.

Subsection 642(3)

To ensure that enough current flows through the complete blasting circuit, the disposable connecting wire between the main lead wires and the detonator’s leg wires must not be less than No. 20 AWG in size.