Electrical safety in opencast mining

The large excavators used for open-pit mining use many different drives. They use both medium voltage and low voltage drives. While portable loads are most often supplied power from resistance grounded sources they are also allowed to use ungrounded power for special drive circuits. The large drives are often ungrounded on the load side of the drive. The secondaries insulation can be monitored by an ISOMETER® with a coupling device where required. For low voltage drives that are not transformer isolated a residual current monitoring system (RCMS) can be used. If the low voltage drive is on an ungrounded supply the ISOMETER® insulation monitoring with EDS, as in both cases the faults occurring are immediately detected and reported to significantly improve availability.

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Electric motors are generally employed for conveyor belts used in mining, which are driven by soft starters or frequency converters. However, these often generate harmonics and DC currents which would normally not be present in an AC system. With regular ground-fault protection or simple insulation monitors (e.g. 3 lamp method or 2 voltmeter process), these circuits cannot be monitored and adequately protected. These specialized applications require more modern monitoring equipment which can recognize and report insulation breakdown and leakage current to ground quickly. Some standards even require a quick shutdown, which can be achieved via ground-fault monitoring devices.

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Heavy haul trucks are increasingly being powered by electric drives or wheel motors. Bender ground-fault monitoring devices monitor the complete electrical drive system for utility vehicles including all electric or hybrid drives (often single-wheel drive). This means they provide effective protection against electric shocks and minimize the fire risk. In addition to onboard electrical protection Bender relays are also installed to protect equipment used to work on these trucks. The Lifeguard can be used on loads fed from welding receptacles in shops and wash bays.

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Crushers are needed in mining to break down the ore extracted from the ground. These use very large, powerful motors which are powered by soft starters or frequency converters due to their size. The resulting harmonics and DC currents are not recognized by GFCIs. The use of a adjustable RCMA residual current monitoring system (ground-fault monitor) is therefore advisable to improve fault detection.

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Mines require power for extraction and processing. This power can be supplied by generators or local utilities depending on where the mine site is located. Larger mines may have utility power crews that work on high voltage but most often mines deal with medium voltage power. The medium voltage is either used directly by large machines or it is transformed to lower voltage for smaller equipment and loads. All three common methods of system grounding (solidly, resistance and ungrounded) may be used on a mine site. For grounded systems the Bender residual current monitors (RCM), Neutral grounding resistor monitors (NGRM’s) and ground-check relays offer sensitive and cost effective protection. For ungrounded systems including the secondaries of large drives, Bender ISOMETER®s are an ideal choice for insulation monitoring and trending.

Complete neutral grounding resistor packages can be supplied by Bender complete with components required for continuous monitoring. Typically the NGR’s are mounted outdoors near the power transformer.

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