Circuit breaker design for safety in underground mining applications

August 12th, 2014, Published in Articles: Vector

 

Circuit breaker applications in underground mining and recommended methods to maximise the effectiveness of these protective devices

In mining, the underground assembly or power centre is engineered to transform and distribute electrical power brought underground. This assembly typically includes a medium-voltage incoming circuit breaker or switch with current-limiting fuse (see Fig 1).

Fig. 1: Low-profile underground assembly including primary fused switch, transformer and low-voltage circuit breakers.

The device protects a close-coupled dry-type vacuum pressure impregnated power transformer which converts the medium-voltage to a lower distribution voltage. The transformer then feeds low voltage (LV) circuit breakers which feed and protect external underground mining loads connected to the power centre via extended trailing cables. These conductions are often thousands of meters long.

LV metal-enclosed ANSI and NEMA switchgear assemblies are typically 2286 m tall while metal-enclosed IEC controlgear assemblies stand 2 m high. Underground power centers, however, typically have half these fimensions to meet clearance requirements.

Voltage drop on generally soft power systems as a result of the long lengths of trailing cable requires higher application voltage. In the USA it is not unusual for secondary distribution voltages to be 3-phase 1000 V AC while operating voltages can reach up to 1240 V AV in countries such as China. Most importantly, the design codes and standards for this assembly are not dictated by ANSI, NEMA or IEC standards. Instead, mining authorities in individual countries generally review and approve these assemblies for application in underground mines.

Special circuit breaker applications

Circuit breakers are used in underground mining equipment on equipment such as shears and continuous miners, as well as in assemblies feeding electrical power to the underground mining equipment.

These circuit breaker components are not necessarily manufactured or tested to traditional standards which apply to assemblies used for above-ground industrial use. One variable here is the operating voltage of the device itself.

Molded-case circuit breakers

Modern molded-case circuit breakers (MCCBs) are applied throughout most, if not all, industries. They offer safe and economic load connection and disconnection from the electrical source and provide both overload and short-circuit overcurrent protection.

MCCBs comprise five major components: the molded-case or frame; an operating mechanism; arc extinguishers; contacts and trip components (see Fig. 2). The manufacturers of these products often seal the molded cases after assembly, calibration and testing. There are typically no internal serviceable parts, and end-user maintenance has been limited historically to mounting, wiring and manual operation.

Fig. 2: Cutaway view of a molded-case circuit breaker.

Fig. 2: Cutaway view of a molded-case circuit breaker.

Mining industry standards globally require both earth fault and trailing cable protection in addition to higher operating voltages. MCCBs in underground mining applications typically require overcurrent relays for phase-to-earth protection. A special earth monitor relay is also applied to sense the integrity of the trailing cables connecting mining machinery to the underground power centre.

Trailing cables are often longer than 500 m and may be severed while energised as they are continually exposed to the mobile equipment used in mining.

The earth monitor relay protects the circuit by sensing the continuity of a pilot wire included in the shielded cable connected through an earth conductor returning to the origin. Should this loop be severed, the monitor would relay the incident and trip the MCCB to prevent injury or fire. The MCCB is relied on to trip, protecting the circuit in both the phase overcurrent protection and in the trailing cable earthing protection circuit. An internal undervoltage release coil is included in the MCCB to accomplish a fail-safe trip circuit.

An internal undervoltage release coil is included in the MCCB to accomplish this. The contacts are opened on a fault condition by de-energising the undervoltage (UV) release coil, which actuates a plunger causing the MCCB to trip.

MCCB trip resets can cause failure

How mining operators in the field reset MCCBs when they trip is often at issue. The MCCB handle is typically moved to the lowest “off” position after a trip; this effectively resets the mechanism.

The breaker handle is then switched on to reset the device. Important to consideration is the availability of control power to the UV release coil. The control power for this circuit is often fed from a power source which was also cleared during the fault condition.When the operator tries to reset the breaker mechanism with the undervoltage release coil plunger still resting on the trip bar, the breaker is effectively being requested to initiate a trip while the operator is trying to reset and energise the device. The breaker will “trip-free” and the main contacts will never close. The trip-free mode prevents the circuit from closing into a potential fault.

The breaker mechanism includes an over-toggle feature which stores a significant amount of energy in the springs. This design ensures that the contacts close with sufficient pressure and force to energise and protect the circuit.

When the breaker mechanism trips free, all of the stored energy is released internally, causing significant stress on the mechanical assembly.

In practice, repeated reset attempts resulting from trip-free operation will destroy the breaker, often after only 50 or 60 operations. In mining, four to five breaker reset attempts per shift is not uncommon. Multiple reset attempts without undervoltage release control power cause cumulative damage to the MCCB mechanism.

The most common failure modes include bent rotational reset pins or broken or cracked components in the steel frame of the mechanism. Adding an LED internal undervoltage trip indicating light is a simple method to ensure that the MCCB is never vaused to be reset and to operate trip-free.

MCCB safety issues

Harsh underground mining environments impact MCCBs’ reliability. Failure can occur frequently when an MCCB is used to clear a downstream, high-level fault with frequent resets in a high-temperature, dusty or moist environment. Breakers in obvious poor condition should be replaced.

MCCBs may be repairable at times but the site maintenance and operations group should avoid MCCB repair centers from third-party companies. Repairs, refurbishing, and modifications of

MCCBs should not be done by independent third parties because:

  • The components used may not be manufactured to factory specifications.
  • Damaged breaker frames are not serviceable.
  • Processes used by third parties often remove lubrication, coatings, insulation, and plating required for proper breaker performance.
  • Abrasive processes promote corrosion and weaken components.
  • Solvents, paints, and other foreign materials deteriorate breaker insulation and degrade breaker performance.
  • Counterfeit labels disguise third-party service and may not describe breaker ratings accurately.

Mine operators are ultimately responsible for all repairs and modifications made to circuit breakers. The mine operator must confirm that all repairs are made in accordance with factory specifications, that the refurbished product is fully tested, and that modifications will not adversely impact the performance of the circuit breaker.

Vacuum circuit breakers

Although LV molded case circuit breakers have been applied in underground mining equipment and power centers for many years, recent changes in system design have opened the door for application of medium-voltage vacuum circuit breakers as well (see Fig. 3).

Fig. 3: New design fixed-mount vacuum circuit breaker applied in 1,2 kV low-voltage and up to 24 kV medium-voltage underground mining systems.

Fig. 3: New design fixed-mount vacuum circuit breaker applied in 1,2 kV low-voltage and up to 24 kV medium-voltage underground mining systems.

System voltages and source MVA for underground applications appear to be increasing gradually. Where underground assemblies were historically operated based on internal 1 MVA transformers (or a maximum of 1,5 MVA at 1000 V AC distribution voltage), newer designs are vased on as much as 5 MVA at voltages of 3,3 kV and higher. This is driven by new, larger continuous mining and conveying equipment applied underground.

The impact of these changes in underground electrical systems is an upward pressure on circuit breaker designs.

MCCBs traditionally applied at 1000 V AC with full load current ratings of 150, 250, 400, 600, 1200, and 2000 A with interrupting current ratings from 10 to 25 kA are now operate at operating voltages from 1,2 to 3,3 kV with interrupting ratings to 25 kA and higher.

Circuit breaker manufacturers are pushing the limits of MCCB design. MCCBs employ arc interruption in air via a proven technology that draws an arc across opening contacts into an arc-chute which divides the arc into several smaller components, eventually cooling and extinguishing it to interrupt a fault.

Higher rating requirements are pushing the physics to successfully interrupt an arc in air. This opens the door to other circuit interruption methods including vacuum and SF6 designs. Both vacuum and SF6 circuit breakers are also used extensively in many industrial applications. These technologies extinguish a fault by interrupting the arc in either vacuum or in sulphur hexafluoride (SF6) gas.

The interruption occurs in a sealed chamber when the circuit breaker contacts part to open the circuit or to interrupt an arc. The arc is extinguished and the fault cleared more readily because the arc is contained and cannot ionise in air. Higher voltage is current and interrupting ratings are therefore possible with this alternative. The vacuum or SF6 circuit breaker is generally larger and more costly than its MCCB counterpart, but these designs offer some advantages.

The first of these is voltage ratings up to 5 kV and current ratings of either 800 or 1200 A. This breaker has totally encapsulated pole units and is designed for 30 000 operations.

The circuit breaker can also be operated electrically with an internal mechanism and optional spring charging mechanism. Electrical operation allows the operator to open and close the circuit breaker via a remote panel as opposed to operating the mechanical toggle type-mechanism of the typical MCCB.

Remote operation improves the safety for this product because the operator can be well-outside the flash protection boundary. Harsh environmental conditions in underground mining are less of a concern because the arc interruption occurs in a sealed chamber.

The new design is also available in a similar form factor in voltage ratings up to 15 kV. This would allow circuit protection in underground power centres for both secondary distribution and primary medium voltage protection at the transformer primary.

Type testing for underground assemblies

Defined test requirements for underground electrical assemblies have generally been unclear. An improved focus on underground miner safety has not translated into safety improvements in underground electrical equipment. This is unfortunate as electrical accidents are the fourth leading cause of death in mining.

The new Australian Standard AS4871 manufacturers of underground equipment to complete type testing of electrical assemblies including surface temperature rise limits, short-circuit withstand tests and verification of protective earth circuit tests. This additional testing is typically performed in third-party, high-power test laboratories and the equipment manufacturer is required to show certification of the specified tests.

Adding new requirements for type testing is expensive, but the authors believe the end result will be a much safer installation with proven capability when electrical systems are called upon to perform in the event of downstream fault or arc flash events.

References

  1. DD Roybal: “Circuit breaker interrupting capacity and short-time current ratings”, IEEE IAS Pulp and Paper Industry Conference, 2004.
  2. ANSI/NEMA Standard C37.20.1 Metal cnclosed vow-voltage AC power circuit breaker switchgear assemblies, 2002.
  3. ANSI/NEMA Standard C37.20.7 Guide for testing metal enclosed switchgear rated up to 38 kV for internal arcing faults.
  4. IEC Standard 61439-1/2, Low-voltage switchgear and controlgear assemblies
  5. UL 489, Molded-case circuit breakers, molded-case switches and circuit breaker cnclosures, 2006.
  6. IEC 60947.2 (AS 60947.2) Low-voltage switchgear and controlgear, part 2 circuit creakers, 2005.
  7. M Higginson and DB Durocher: “Proper application and maintenance of molded-case breakers to assure safe and reliable operation”, IEEE IAS Electrical Safety Workshop, 2009.
  8. National Fire Protection Agency: NFPA70E Standard for Electrical Safety in the Workplace, 2009 Edition
  9. National Institute for Occupational Safety and Health: www.cdc.gov/niosh/mining/topics/topicpage1.htm
  10. AS/NZS 4871.1 Electrical equipment for mines and quarries, 25 May 2010.
  11. IEEE Standard 1584 Guide for performing arc-flash hazard calculations, 2002.
  12. US Environmental Protection Agency: “Byproducts of sulfur hexafluoride (SF6) use in the electric power industry”, 2002.

Contact Marlene Coetzee, Eaton,  Tel 011 824-7400, marlenecoetzee@eaton.com

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