ExTA Arc Faults in FLP Enclosures

Friday, 17 March 2017

The incidence of phase-to-phase arc faults in flameproof enclosures is relatively rare. By design, the majority of electrical faults manifest as phase-to-earth faults which, in an earth fault limited system, enables detection and isolation without catastrophic consequence.

 

In those instances when phase-to-phase faults do occur, or where earth fault currents are not limited, it is typically assumed that the type tested pressure withstand characteristics of Ex d enclosures is sufficient to withstand the electrical fault.

 

This assumption omits consideration of the very high temperatures that characterise arc faults, which far exceed those of flammable gas ignitions. The fundamental explosion protection principle of Ex d - the cooling effect of controlled flame paths to prevent external ignitions – is not immediately applicable to the case of arc faults.

 

Research developed in the early- to mid-1990’s investigated the performance of Ex d enclosures when subjected to arcing faults. The findings, at that time, suggested that typical enclosures of the period were able to withstand the faults associated with power levels of the era. The reports emphasised that the findings were necessarily only indicative, and there are hints that further work was required to establish more rigorous results.

 

While some literature was published, few directives, guidance or recommendations found their way into published standards for electrical equipment for mines and quarries.

 

Since the original investigations, a number of factors have impacted on the potential performance of Ex d enclosures subjected to arc faults: 

 

·         Fault levels have increased markedly as larger and more powerful electrical equipment is brought into use. Since the energy of an arc fault is typically proportional to the square of the fault current, the consequence of elevated fault currents further aggravates the consequences of an internal arc fault.

·         Operating voltages, and hence conductor separations have increased from 1 kV a decade ago to 3.3, 6.6 and 11kV in contemporary equipment. The increased conductor separation also increases arc energies (approximately linearly with arc length).

·         The use of finite element tools in the design of Ex d enclosures has sought to optimise material strengths and structural elements to minimise manufacturing cost. In essence, this has eroded safety margins inherent in more robust enclosures (although it is difficult to quantify that effect).

·         Improved machining tolerances, together with the use of sealing ‘O’ rings, has reduced flamepath gaps and minimise ingress into enclosures. This has raised reference pressures from those measured a decade or more ago.

 

The issue of concurrency of methane ignitions and arc faults also warrants further examination. Both are foreseeable events, and should be considered as part of the design brief. However, there is argument as to whether the two events should be considered statistically independent, or should be considered to compound in terms of the pressure withstand capability of the enclosure.

 

The subject matter has been examined in some detail in previous decades. Three ACARP reports and multiple technical publications have investigated various aspects. Significantly, the most recent immediately applicable publication dates to 2004:

 

  • Previous ACARP Projects:
  • Technical literature:
    • Present and future coal mine fault levels, Schroder, F., Power Systems Computing (a report for SIMTARS), 1990.
    •  Pressure rise due to arcing in flameproof enclosures, Murphy, A.B. and Lowke, J.J., Division of Applied Physics, CSIRO (for SIMTARS), 1990.
    • Arc fault containment in flameproof enclosures: a literature review, Littler, G.E., National Energy Research Development and Demonstration Program, 1990.
    • Arc fault containment in flameproof enclosures, Stage 1, Birtwhistle, D. and Byers, K., National Energy Research Development and Demonstration Program, 1990.
    • Arc containment in underground flameproof enclosures, Byers, K., SIMTARS, 1994.
    • Managing electrical hazards in underground coal mines, Crisp, J., SIMTARS, 1996
    • Code of practice for the control of fault arcs in flameproof enclosures used in underground coal mines, Mining Electrical Mining Mechanical Engineering Society, 2004.

 

The age of the literature is of concern, as explained above.

 

In line with the principles of continuous improvement and risk management, it is pertinent to re-visit the risks presented by arc faults in flameproof enclosures.

 

Outcomes of such a review might include:

 

  • Quantification of modern fault current levels and the associated arc fault energies.
  • Identification of operating parameters and requirements that have changed markedly, including:
    • operating voltages, conductor clearances and impacts on fault energies
    • fault currents
    • equipment densities inside enclosures and resulting pressure piling
    • flameproof wall thicknesses and inherent strength
    • tighter flamepath clearances and increased reference pressures
    • use of viewing windows and their strength relative to body of enclosure 
  • Guidance on how to consider the likelihood of concurrent methane ignitions and electric arc faults.
  • Design parameters and assumptions for designers, suppliers and operators of Ex d electrical enclosures.

 

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