The dangers of uncontrolled arcing of electrical equipment and the resulting flash and blast effect that can destroy plant and severely injure or kill personnel are well recognised in industry throughout the world.
The use of the North American Standard1 to estimate arcing fault energy and specify personal protective equipment (PPE) has become the primary guide by which plant owners and operators attempt to deal with the hazards posed by arc flash incidents. Pre-assessment of arc flash hazards is a mandatory requirement within the United States for electrical work.
Among its many limitations, IEEE 1584 only considers the consequence of an arcing event and gives little attention to the likelihood of these faults occurring. Mitigation of arc flash effects on personnel under IEEE 1584 is primarily directed towards consequence reduction through the application of thermal resistant PPE and work clothing.
In working with clients, Sinclair Knight Merz (SKM) has sought to bring the evaluation of arc flash hazards into focus by:
- Proposing a method of evaluating the likelihood of an uncontrolled arcing fault, and thereby a more inclusive picture of risk incorporating both likelihood and consequence measures
- Going beyond the de facto American approach, which tends to focus on PPE as the principal method of preventing personnel injury, towards considering a broader range of mitigation techniques
Conventional wisdom is that reliance on PPE has multiple shortcomings and should be considered as the “last line of defence”. We believe a far more robust approach to arc flash mitigation is possible through understanding the physical installation, identifying what can be fixed or modified to reduce both likelihood and consequences, and tailoring additional levels of mitigation in accordance with the Hierarchy of Controls2.
Arc flash evaluation and mitigation
It is vital that all designers and operations personnel who use arc flash calculation modules in IEEE 1584 and those available in the public domain understand how they work and the limitations in the results they produce.
Recent theoretical and practical research undertaken in Australia3&4 has questioned the following assumptions inherent in IEEE 1584 methodology:
- An arc is a spherical point source radiator of fixed diameter
- 100 per cent of the arc energy is radiated
- The separation of electrodes is significant in calculating arc energy
- The maximum power transfer theorem applies
In contrast, the Australian studies have demonstrated that:
- Far from being a point source, an arc fault can generate a projected plasma cloud that continues to expand even after the fault is cleared
- Less than 20 per cent of the arc energy is radiated with more than 80 per cent expended in complex energy forms such as convective heating due to the plasma cloud, electro-magnetic energy, electrode dissipation energy etc
- The heat load of an object in the plasma cloud is more than three times greater than IEEE 1584 predictions
- Electrode separation is not a significant factor in the generation of arc flash energy
- The IEEE 1584 test rig is not representative of all switchboard electrode configurations
- The IEEE 1584 calculations tend to underestimate low voltage (LV) exposures and overestimate high voltage (HV) exposures
- The equations become less accurate with increasing voltage and fault levels and fail the extreme values test
It is understood that many of these deficiencies have now been recognised by the IEEE Committee and the philosophy underpinning the energy calculations is being reconsidered. However, it is not known when a revised Standard will be issued, or what changes may be made, as the existing procedure is firmly embedded in North American legislation and practice.
Possibly of more significance is the propensity of IEEE 1584 and its companion standard NFPA 70E, to encourage switchgear owners and operators to rely on PPE selection as their principal mitigation against arc flash risk. In some older installations this is analogous to fitting seat belts to a Model T Ford travelling at speed on a congested freeway. While providing some additional protection it can also engender a false sense of security in potentially risky situations.
In spite of these deficiencies, arc flash calculations can have a valuable application. Used with care, they allow switchgear owner/operators to quantify and compare the arc energy potentials present in their equipment across their sites. This in turn can focus attention on those pieces of equipment that will expose operators to the highest potential of injury from an uncontrolled arcing event and indicate the minimum safe distances that should be observed when working in the vicinity of the equipment.
Physical condition assessment
While working with clients in resource and utility industries, SKM has often been requested to consult and make recommendations on switchboard condition, operator safety and the possibility of equipment life extension. This usually requires the simultaneous assessment of multiple HV and LV installations so that a common approach to switchboard safety or site development could be initiated.
In most cases there has been very little qualitative maintenance data to rely on such as thermographic and partial discharge testing and the opportunity for switchboard partial disassembly and close physical inspection is limited.
The need for a simple, consistent protocol to quantify the “likelihood” of a switchboard failure was clearly evident. SKM has developed a list of 50 inspection elements considered to be important condition indicators, spread over the following major areas affecting safety and condition:
||Age and fault rating
||(considered the most critical)
||(affecting switchboard deterioration)
||Design and installation
||(affecting intrinsic capability and design safety)
||(maintenance and documentation evidence)
||(considered loading, switching and past failures)
||(type, condition and capability of systems installed)
Each criteria element is assigned a value of 0, 1 or 2 depending on the full compliance, partial compliance or non-compliance with the element criteria.
As some elements such as switchgear age, arc flash containment design, busbar insulation, fault rating etc may be considered more important indicators of failure “likelihood”, a facility is included in the protocol to adjust the weighting of these elements to meet practical or client considerations.
After summating the weighted value of the individual criteria elements, a ranking value between 0 and 200 is determined to allow an owner/operator to identify which switchboard installations need immediate mitigation and/or more in-depth assessment.
As an interim measure, a revised PPE regime and operational procedures can be considered for immediate personnel protection.
There are many options for arc flash mitigation, all of varying effectiveness and cost in terms of both capital and production interference. An appropriate solution can be tailored for any situation from the available options. The selected mitigation solution should preferably use the multi-layer approach.
Elimination of arc flash risk is generally not possible where high energy electrical supplies are used.
Substitution of electrical equipment can be highly effective in lowering the risk of arc flash. Some of these options are most effective and relatively cost neutral when applied during the design phase of the project. Commonly employed substitution methods involve the:
- Replacement of old style uninsulated switchboards with more capable Form 3b+, fully insulated, segregated and IAC tested models.
- Replacement of old oil-filled HV circuit breaker modules with new faster vacuum or SF6 circuit breakers
- Replacement of large MVA rated transformers with multiple smaller MVA transformer units.
Engineering solutions are often the easiest to apply on existing installations but are usually directed towards detecting and ameliorating the affect of arc flash or removing the operator from the arc flash zone.
Somewhat less effective, but quickly applied mitigation can be achieved by administrative means such as improved maintenance and switching procedures to detect and minimise high risk situations, and improving operator training and awareness programs.
While appropriate selection and enforcement of PPE is possibly the fastest form of mitigation to implement, it is the least effective. PPE may not provide the expected level of protection and there is a common reluctance in hot an uncomfortable situations for personnel to use all of the necessary PPE.
1 IEEE 1584 – IEEE Guide for Performing Arc Flash Hazard Calculations, Institute of Electrical and Electronics Engineers (2002), New York
2 Occupational Health and Safety Regulation 2001, Chapter 5, Part 5.1, Hierarchy of Controls, NSW Government (2001), Sydney
3 Stokes, A.D., Sweeting, D.K. – Electric Arcing Burn Hazards, IEEE Transactions on Industry Applications, Vol.42, No 1, January/February 2006, New York
4 Sweeting, D.K., Stokes, A.D. – Energy Transfers with Arcing Faults in Electrical Equipment, International Conference on Fuses and their Applications, Clemont-Ferrand France, 10-12 September 2007.