Arc flash safety: A comprehensive approach to risk management
18 June 2015
Arc flash can occur at a wide range of installations, often causing severe damage to electrical systems and injuries or death to personnel. Paul Welford of GSE Systems, highlights the importance of risk management and mitigation to protect systems and staff.
When an arc flash occurs, large amounts of energy can be dissipated in a very short time. This energy causes the ionisation and vaporisation of the conductive metal materials (highly conductive plasma). This, coupled with the heating of the ambient air, creates a rapid volumetric expansion, known as arc blast, and consequently an explosion [1, 2].
An arcing fault usually occurs between phase bus bars or from phase to neutral or ground. During this event, current is conducted through the plasma, and the major factor that affects the current magnitude is the impedance of the arc. Intense light, sound waves, shrapnel, molten metal, toxic gases and smoke are all components of the arc flash as indicated in Fig. 1. Due to the aforementioned hazards, human and business consequences can be severe. Personnel exposed to arc flash can suffer severe burns, lung damage, vision loss, eardrum ruptures and even death [3-6].
The duration and current magnitude of an arcing fault can vary widely. An arc can be initiated by a flashover due to a failure such as breakdown in insulation or by the introduction of a conducting object that accidentally bridges the insulation. Under certain conditions that sustain the arc, faults develop into dangerous arc flash incidents. Generally, systems with voltage magnitudes lower than 120 V AC or 50 V DC cannot sustain an arc.
Within a risk assessment, arc flash hazard analysis is crucial in evaluating the risk and deciding which mitigation techniques are appropriate to control it. There are ongoing efforts to improve codes, standards and regulations for arc flash so that they include state-of-the-art methods to provide better electrical safety and performance .
Risk Management Process
Risk management is an iterative and continuous process that aims to reduce the risk and incorporate best practices . Risk assessment involves identification, analysis and evaluation of the arc flash. Risk analysis is performed to estimate the likelihood of an arc flash incident and the severity of arc flash hazards. Likelihood is generally affected by two main factors: the number of interactions with energised electrical equipment and the condition of electrical equipment. Severity is the degree of harm that results from the exposure. Risk can be high even in cases of high likelihood, in cases of high severity or both.
Risk evaluation should be conducted to determine if the risk is acceptable or manageable to protect equipment and operators. If not, risk treatment, risk control and mitigation procedures are required to reduce arc flash risk and hazard. Selection of personal protective equipment (PPE), placement of warning labels indicating the hazards, and training of operators are also essential parts of risk management and this should always be combined with mitigation measures to improve efficacy.
Available techniques for reduction of hazard levels in existing systems, including replacement or addition of equipment, are limited  but regularly developing. For new installations, innovative equipment and design methods are constantly being developed and implemented. Before implementing any method, its applicability to the specific electrical system must be considered  along with the nature and magnitude of the potential arc hazard.
• Protection schemes
Normal protection schemes aim to detect short-circuit current in order to interrupt the fault. Arcing fault current is always less than short-circuit current. Thus, an efficient protection coordination study should be performed in order to select the proper settings of the overcurrent protective devices to sense the arcing fault. The difficulty of this task is to reduce arc flash incident energy without sacrificing selectivity, in which the effect of a short circuit is minimised while the portion of the power system which is disconnected and therefore disruption to operations is minimized.
The main categories of overcurrent protective devices are fuses and circuit breakers. Their role is to clear fault current, but they will not react as quickly or at all if the arcing current is less than their fault threshold. A modern piece of equipment that provides even faster clearing time of the fault is the arc fault detection relay, illustrated in Fig. 2. A combined detection of light and current (and sometimes sound as well) allows clearing times of the order of 1/4 cycle or even lower.
• Arc flash impact reduction
Remote operation and arc-resistant equipment are methods that do not reduce arcing fault energy but can protect personnel from arc flash . Remote operation, such as remote racking and switching, effectively removes the operator from the hazard, thus the operator is outside of the arc flash boundary. This method is safer for operators and does not affect system selectivity but may be costly and difficult to retrofit into existing systems although proprietry devices are available on the market. Also, remote operation does not reduce the risk of damage to adjacent equipment. Arc-resistant equipment consists of switchgear that has the capability to confine the incident energy of the arc flash event in the specific area of the switchgear and provides a system for pressure relief.
• Arc flash current path alternation
A different approach to mitigation is to introduce an alternate current path in order to transfer the arcing fault and capture it. Providing a lower impedance path than the fault path and fast transfer of the arcing fault are important factors for the success of the method. There are two methods: ‘crowbar method’ and ‘lower impedance arcing current path method’. The first introduces a bolted fault, as an alternative current path, that provides a current bypass to extinguish the arc. The second, an example illustrated in Fig. 3, introduces the breakdown of an air gap between the phase electrodes and a plasma gun. Both kinds require a proper arc fault detection system, as described earlier, and they can offer protection from arc flash and blast energy .
• Temporary mitigation methods
Drawing on engineering experience, operators may temporarily reduce the settings in protection devices during system maintenance or may operate with an open tie when buses are connected via the bus-tie breakers. Reduced settings provide more sensitive and faster interruption of the arcing fault, but usually this cannot be implemented as a permanent solution because it could result in undesired tripping under normal operating conditions. Similarly, operating with open bus ties reduces overall power plant reliability and usually cannot be implemented as permanent solution.
Arc flash hazards are a real threat to personnel, equipment and operations. Identifying risks is essential. However, operators must not stop there. To ensure safety and regulatory compliance, operators must develop and implement feasible solutions to mitigate workplace hazards. Because every operation is different, the risk management process will vary from one organisation to the next. If the job is too immense or the situation potentially too dangerous for internal personnel to handle alone, professional hazard mitigation experts can provide a customised approach to meet all safety requirements through a cost-effective programme.
Mitigation measures are also essential to reduce the necessity of over-reliance on PPE to protect against arc flash hazards. For higher protective categories, PPE is often restrictive to operations and can be cumbersome and uncomfortable thereby introducing further risks in regular switching and maintenance. The reduction of the hazard through mitigation allows categories of PPE protection to be reduced to a manageable level as a suite of solutions to the arc flash problem.
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