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Mike Frain and Jim Phillips explained the arc flash hazard and how it can be predicted in last month's Electrical Review. In this month's article the authors speak about the various measures that can be adopted to protect organisations against the sometimes catastrophic effects of an electrical flashover, and also to prevent injuries to workers

There are many ways in which the hazards associated with arc flash can be reduced and in some cases eliminated. A risk assessment must be performed where there could be danger which may include performing an arc flash calculation study to define the severity of the hazard. It is a fact many workers are put to work on very high incident energy equipment without a thought for the consequences which results in a huge risk to the company and the individual. There is a great deal of helpful information on the HSE website about risk assessment starting with the five step guide referenced at the end of this article. There is a need to evaluate the arc flash risk as a part of the process and also the methods for controlling that risk. The amount of energy that could be released in an electrical flashover is the starting point in that evaluation process. Once that is known then risk control measures can be explored to determine the test of reasonableness in working live and suitable precautions as laid out in the Electricity at Work Regulations 1989.

Hierarchy of risk control measures
The need for risk assessment is embodied in European Law through directive 89/391 and is transposed into UK Law through Management of Health and Safety at Work Regulations. Most people are familiar with the general principles of prevention as laid down in these documents and in other UK regulations. They say that "Where an employer implements any preventative measures, he shall do so on the basis of the principles of prevention" shown below. The authors' thoughts are shown in italics on how these principles can be interpreted when it comes to arc flash prevention.

1. Avoiding the risk - which means dead working. Not energised = no danger
2. Evaluation the risks which cannot be avoided - by arc flash assessment
3. Combating the risks at source - by designing out the problem
4. Adapting to the individual - the avoidance of monotonous work
5. Adapting to technical progress/information - take advantage of technological and technical progress to improve both safety and working methods. The evaluation of the hazard has progressed as have mitigation and protection techniques in respect of arc flash.
6. Replacing the dangerous by the non dangerous -
replace vulnerable legacy switchgear and control panels preferably with arc protected equipment.
7. Developing a coherent overall prevention policy - specific to structure environment, workforce & equipment issues.
8. Giving collective protective measures priority over individual protective measures - by screening live parts, by good design, create a safe place approach.
9. Giving appropriate instruction to employees - create a safe person approach.

The general principles of prevention given previously should be considered against a hierarchy of risk controls with priority as given below. The top of the list should always take priority with PPE as a last resort.

1. Elimination of the arc flash hazard
2. Reduction
3. Information and training
4. Control the risk
5. PPE

All these measures should be properly monitored and reviewed and this is particularly important when considering the lower order risk controls. Each of these risk controls are expanded in the context of arc flash as follows.

1. Elimination of the arc flash hazard
The best way to prevent injuries from occurring is to de-energize equipment before beginning work and as pointed out last month, a fundamental safety principle, which is embodied in U.K. legislation, is to design out, eliminate or remove the electrical hazard at its source. Designers of electrical systems should consider the need to eliminate live work as part of the overall system design. Some of the elimination measures include the segregation of power and control circuits, safe control voltages and currents, finger safe shrouding of terminals and built in test points. Even then, the condition of the electrical equipment must be verified before work commences. It is not uncommon to have accidents occur on equipment that has been rendered dangerous, because electrical workers have not reinstated vital safety components such as door interlocks and insulating shielding after completion of work.

2. Reduction of the arc flash hazard
Since incident energy is a function of short circuit current and the protective device clearing time, a reduction of the arc flash hazard may be achievable by evaluating the protective device sizes, settings and time current curves. Many times where the incident energy is at dangerous levels, it is because the upstream protective device's instantaneous adjustment is set too high and the device is operating in the long time delay region. Lowering the instantaneous setting may allow it to trip faster resulting lower overall incident energy.

Caution should be exercised however, because lowering a device's trip setting can create a reliability issue by compromising selective coordination with other devices. This could cause multiple devices to trip during a short circuit and lead to a more widespread outage. The problem can be solved by changing the device settings only when live work is going to be performed. After the work is completed, the original settings can be restored to maintain existing selective coordination. These types of temporary setting changes are often referred to as ‘maintenance settings'.

Protection arrangements should be explored at design stage to minimise the effects of electrical flashover through the use of fast acting and/or current limiting devices. Other solutions may include retrofitting circuit breakers with instantaneous trip units. Another protection method that has been developed uses an optical sensor that detects the ultraviolet light emitted from an arc flash and causes the protective device to instantaneously trip. Equipment manufacturers are continually developing better methods to reduce or eliminate the arc flash hazard.

Incident energy levels are roughly proportional to the inverse square of the distance. This means a small increase in distance between the live part to be worked upon and the torso of the worker can result in a significant decrease in incident energy. This is therefore, a valid reason to keep workers as far away as possible when undertaking work including inspections on or near to energised circuits. Enclosed switchgear which has a front cover removed and facing the worker acts to magnify the effects of the arc flash and will direct the energy outwards.

3. Information and training
Many times electrical workers and even management do not fully understand the nature of the arc flash hazard and the seriousness of the injuries that can be sustained. Experienced personnel are frequently involved in these types of accidents. Competent electrical workers should be trained in the decision making process necessary to determine the degree and extent of the hazard and the PPE and job planning necessary to perform the task safely.

A review of electrical procedures and safe systems of work can raise the profile and understanding of the hazard and associated control measures. Periodic awareness and refresher training, toolbox talks and specific training in the policy, rules and procedures.

Warning notices were mentioned in last month's article, and arc flash studies will give information about the incident energy level, PPE and various approach boundaries. The Health and Safety (Safety signs and signals) Regulations 1996 is the relevant legislation on the provision and use of safety signs at work the purpose of which is to encourage the standardisation of safety signs throughout the member states of the European Union so safety signs, wherever they are seen, have the same meaning. If you are designing labels you ought to consider the requirements of this legislation as warning and prohibition signs will need to be in accordance with this directive. Warning signs for instance must be yellow (or amber), they must be triangular and the yellow must take up at least 50% of the sign.

4. Control the risk
Safety rules and procedures with clear responsibilities are essential parts of a safe system of work. They should deal with properly assessing and authorising only competent people with systems for risk assessment. Where live working cannot be avoided, then the safe working systems should stipulate the use of the correct equipment and instruments. Electrical flashover accidents are very often caused by the operator dropping un-insulated tools or metal parts or by using incorrectly specified instruments. There should be a rule that no live work will be allowed on equipment that is damaged even for the reasons of proving dead. There have been many incidents involving damaged cables where an approach has been made to prove dead when the damaged cable was in fact still live.

The main objective of an electrical maintenance program is to keep electrical equipment in good working condition so that it can reliably and safely operate within its design criteria. This includes testing and maintenance of circuit breakers, protective relaying and associated equipment on a regular basis. Improper maintenance of equipment can contribute to the severity of an arc flash. When a protective device such as a circuit breaker or relay is not properly maintained, the likelihood of it operating slower (or not at all) increases, which would also increase the duration of exposure to the arc flash. Although not technically part of an arc flash calculation study, a proper equipment maintenance program is vital in making sure the protective devices will operate correctly.

5. PPE
The final risk control measure in the hierarchy of controls is PPE. You will notice PPE is not mentioned in the principles of prevention given above and PPE alone will not prevent the accident. It is seen as a last line of defence but where used properly has prevented injury to individuals and reports among utility companies have confirmed this. There has been research that shows that the hands, arms and face are the most commonly affected parts of the body in an arc flash incident so face and hand protection should also be considered.

Flame resistant PPE is surprisingly comfortable nowadays and can be worn as everyday workwear. Non flame resistant clothing may ignite or melt at low incident energy values and once ignited will continue to burn after the electrical arc has been extinguished. Three seconds of burning material next to the flesh can result in serious full thickness burns. This actually means that ordinary clothing could actually become a hazard and for this reason it is well worth considering a policy of comfortable FR clothing.