Chapter 15:
Why Do Arc Flash Calculations?


I felt compelled to write this chapter after listening to a colleague who questioned the need for carrying out arc flash calculations. He mistakenly thought that calculations = PPE which cannot be further from the truth. Here are ten solid reasons for carrying out calculations in order to prevent the hazard from causing harm.

15.1 Root out danger.

Arc flash is just another hazard and I have been consistent over the years that the major consideration is to understand it and root out real danger. The analogy of the hidden raging tiger lurking inside the electrical equipment works well for me as my prime consideration is the identification and evaluation of the hazard as the first steps in risk assessment. Calculation forces us to have a look at the upstream protective devices and in the same way as protecting against electrical shock, impedance is the enemy of good protection. So, we need to consider whether an arcing fault is going to be cleared quickly with minimal damage or, the fault will persist with catastrophic harm to the equipment, and anyone stood nearby.

15.2 Keeping people out of restrictive PPE.

The difference between moon suits and no PPE at all is often just a few amps. In most of the studies that I have carried out, I can point out where 90% of high hazard levels could be averted by a competent engineer reconfiguring circuits or in most cases protection alterations. In one such case I was working for one of the largest technology factories in the UK where there were extremely high specification clean room environments. Following the initial calculations, it was discovered that there were 120 low voltage panels which were identified in the clean room production areas, that had high levels of incident energy. We managed to successfully reduce the levels at all these panels to below 1.2 cal/cm2 at a distance of 450mm. Measures such as new protection settings, replacement of circuit breakers, installation of additional local switch fuses and re-feeding equipment were all adopted with relatively little cost and effort. The motivation there was partly reflected in the fact that clean room flame resistant PPE is very expensive but why should this approach not be applied to all industrial settings? This was carried out in 2008 and for such a large site, their safety performance has since been excellent.

15.3 The first step in risk assessment.

When carrying out a risk assessment, as a minimum we must:

  1. Identify what could cause injury (hazards)
    • This is derived from system parameters such as voltage, fault level and protection arrangements.
  2. Decide how likely it is that someone could be harmed and how seriously (the risk)
    • This is derived from system conditions such as the condition of the equipment, the quality of the installation, how well it has been maintained and whether it is being operated in accordance with its original design and:
    • It is also directly related to the task to be performed.
  3. Take action to eliminate the hazard, or if this is not possible, control the risk.
    • Use the four P approach to eliminate or control the risk.

Where we have the means to evaluate the hazards in the workplace, we must take advantage. As I have said earlier, it is a matter of a few amperes that can be the difference between an arcing fault being cleared quickly and harmlessly to a situation that results in catastrophic damage, losses and severe personal injury.

15.4 Examples of where calculations would have prevented accidents.

I give several examples of failure of protection systems in Chapter 12: Myths and Mistakes where calculations would have picked them up in my view. There is the catastrophic failure of a capacitor that raised a main substation to the ground and caused an environmental incident and also the apprentice who was burned whilst connecting cables into a distribution board. Not featured in Chapter 12: Myths and Mistakes is an 11kV circuit breaker which exploded at a waterworks with no injuries but the costs were huge. All three incidents would have been averted had the upstream protection arrangements been examined as part of an incident energy study.

15.5 Helps to comply with the law.

Whether it is the European Framework Council Directive 89/391/EEC (Workplace Health and Safety Directive) or the UK Management of Health and Safety at Work Regulations 1989 or the Irish Safety, Health and Welfare at Work (General Application) Regulations 2007, the cornerstone is risk assessment which means that any employer must evaluate all the risks to the safety and health of workers. Arc flash risk assessment for workers who operate in proximity to or on energized electrical equipment, cables and overhead lines, is an essential part of electrical safety management. Calculations give us the means to evaluate the hazard and also the means to provide prevention techniques such as automatic disconnection of supplies for arcing faults.

15.6 90% of the effort for a study is what we should be doing anyway.

When carrying out system studies, the vast majority of the basic electrical system information gained is what any self-respecting electrical engineer ought to have at his or her fingertips. A precursor to an arc flash study is to have a single line diagram of the electrical system, a short circuit study and a protection coordination study. Appendix A from IEEE 1584 2002 described all the data that would be required in order to undertake an arc flash study. It broke the data down into four separate categories: 1. The diagram, 2. Short circuit study, 3. Coordination study and 4. The arc flash study. I have represented the data headings into a chart shown in Chapter 12: Myths and Mistakes and as demonstrated, the amount of effort required for the arc flash study is less than 10% of the total. That means that the vast majority of information is what most engineers who have responsibilities for an electrical installation will choose to keep. I would certainly feel uncomfortable as a duty holder if I did not have these records in place.

15.7 Design problems are averted.

I describe in Chapter 12: Myths and Mistakes a fault level study, protection coordination study and an arc flash study that I carried out for a large teaching hospital about 10 years ago. I uncovered a design flaw where 2 MVA voltage optimisation units had been installed into the busbars between the two main transformers and the main LV switchboards for the critical care block. The additional dynamic impedance meant that in the event of an arcing fault, protection arrangements would not operate, and also downstream protection could be compromised as a result. The results were frankly quite startling, and I calculated that the incident energy at the low voltage switchboards could be as much as 73 cal/cm2 against 5.1 cal/cm2 for not having the units in service. Because of this and also the vulnerability of the patients in this unit, the units were removed at great expense. To have carried out the study at design stage, would have revealed this but since only zero impedance faults are considered in present building services standards, an opportunity was missed.