Myths & Mistakes
Writing this chapter has allowed me to draw on my own practical experiences to reinforce my belief that electrical engineers, whether involved in design or putting people to work on high-power systems, must consider the effects of arcing. With all the advances in predictive techniques, it is really not a defence to be unaware of the severity of arcing faults should they occur. We must take t into account not only in designing protection for workers but also for designing protection for electrical systems which are increasingly more complex and subsequently the losses are considerably greater.
12.1 Arc Flash is much more than PPE.
The arc flash debate has been polarised around the need for personal protection and, in the UK especially, the aversion to PPE has sometimes deflected attention away from the true losses due to arcing faults. The following is an account of an arc flash that did not cause personal injury and, to my knowledge, did not result in any subsequent serious illness but did result in huge losses which could have been much worse.
One Sunday afternoon in July, some years ago, the Chief Engineer of a large steel rolling mill called me to say that there had been a serious explosion in one of the high voltage substations. I immediately went out there to see if I could be of any assistance. On arrival the subsequent fire was already being suppressed by the fire brigade, so I started to assess the damage. It became apparent to me that the cause of the fire was a large 11,000 Volt capacitor which was insulated by polychlorinated biphenyls (PCB) and was surrounded in turn by mineral insulating oil. Polychlorinated biphenyl was used for many years as an electrical insulator and is now banned because it is a non-biodegradable poison. There had been an electrical flashover inside the capacitor which punched a hole through the inner envelope of liquid PCB and then through the steel tank holding a large quantity of oil. This ignited the oil and surrounding building fabric which severely damaged adjacent switchgear and transformers putting the whole substation out of action. Although the fire had been suppressed, I was concerned about the hazard that the fire brigade would have been exposed to due to the burning of PCB. I knew that PCBs could give off dangerous cancer-causing dioxins and that is why PCB filled equipment could only be disposed of at licensed waste sites where the temperature of incineration was extremely critical. As a result, the fire crews were dispatched to the nearby hospital to receive health checks and tests.
Unfortunately, the crisis did not end there. The weather had been unseasonal, and it was raining heavily. It was drawn to my attention that the oil together with the PCB was being washed into a nearby lagoon and was forming a slick on top of the water which was only hours away from being discharged into the river Don in Rotherham. Fortunately, I was able to summon a specialist treatment firm to dispatch a tanker to capture the slick and thus averted what would have been a very serious and costly environmental incident. The damage from the incident ran into hundreds of thousands of pounds and that was not counting the production losses. As it happened, my client closed the plant early for the summer holidays which gave me some time to organise emergency repairs to get the plant up and running again.
12.2 “You have to be a maths genius in order to carry out arc flash calculations!”
Somebody said that to me some 14 years ago. At that time, the arc flash calculations were no more complex than the adiabatic equations needed to work out cable dimensions. It is true that the latest version of IEEE 1584 does carry equations that on the face of it do look a little scary as some have up to 13 correction factors. The abundance of correction factors is due to the fact that the equations are empirically derived, in other words they are curve fitting equations from actual arc flash explosions in the laboratory. The fact is, I am no maths genius but have been quite comfortable carrying out longhand calculations for arcing current, incident energy and arc flash boundaries. There is no calculus involved and the height of complexity of the equations probably involves the log button on your calculator, you know, the one that has not been pressed in a while. The engineer in me says that by being able to understand the formula could make sure that I will not be blindly stabbing numbers into software packages and just trusting in the output. The IEEE 1584 Guide for Performing Arc Flash Calculations carries all the calculations and also useful worked examples. Jim Phillips provides online classes to carry out IEEE 1584 arc flash hazard calculations and has produced really simplified spreadsheets which will breakdown the calculations into small chunks and will aid understanding of the process.
The simple calculators that are available with this guide will allow you to arrive at an accurate assessment of the severity of the hazard, arc flash hazard boundaries and levels of PPE if required for the task. They also allow the calculation of fault levels and take into account the contributions from induction motors.