2.4 Which Factors affect the Severity of an Arc Flash?

An understanding of factors which will affect the severity of an arc flash is the key to the management of the phenomena. The distance from the arc, the arc duration and arcing current all have an effect on the severity of the arc flash in terms of incident energy but also determine the severity of ultraviolet radiation, sound pressure and blast pressure. The following describes, in simple terms, the relationship between each one of these factors.

2.4.1 Time and Current

Incident energy is directly proportional to arcing current and also the duration of the arc. In other words, if we double the arcing current then the incident energy will also double providing the arc duration stays the same. If we halve arcing current, then the incident energy will also reduce by a half.

Exactly the same thing happens with the duration of the arc and by reducing the time to trip by a half will halve the incident energy. Whilst it is not that easy to attenuate the arcing current in most cases, the arc duration can be limited by faster disconnection of protective devices. This is the one very commonplace answer to reducing incident energy and therefore severity.

It may be worth mentioning here that higher fault levels may lead to faster disconnection of a protective device which can lead to a reduction of incident energy. This phenomenon, which I first identified and have referred to for many years as the “fault level paradox” will be explored in more detail in later chapters.

2.4.2 Arc in an Enclosure

When an arc occurs within an enclosure, the emission from the arc is focused outwards towards the operator. There is more likelihood that the directed effect of heat, but also the effect of hot metal particles and splashes that accompany actual electric arc faults may cause greater injury than an arc in open air. Furthermore, the size of the enclosure has an effect as well. Calculations of incident energy take into account not only whether the arc is in an enclosure or in open air, but also the dimensions of an enclosure.

2.4.3 Conductor geometry

Current studies and standards have also taken into account the configuration of the electrodes. The intensity of an electrical arc and the resultant level of incident energy is affected by the conductor geometry or electrode configuration. For instance, horizontal versus vertical conductors, open air versus enclosed and also how the conductors terminate. As we explore the topic of predicting incident energy, there will be more detail about how to apply electrode configuration in a practical setting.

2.5 Blast pressure, sound attenuation & distance

As mentioned previously, there could also be an explosive force called arc blast which can be responsible for blunt force injuries. (There are a few myths around the subject of arc blast which will be explored in more detail later) Increased distance has the effect of attenuation of the blast pressure wave and according to theoretical formulae developed by Ralph Lee in 1987 the pressure is inversely proportional to the distance to the power of 0.9. In other words, to double the distance will roughly halve the blast pressure.

According to the same formulae, the blast pressure is directly proportional to the arcing current. Therefore, a doubling of the arc current will result in a doubling of blast pressure.

Although there is much debate about how Lee’s theoretical formulae can be interpreted in the real world, I have first-hand knowledge of where the ballistic effects of arc flash have caused tremendous damage and potential for injury. In addition, the fact that doors are closed on equipment will most certainly affect the likelihood of an arc flash event but will not guarantee reduced injury as closed doors will not contain the blast unless the equipment is arc protected. Arc protected equipment is discussed in Chapter 5: Prevention.

The accompanying sound pressure wave can cause permanent deafness through the rupturing of eardrums. It is sometimes incorrectly assumed that damaging sound pressure decreases as an inverse square of distance in the same way as sound intensity. In fact, it approximates to being inversely proportional meaning that a doubling of distance will result in a halving of sound pressure.

The intensity of any illumination declines as the inverse of the square of the distance. Twice as far 1/4 as strong three times as far 1/9 as strong etc. If there is smoke or dust suspended in the air it would be even faster, but these are things you would not want to count on.

Learning Points
• It is very easy for an arc to be initiated when working on energised equipment.
• It cannot be taken for granted that the electrical protection will limit the damage caused.
• The consequences of arcing incidents are far greater than personal injury.