In the simplest of terms an arc flash is when electricity is discharged from a source, causing an explosion, ionisation of the air and severely high temperatures, and if people are exposed to it serious injury and even fatality.
Some basic definitions
An electrical arc is a bridge formed between two electrodes. In other words, electricity is discharged through a fault in a system and ‘arcs’ between the source and another, or goes to ground, which can be through a human being. An arc flash is when a fault in an electrical system causes a discharge of unwanted electricity which ionises the air and allows the current to flow through it.
Causes of arc flash
The causes of arc flash can be simple human errors such as dropping tools into the wrong places in a system. Another cause could be the failure of insulation through age resulting in degradation, or other improper maintenance which could also be another cause.
If insects and rodents or other small animals can gain entry to a system and hardware, they could also cause arc flash as they provide an alternative root for current, as can dust and erosion of materials, and can cause faulty connections.
Generally a system running at around 480 volts or above is required to create a possibility of an arc flash. In general terms, the higher the voltage used the higher the risk. This is why power generating equipment is more susceptible to producing arc flash than equipment of lower voltage output, but this is also susceptible.
As the electrical discharge causes ionisation of the air, normally a non conductive gas, the discharge is able to travel through the air which then acts as a conductor.
An arc flash ionises the air around it and the temperatures that can be generated can reach an astonishing 35,000 degrees Fahrenheit. Such severe temperatures can obviously burn human skin and clothing from considerable distances away. An arc flash also set light to flammable materials in the vicinity and can even melt copper, aluminium and even steel components in the vicinity. As the materials expand as they vaporise they produce explosive pressure and sound waves which can severely damage the human ear.
The explosive power of an arc flash, more commonly called the arc blast, will obviously have many more effects than directly injuring workers and those nearby; just as with any other explosion the structural integrity of the immediate area and building can be compromised, causing further injuries and objects that are displaced and turned into projectiles can clearly cause damage and injury. As metal is turned molten it can also be hurled considerable distances presenting a further hazard. The forces involved in an arc flash can exceed 100 KiloPascal and project debris to distances of up to 300 metres.
Far infrared to ultraviolet radiation is generated and the effects of this can be extreme on humans and surroundings as they absorb the energy.
Protecting against arc flash
Regular thorough inspection and maintenance programs can mitigate in favour of reduced risks of arc flash. Additionally, ongoing training and awareness programs for workers can ensure that they are best informed to carry out work in a safe and appropriate manner bearing in mind potential risks.
Protective clothing that can withstand the effects of arc flash is available.
When choosing protective equipment there are several factors to be taken into consideration and these will include:
• The phase/ground rated voltage at the site
• Calculated distance between arc source and receptive surface
• The location where the arc may be produced
• The maximum fault current value
• The AC cycle number and the mono of three-phase circuit type
There will be other factors but it can be seen that it is possible to reduce the possibility of arc flash as well as provide effective protective clothing for those who may be working in an environment where arc flash can occur.