Over the years I have come up against this problem about 5-6 times. In 50% of the times I have been castigated for even bringing up the question, however when doing HSE commissioning I have never had a problem. I have “witnessed” 2 deaths due to arc-flash, one in South Africa and one abroad. Although it is primarily an electrical engineering function, the HSE Manager also has a role to play. I am not an expert yet, understanding the basics will help the HSE Manager – A simple explanation is offered below – What is arc flash?
Arc Flash is the result of a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. During an arc fault the air is the conductor. Arc faults are generally limited to systems where the bus voltage is in excess of 120 volts. Lower voltage levels normally will not sustain an arc. An arc fault is similar to the arc obtained during electric welding and the fault has to be manually started by something creating the path of conduction or a failure such as a breakdown in insulation. The cause of the short normally burns away during the initial flash and the arc fault is then sustained by the establishment of a highly-conductive plasma. The plasma will conduct as much energy as is available and is only limited by the impedance of the arc.
This massive energy discharge burns the bus bars, vaporizing the copper and thus causing an explosive volumetric increase, the arc blast, conservatively estimated, as an expansion of 40,000 to 1. This fiery explosion devastates everything in its path, creating deadly shrapnel as it dissipates. The arc fault current is usually much less than the available bolted fault current and below the rating of circuit breakers. Unless these devices have been selected to handle the arc fault condition, they will not trip and the full force of an arc flash will occur. The electrical equation for energy is volts x current x time.
The transition from arc fault to arc flash takes a finite time, increasing in intensity as the pressure wave develops. The challenge is to sense the arc fault current and shut off the voltage in a timely manner before it develops into a serious arc flash condition.
Calculating this energy is the key to control measures, software is available online, and some is needed to be purchased whilst other software is free. The HSE professional requires working within the constraints of arc flash procedures and the calculation results. Not many industries have specific procedures for these events. I normally refer to the OSHA Regulations for Arc Flash – NFPA-70E. All companies should have an arc flash analysis if this is identified in the Company Baseline Risk Assessment and done by a professional.
NFPA 70E defines arc flash hazard as “a dangerous condition associated with the possible release of (thermal) energy caused by an electric arc.” NFPA, NESCT, CSAT Z462, MSHA, OSHA and IEEET all deal with arc flash
Depending on the calculated value determines what PPE should be provided for the persons who are performing the work.
This however is not the end of the standard, it includes labelling and signage, procedures and limitations, boundary determination, see examples below:
The effect on the worker is dependent on the controls put in place.
1.2 cal/cm2 is considered the threshold for acurable (second-degree) burn.
The above is therefore our starting point, and a series of engineering solutions is possible however we cannot ever exceed a value of 40 cal/cm2. Any value above this and special measures such as remote switching needs to take place. Abroad this is now the most popular method, although in developing countries the old switching method still takes place.
This article is focused on the basics and is intended to highlight the hazards that may be in your organisation. When you next do your RISK Assessment, take your electrical engineer with you. You may be surprised.