When working, testing or fault finding on energised electrical equipment, a fault current of up to 20 times the rated current of the supply transformer can flow for short duration during fault conditions. You should only perform work tasks that you are trained to perform.
Arcs can have the energy to cause an explosion and/or melt metallic switchboard cubicles and equipment. Arcs may cause severe burns to the skin and flash burns to the face and eyes. Inhaled hot gases and molten particles can cause serious internal burns to the throat and lungs. Injury can also occur through the impact from flying debris and dislodged components. Circuit protection devices may not operate in such circumstances.
Arc flash can be created by a short circuit between two high current conductors, for example by dropping a screw driver across two bus bars operating at 40kA. The energy released not only vaporises the screwdriver, but forms a gaseous plasma that conducts electricity for a short period; typically less than a second. The temperature of the plasma can exceed 6,000oC. Close exposure to an arc flash is catastrophic.
Arc flashes are commonly associated with work around meter boards, e.g. when racking circuit breakers in or out. Arc flash can occur when switching or fault finding in circuits with high potential fault currents. The risk is the production of an arc flash which may produce:
- Plasma (super-heated ionized gas at over 6,000oC)
- Molten metal, droplets, spray
- Metal vapor
- Debris
- Pressure waves – typical when arcing occurs in enclosed switchgear, where pressure builds leading to sudden released.
- Anyone in close proximity to the arc flash will be exposed to incident energy that could lead to severe burns or death.
Note – arcs developed from low voltage conductors generally have longer clearing times that HV arcs and can therefore deliver more heat – i.e. longer exposure to the heat.
Additional protective clothing should be worn by electrical workers if working on or near energised electrical equipment. This includes tasks such as fault-finding, supply verification and testing on low voltage equipment. The protective equipment shall be selected in accordance with a risk assessment and guidelines of Managing Electrical Risks in the Workplace, AS 4836: (Safe Working on or Near Low Voltage Electrical Installation) and ENA NENS 09-2014 National Guideline for the Selection, Use and Maintenance of Personal Protective Equipment for Electrical Arc Hazards.
Selection of PPE
There are a number of different ways to select the correct arc flash PPE. A simple and quick way is to make this assumption of fault levels. By using the kA rating of the main circuit breaker in the switchboard that you are working. The value of the kA rating determines how much current the circuit breaker can withstand under fault conditions. The circuit breaker only has to withstand this for a brief period, usually the time it takes for the circuit breaker to trip. For example, a value of 6kA means that the circuit breaker can withstand 6,000 amps of current during the brief time it takes to trip.
Line up your kA rating with the most appropriate row in the table below and utilize the recommended PPE. This table will also provide an estimate of incident energy (Cal/cm2). This can be used to determine minimal Arc Thermal Performance Value (ATPV). Your ATPV value needs to be greater than your estimated Cal/cm2
Under Garments
These are not considered as providing protection. They can be wool or cotton, but not synthetic fibres which could melt/burn and add to injury in the event of an arc flash.
Base Garments
These are long sleeved shirts and trousers, or coveralls. They should have a minimum ATPV of 5.6 cal/cm2
- Flame-retardant clothing covering the full body (including legs and arms) worn by electrical workers working on or near exposed energized conductors or live conductive parts
- Arc rated fabrics self-extinguish when exposed to an arc event. Preferably, fabrics that are inherently flame resistant – performance cannot be washed out or worn away over time.
- Shirts manufactured with a ‘closed front’
- Where button are used on shirt openings, they should be covered by a placket of the shirt material (known as a fly front design)
- The sleeve cuff should be designed to include a full gusset
- Retro-reflective materials used on garments should not contain conductive materials and should comply with the requirements of AS/NZS 1906.4.
- Labelling should include the APTV including the unit of measure
Over Garments
These are worn over base garments and provide added protection in the event of an arc flash. They include:
- Coveralls
- Switching jacket
- Switching coat
- Leggings
- Overpants
- Bib and brace coveralls
Gloves
Insulating gloves may be worn under arc rated gloves. Alternatively, composite gloves may be worn
Eye, face and head protection
Arc rated face shield with chin cup or shroud should be worn if the potential incident energy is over 3 cal/cm2