Arc Flash Protection

Introduction

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.

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 5,000 Amps. 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 6,000^oC. 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.

References

Managing Electrical Risks in the Workplace Code of Practice,

AS 4836:2011 section 9 (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.

NECA NSW Clothing policy

NECASafe Electrical safety training

Possible hazards and risks

The Hazard

Exposed conductors with high potential fault currents. Found on the supply of switchboards and elsewhere. These may be high, low or extra low voltage.

Pathway

A conductor, such as a wire, tool, etc. short circuits two conductors.

Risk

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.

Procedure

Set up administrative controls

Ensure that the area is clear of all obstacles, and that signage/barriers are in place to prevent other people from entering the area. Where possible, insulate any exposed conductors.

A first aid kit must be available on site.  The kit must contain items that are appropriate for treating burns.

Arrange for an observer to be present on site when the work is carried out. The observer should be trained in rescue procedures and in the administration of first aid – particularly in relation to burns.

Appropriate firefighting equipment must be available on site.

1.      Site Walkabout

Because conditions may have changed since the original installation, it is critical that the existing conditions be field verified to ensure that the arc flash analysis is performed using accurate breaker settings and field conditions. Start by walking around the site identifying locations of transformers or substations and their proximity to the equipment to be worked on. Also, look for solar array systems, inverters, battery backup/storage and UPS equipment. Include power floor plans showing locations of electrical equipment, and single-line diagrams indicating the overcurrent protective devices and cable sizes for all relevant areas.

2.             Make an Assumption about Fault Levels

To make this assumption of fault levels use the kA rating of the main circuit breaker in the switchboard that you are working and the information you collected in step 1.  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.

If this information is not available, you can assume 12kA for single domestic dwellings or 40kA for all other commercial, industrial or residential units. To err on the side of caution you should over classify your level of protective gear

3.             Select your PPE

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

4.             Calculate Incident energy (step 4 – 9 optional)

Because the process of calculating prospective fault current requires the testing of energized circuits, this step is optional. If you believe that the controls and PPE that have been put in place are sufficient and practical then you can skip to step 10.

It is important that sufficient PPE from the above table be in use before this test is undertaken. Once the calculation is complete, you will be able to predict the level protective gear actually needed.

5.             Time of Day (step 4 – 9 optional)

The time of day is important when undertaking this risk assessment, you should pick a time when the property is at full load and solar array systems are producing the most wattage.

6.             Test for Prospective Short Circuit Current (step 4 – 9 optional)

Perform a short-circuit analysis at the most upstream point of the exposed equipment being worked on or the switch board— The prospective short circuit (PSC) or fault is the current that would flow in the circuit if no circuit protection operated and a complete short circuit occurred. The supply voltage and the impedance of the path taken by the fault current determine the value of this fault current. Measurement of PSC can be used to check that protective devices within the system will operate within safety limits and as per the safe design of the installation. The kA rating of a circuit breaker is a very important safety aspect to consider when designing a circuit. Without it, there is a good chance that a serious accident will occur. PSC is also used to determine Incident Energy (IE), which is the purpose of this test. Ensure you follow the instructions of your testing device to obtain the PSC.

7.             Calculate Incident Energy (step 4 – 9 optional)

IE of an arc flash is dependent upon the length of the flash, the available PSC, and inversely (and exponentially) related to one’s distance from the origin of the flash.

NENS 09 proposes the following formulas to calculate the incident energy likely to be developed 450mm directly in front of the conductors, where:

IE:           Incident energy (cal/cm2)

t:              Fault duration (sec) – this is typically 0.1 for molded circuit breakers, 0.4 for fuse protected equipment and 0.5 seconds for HV – check the fuse clearing time for the circuit involved.

r:              Distance from arc (metres) this is typically 450mm

Irms:       three phase fault current (amps).

The formulas are:

•     Copper electrodes: IE = 3.8 x 10-4 x t x Irms1.12/r2

•     Aluminium electrodes: IE = 4.4 x 10-4 x t x Irms1.12/r2

If this seems to hard, you can complete this calculation using the NECA incident energy calculator.

8.             Add a Safety Factor (step 4 – 9 optional)

Because variation in the PSC value due to the time of day and connected load, it is important to add an additional safety factor to your target score. This safety value will also compensate any changes to the field conditions as maintenance and upgrades occur during the year. By adding 1.2 Cal/cm2 to the calculated value should be sufficient to compensate for any variation for your target safety/PPE value. If you are using the NECA incident calculator this will automatically be calculated. This new value is known as the ‘Minimum Arc Rating Protection’ value. PPE protection must be greater than this value.

9.             Repeat Step 3 to Select Your PPE (step 4 – 9 optional)

Repeat step 4 but now use the appropriate Cal/cm2 column instead of the kA column

10.          Label the board

Label the board / equipment that you were working in. Labeling will help the next time you work from that board or help other electricians with their PPE choices. You can download a ready to print label from the NECA NSW website.

Selection of PPE / Uniform

Base Garments

These are long sleeved shirts and trousers, or coveralls. They should have a minimum ATPV of 4 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

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.

Arc Rated Outer 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 must be worn if the potential IE is over 3 cal/cm2

Helmets

Type 1 helmets that comply with clause 4,4 of AS/NZS 1801.

Footwear

Footwear will comply with AS/NZS 2210.1.