Guide to Arc Flash Analysis

ELECTRICAL SAFETY PROGRAM 
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. 

WHS Regulation 158 and 161 both states that if electrical work is to be carried out on energised electrical equipment an electrician must ensure that risk assessment is carried out by a competent person who has tools, testing equipment and PPE that are suitable for the work and have been properly tested and are maintained. 

To clarify what work on energised electrical equipment means, it is any task that includes the following:

  • Isolating

  • Switching

  • Removing fuses or links

  • Isolation verification (testing for dead)

  • Testing

  • Fault finding

  • “Live” work

This is work that electricians do every day. Having a “No Live Work” Policy cannot rule out all the circumstances when electrical hazards exist in the electricians environment. If you employ electricians, you cannot say that they never work live.

A risk assessment involves considering what could happen if someone is exposed to a hazard and the likelihood of it happening. Risks associated with electrical work may arise from the properties of electricity. Electricity is particularly hazardous because electrical currents are not visible and do not have any smell or sound. The risk assessment should also consider how and where the electrical work is carried out. Electrical work may be carried out in difficult conditions, including in wet weather conditions, confined spaces and in atmospheres that present a risk to health and safety from fire or explosion.

It is becoming more and more common for larger transformers to be used to power domestic installations. This has created a situation where the short circuit currents are much higher than they used to be. A typical transformer to power a street of houses may have been 100kVA. Now it is not uncommon to see 300kVA or even 500kVA transformers. This can result in short circuit currents of 20kA and above. To make matters worse, a number of well known manufacturers sell cheap low quality circuit breakers that have a rating of only 3kA. Electricians are continuing to use these without understanding the full ramifications of their decision.

The proliferation of solar panels, energy storage and uninterrupted power supplies (UPS) used in industrial, agricultural, commercial building and even the residential sector increases the risk of exposure to safety hazards that are inherently associated to nonlinear power sources. With more complicated systems being installed increasing the likelihood and severity of an electrical arc flash the need for a systematic approach to managing occupational health and safety in the electricity industry is needed more than ever. 

All electrical contractors and businesses should have in place an Electrical Safety Program which includes training, procedures, electrical risk assessments, appropriate rated PPE and the right tools and equipment to do the task. Clothing should be flame resistant. Do not settle for minimal performance, make sure you do your homework and have options that meet the needs of work. Components of an Electrical Safety Program include:

 

  • Defining safe work practices and use requirements for all people who work with electrically energized equipment as part of their normal job / research duties. 

  • Establishing training requirements for "qualifying" and "authorising" persons who work on or near energized electrical circuits and components, and establishing "qualification" requirements for electrical contractors. 

  • Establishing a process for evaluating and calculating the hazards of every potentially energized electrical work task and for determining appropriate hazard controls. 

  • Establishing a formal process for controlling energized electrical work through an assessment and documented "energized work" process. 

  • Chartering an "Electrical Safety Committee" to oversee work practices and procedures. 

  • Consulting with electrical safety experts at NECA, Safety Regulators and Network providers.

 

A good risk assessment process will include a calculation of incident energy which is used for the selection of PPE and controls. Most electricians are daunted by the prospect of an Arc Flash calculation and normally bypass the important risk assessment procedure because they have never been trained on how to undertake it. Sure, to get super accurate results you probably need to have an engineering degree and some complex software. However, a basic calculation to determine a safety control is much easier to complete.  

Calculating incident energy also has other benefits, it helps identify one of the most common design faults found in electrical circuits by distinguishing incorrectly sized circuit breaker fault current ratings (commonly called the kA rating). You can also use this calculation to know when to implement other traditional controls such as safety observers, low voltage rescue, insulation mats etc. 

Once it is determined that PPE is required, choosing the PPE that is best for the situation seems like a daunting task. Working around electrical equipment ranging from 100-amp residential panels to 4,000- amp commercial and industrial switchgear offers a wide range of available fault current that could lead to injury in the event of an arc flash or electric shock. 

Although this type of basic calculation is useful as a guide to select PPE where there is no other information available, it is clear that assumptions will have to be made and may not necessarily be applicable to the systems you may be analysing. As such, they will often lead to over classification of protective gear for work on a specific piece of equipment. However, it is of even more concern that there is a potential for under classification of the protective gear recommended for work on a specific piece of equipment.

Therefore this advice should be considered as guidance only and the table below should not be blindly accepted.

1. Time of Day
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. 

2. 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. 

3. Make an Assumption about Fault Levels
Because the process of calculating prospective fault current requires the testing of energized circuits, it is important that sufficient PPE be in use before this test is undertaken, but without the calculated value of incident energy, you will need to make an assumption. To err on the side of caution you should over classify your level of protective gear for the initial test. Once the calculation is complete, you will be able to predict the level protective gear actually needed.

To make this assumption use 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. 

If this is still not available, you can assume 12kA for single domestic dwellings or 40kA for all other commercial, industrial or residential units. 

4. Select your PPE
Line up your kA rating with the most appropriate row in the table below and utilise the recommended PPE.

5. Test for Prospective Short Circuit Current
Perform a short-circuit analysis at the most upstream point of the exposed equipment being worked on or the switchboard— 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.

A Prospective Short Circuit (PSC) or Fault

 

6. Calculate Incident Energy

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 below: -

7. Add a Safety Factor 

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. 

8. Repeat Step 4 to Select Your PPE
Repeat step 4 but now use the appropriate Cal/cm2 column instead of the kA column 

9. Document your findings
Assemble a report including floor plans, calculation, risk assessment and provide to client and keep on file for length of job or 5 years. 

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.  

Sample Arc Flash & Shock Hazard Appropriate PPE Required label - ready to download HERE

For a copy of this document, CLICK HERE.

 

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