Risk Assessment
Exact statistics about arc flash accidents are not available. Still it is possible to make an estimate when looking at all electrical accidents in the Netherlands. News reports about work injuries involving electricity often note that burns are part of the injuries. It is likely that arc flash accidents make up a significant part of electrical accidents resulting in heavy injuries or death.
Assuming 100.000 employees who are exposed to electrical risks in their work, 1 death and 10 heavy injuries per year gives the following average accident rate per year:
| Consequence category | Netherlands average accident rate per year | Acceptable accident rate per year |
|---|---|---|
| Multiple loss of life | 1x every 10 years (10-6) | 1x every 10 years (10-6) |
| Loss of a single life | 1x per year (10-5) | 1x per year (10-5) |
| Major injuries with lasting damage | 10x per year (10-4) | 10x per year (10-4) |
At first sight there doesn’t seem to be a pressing reason for taking measures to reduce arc flash risk, the accident rate is the same as the acceptable rate after all. But this is an average rate of all employees who are exposed to electrical risks in their work. Any situations with an above-average chance of accidents would still require measures.
As an example, work activities such as putting large distribution equipment in an electrically safe state and reenergizing are in some organizations performed by a small group of employees. Contractors and other employees will only start working after the equipment has been released. This small group of employees will be exposed to a much larger risk than the average electrically qualified worker.
Risk Evaluation Method
Both NFPA 70E and EN 50110 require arc flash risk to be evaluated but do not provide a method. One possible method is using a risk matrix. In a typical matrix the chance and effect of an arc flash accident are combined to obtain a risk classification that tells if the current risk level is acceptable or risk reduction measures are required.
A risk matrix specifically for arc flash is shown below, the acceptable accident rate has been indicated with a *.
| 10-6 | 10-5 | 10-4 | 10-3 | 10-2 | |
|---|---|---|---|---|---|
| Multiple loss of life | * | ||||
| Loss of a single life | * | ||||
| Major injuries with lasting damage | * | ||||
| Major injuries without lasting damage | |||||
| Minor injuries |
Classification of Consequence Severity
To determine arc hazard level the IEEE Standard 1584 can be used to calculate the incident energy and arc flash boundary. The calculation takes the short-circuit current, protective device characteristics and equipment design as input.
| Incident Energy | Consequence Severity |
|---|---|
| Less than 1.2 cal/cm² | Minor injuries |
| 1.2 – 12 cal/cm² | Loss of a single life |
| More than 12 cal/cm² | Multiple loss of life |
Classification of Likelihood of Occurrence
The likelihood of occurrence for arc flash will depend on the task that’s performed as well as other factors such as equipment state. NFPA 70E table 130.5(C) can be used as a guide for determining the likelihood of certain activities.
| Task Description | Likelihood of Occurence |
|---|---|
| Inspection and opening panel doors that does not lead to exposed conductors. | 10-6 |
| No contact inspection tasks like infrared thermography. | 10-5 |
| Any activity near potentially energized exposed conductors, including opening doors, removal of barriers and voltage testing. | 10-4 |
Risk Evaluation
Using the tables above we obtain the following risk classes for electrical arcs:
| < 1.2 cal/cm² | 1.2 - 12 cal/cm² | > 12 cal/cm² | |
|---|---|---|---|
| Inspection and opening panel doors that does not lead to exposed conductors. | |||
| No contact inspection tasks like infrared thermography. | |||
| Any activity near potentially energized exposed conductors, including opening doors, removal of barriers and voltage testing. |
- Red: Undesirable: tolerable only if risk reduction is impracticable or if the costs are grossly disproportionate to the improvement gained.
- Yellow: Tolerable if the cost of risk reduction would exceed the improvement.
- Green: Acceptable as it stands, though it may need to be monitored.
