The likelihood of an arc flash/arc blast event is dependent upon the type of activity being undertaken at the time. It is generally accepted that the greatest likelihood occurs when there is direct interaction by an individual on energised equipment such as a switching or testing activity. After such activities are completed (i.e. isolated), the residual risk of an arc flash/arc blast event is significantly lower.
However, there are some circumstances where indirect events may pose an unacceptable risk to an individual. Such events may occur randomly and without direct interaction by an individual; such as that caused by insulation/thermal failure, moisture/rodent ingress, and failure of a drive during start-up or over-voltage due to a lightning strike as examples. These types of events whilst considered rare, could affect operators, technicians or passers-by who may be in the vicinity of energised equipment at the time of the arc flash/arc blast event.
A risk assessment must be undertaken to determine the appropriate arc flash/arc blast PPE for all credible scenarios by considering the activities other factors that may increase the likelihood of an arc flash/arc blast event: that is consider both direct and indirect exposure risks. The risk assessment must:
- determine if an arc flash/arc blast hazard exists by considering the equipment integrity (properly installed, maintained, equipment doors and covers closed and secured, no evidence of impending failure);
- determine safety-related work practices taking into account the proposed activities;
- identify the incident energy, arc flash/arc blast boundary; and
- nominate the PPE to be used within the arc flash/arc blast boundary.
The risk of arc flash/arc blast can be reduced through:
- lowering potential fault currents where possible (i.e. opening bus ties);
- lowering switching currents where possible (i.e. load shedding); and
- minimising incident energy levels where possible (i.e. temporary protection setting changes).
When conducting an arc flash/arc blast risk assessment, the factors which should be taken into consideration include:
- prospective fault current, and the fault clearing time in the event of an arcing fault;
- calculated incident energy level;
- switchboard/equipment age, construction and maintenance history;
- the nature of the task being undertaken; and
- the frequency of the exposure and the potential severity of the consequences.
As a guide, the following tables have been developed to separate typical activities into two classes; high exposure (activities directly involving the primary energised circuit), and low exposure (activities involving non-primary or secondary circuits).