An arc flash incident can be avoided by understanding their causes and taking steps to reduce the risk of initiating an arcing fault. The various causes of arc flash are summarized below and the mitigation measures are described in this chapter. An arc-flash risk assessment procedure requires that hazards are identified, risks are assessed, and risk control is implemented based on the hierarchy of risk control methods described in Informative Annex F of NFPA-70E.
An example of the hierarchy of methods in order of effectiveness is shown in Figure 5.1:
Preventive Electrical Maintenance
Preventive electrical maintenance practices are employed in most companies that require high reliability and safety on their electrical power supply and for process continuity. Preventive maintenance also provides for a safer workplace. Consider improving maintenance procedures for inspections, performing preventive maintenance, or performing breakdown maintenance, by including procedures that address the arc flash hazards with the use of personal protective equipment (ppe). This can reduce the overall cost of implementing an arc flash program and your electrical safety program. Also see NFPA 70B Recommended Practice for Electrical Equipment Maintenance for reference.
The following list summarizes some of the common causes of arc flash and suggests mitigation measures that could be used.
• Dust, impurities, and corrosion at contact surfaces that produce heat, loosen contacts, and create sparks.
• Sparks produced during the racking of breakers, replacement of fuses, or when breakers or fuses close into faulted lines.
• Failure of insulating materials.
• Snapping of leads at connections due to force—human, rodents, or birds.
• Accidental touching and dropping of tools, nuts or bolts, or metal parts.
• Rodents and birds entering panels and switchgear are common initiators of arcing faults. These can lead to short circuits that often result in arc flash. This risk can be reduced by closing all open areas of equipment with wire net or sealant so that rodents and birds cannot enter.
• Corrosion can lead to the snapping of small wires, which in turn may create sparks and fumes when the tip of the wire hits the metal enclosure or another phase conductor. Check for corroded terminals and parts regularly if the electrical equipment is located in harsh environments. Use corrosion-resistant terminals.
• Heating of cable insulation can damage the insulation—another cause of flashover. Check for loose connections and overheated terminals. Infra-red thermography and ultrasonic inspections can provide valuable data on poor connections and overheated electrical conductors or terminations.
• Insulate exposed energized parts to the extent feasible. Insulation prevents arcing. For example, if a worker drops an uninsulated wrench that touches the bare bus bars of two phases, a short circuit will occur. However, this does not happen if the wrench or the bus bar is insulated.
• Perform acceptance and maintenance testing of relays and breakers to verify proper operation. Arc flash incident energy calculations are based on the proper operation of the upstream protective device. If this upstream device should fail to operate properly, the arc energy released during an arcing fault could be many times the calculated value. The frequency of testing varies depending on the type of equipment, service conditions, and equipment age. Consult the NETA recommendations and the manufacturers’ recommendations to develop a testing schedule. Also review NFPA-70B – Recommended Practice for Electrical Equipment Maintenance. Verify that all protective devices are being operated within their short circuit ratings. If a circuit breaker or fuse is subject to more fault current than it is rated to interrupt, it cannot be relied upon to safely clear a downstream fault. In addition, the device itself could fail and create an arcing fault when attempting to clear a high magnitude fault.
• Pitting of contacts can occur when breakers and fuses are operated. For devices with visible contacts, replace contacts when pitting is noticed.
When a protective device trips or a fuse melts, make sure that the cause of the fault has been found and cleared before re-energizing the circuit. While it is
common practice to try to immediately re-energize a circuit after a fault since many trips are “nuisance” trips, this is a potentially dangerous practice that
should be discouraged. Closing back into a fault can produce an arc flash and equipment damage.
• Check for water, excessive moisture, or ice on insulating surfaces of equipment. This may cause flashover, especially on high-voltage equipment.