Arc Energy Reduction Considerations

Protecting workers from the hazards of electric shock has been understood for decades. Recently though, more attention has been given to the role electrical equipment can play in minimizing arc flash hazards. The energy exposure of an arc flash incident can be significantly reduced when attention is given to the type of equipment specified and where it is installed in an electrical system.

The 2014 edition of the National Electrical Code® (NEC®) addresses arc energy reduction directly in Section 240.87. Previously, the 2011 edition of the NEC® required arc energy reduction whenever a circuit breaker did not have an instantaneous trip function. This would apply to a circuit breaker with no instantaneous trip function at all, but questions lingered about other applications.

Fig-1.gifThis was cleared up in the 2014 edition of the NEC®. The updated code requires arc energy reduction where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher (see Figure 1 at right). This means that even though an electronic trip circuit breaker with a 1200 A sensor has its current rating switch set to, for example, 0.5 (600 A), it will still need an arc energy reduction means (see Figure 2 below). If the overcurrent device in the circuit breaker meets this criteria, then documentation and a method to reduce the clearing time must be provided.

Fig-2.gifAPPROACHES FOR LOWERING CLEARING TIME

To see the connection between clearing time and arc energy reduction, we need to understand incident energy. Incident energy, as defined in NFPA 70E, Standard for Electrical Safety in the Workplace®, is “the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per centimeter squared (cal/cm2).” A method used to calculate incident energy may be found in IEEE 1584, Guide for Performing Arc Flash Hazard Calculations.

Incident energy is a function of current and time. If the arcing time is reduced, then the incident energy will be reduced. Clearing time refers to the length of time necessary for an overcurrent protective device to completely extinguish the arc.

There are five clearing time reduction means:

 

1. Zone Selective Interlocking

Zone selective interlocking (ZSI) preserves the desired coordination between main, tie, and feeder protective devices, and it allows fast tripping for faults within the protected zone – the conductors between the interlocked devices. This is accomplished through wired connections between circuit breaker electronic trip units, ground fault relays, or protective relays. If a feeder device detects an overcurrent condition, it sends a restraining signal to the upstream device(s). The upstream device(s) then follows its normal time-current characteristic and serves as a backup. However, if the upstream device(s) detects an overcurrent condition above its short time – or ground fault – pickup setting but the downstream device(s) do not (e.g. due to a main bus fault), then the main circuit breaker will not receive a restraint signal and trip with no intentional time delay. In this way, ZSI offers the “best of both worlds” – fast clearing of fault currents without sacrificing coordination. ZSI is also available on both low- and medium-voltage equipment and can be applied for both phase faults and ground fault protection.

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