How to Perform an Arc Flash Study – Part 3
Arc Duration

Time is everything when an arc flash occurs.  The longer the duration, the greater the incident energy exposure – which is directly proportional to the duration.

The 2018 Edition of IEEE 1584 defines the arc duration, also referred to as clearing time as:

The total time between the beginning of a specified overcurrent and the final interruption of the circuit at rated voltage.

The term clearing time is also referenced because the arc duration is normally based on how long it takes an upstream overcurrent protective device to interrupt and clear the arcing fault current.  A simple enough concept except how do you determine which device would likely interrupt and how do you determine the actual duration? 

Determining Which Device
When the arcing current from an arc flash jumps across an air gap, it results in a conducting plasma which could engulf other conductors and escalate as well as propagate to other locations.  If the plasma propagates to the line side of the main device such as the main in panel PP-1 shown in Figure 1, then even if the main does trip, the arc flash may not be cleared. It may continue until a device further upstream such as the feeder in the Main Distribution Panel (MDP) trips. A similar situation may occur if the arc flash originates on the line side of the main device.

One Line Example - Brainfiller

FIGURE 1. One Line Diagram – Which Device Defines the Arc Duration.

Because of the uncertainty about whether the main device would interrupt and clear an arc flash, a common approach is to consider the clearing time of a device outside the area that could be impacted by the arc flash.  This would be a device in a separate enclosure such as the feeder located in MDP.

For equipment such as metal clad switchgear where devices are located in individual compartments, engineering judgment must be used when considering whether the main would be unaffected by an arc flash on a feeder.

Determining the Duration
Determining the protective device clearing time depends on the device’s tripping characteristic as well as the magnitude or arcing current.   As a minimum, overcurrent protective devices will have two distinct tripping characteristics.  The overload region which provides a time delay for lower magnitude currents and the instantaneous region which will trip for higher magnitude currents.  In the electrical power world, the term “instantaneous” means no intentional time delay.  There is an unintentional delay of up to several electrical cycles which is the time it takes the device to physically open and clear the arc flash.

The device type such as whether it is a molded case or electronic trip circuit breaker, current limiting fuses and whether it has adjustable settings all help define the tripping characteristic.  In addition, the condition of maintenance of the device is important.  If a device is old and poorly maintained will it perform as expected?  Maybe not which could lead to a longer arc duration.   

Too long – Two Seconds
If the arcing short circuit current is a lower magnitude such as what happens with higher impedances of long conductors or smaller transformers, analysis of the device may indicate it may trip in the overload region for the arc flash – which can take several seconds or more.  In this case, even though the lower current results in a less intense arc flash, the duration could lead to a larger total incident energy.  

For cases where there are long clearing times, IEEE 1584 contains language that many people refer to as the “2 second rule”. Although not actually a rule, this language permits capping the arc duration used in the calculation at 2 seconds.  The actual language states:

If the total protective device clearing time is longer than two seconds (2 s); consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible, and 2 s usually is a reasonable assumption for the arc duration to determine the incident energy.

The language contains caveats such as the decision to use two seconds requires engineering judgment based on the task, location and the person’s ability to move away.  The concept is based on human reaction and response.  If there is a threat such as an arc flash, the person reacts and responds automatically.  Also, even though the term “arc duration” is used in this language, two seconds does not mean the arc extinguishes in two seconds, it is referring to a person’s exposure.

Part 4 will address the use of time-current characteristics to determine the arc duration.