“Don’t touch that dial” is an old phrase from the 1960s television era that an announcer would say just before “Batman” or another program cut to a commercial. They would pronounce it so authoritatively that you wouldn’t dare change the channel. (Changing the channel was also more of an effort back then, since televisions had rotary knobs and no remotes.) Today, however, the phrase has a different meaning when it comes to the arc flash hazard. Maybe this analogy is a stretch, but follow along anyway.


“The dial”—adjustable 
protective devices


Many circuit breakers have setting adjustments that can be used to define how the device responds to short-circuit currents. Adjustable circuit breakers, as a minimum, will have an adjustable instantaneous setting that defines the level of current where it will trip with no intentional time delay, aka instantaneous trip. If the abnormal current exceeds this current level, the device trips instantaneously. If the current is less than this level, it will trip in the time delay or overload region.


Other devices, such as electronic trip and digital breakers, can have many settings that are used to define an elaborate tripping characteristic. The preferred method for determining the settings for these devices is to perform a coordination study (for more, check out “Don’t Be Left in the Dark,” September 2014, 
ELECTRICAL CONTRACTOR or online at www.ecmag.com/section/systems/dont-be-left-dark-selective-­coordination). This requires analyzing the various protective devices in the electric power system to determine how they should be set so only the device nearest the load trips in the event of an abnormal current. This setup allows devices closer to the source to remain closed and results in the minimum amount of an outage.


However, another objective is to ensure that the protective device also responds quickly enough to provide suitable protection of the conductors, transformers, motors and other electrical equipment. The recommended settings should take into account selective coordination and protection.


It tripped—that was then


Here is where “don’t touch that dial” fits in. When a protective device trips, often the first response is to reset the circuit breaker and see what happens. It isn’t necessarily the best approach, but sometimes little thought goes into the cause when the focus is on getting the power back on.


If a breaker trips once, it will probably trip again; resetting it will not solve the root cause. The second time the device trips, many otherwise experienced people will increase the setting without considering the trip’s cause. Changing a device setting without proper analysis of its impact could reduce selective coordination with other devices and reduce the level of protection.


This is now


Increasing a protective device setting can create a potentially larger and more dangerous problem: the arc flash hazard. Depending on the arcing short-circuit current that could flow during an arc flash, increasing a protective device setting may cause the protective device to take longer to trip during an arc flash. The amount of incident energy from an arc flash is directly proportional to the duration, which is defined by the time a protective device takes to operate. An increase in the device setting does not guarantee it will take longer to operate, but it could happen. It is necessary to get a detailed analysis of the effect that the circuit-breaker setting change has on the arc flash hazard.


As an example, let’s say a calculated incident energy is 5.4 calories per centimeter squared (cal/cm2) and the protective device takes three electrical cycles (3/60th of a second) to operate. If a setting change causes the device to now take 15 cycles (15/60th of a second) to operate, the incident energy would increase by a factor of five to 27 cal/cm2!


The protective device setting’s impact on the arc flash hazard is so important that NPFA 70E 130.5(2), Arc Flash Risk Assessment, requires such an assessment to be reviewed periodically and updated when a major modification or renovation takes place. This is to account for changes that can include protective-device settings that can affect the results of the arc flash risk assessment.


So, if a circuit breaker trips, don’t touch that dial until you diagnose the cause. If you must make a setting change, review its effect on the calculated incident energy, because the next time it trips, there could be a very large POW, BAM or KABOOM!