In the Zone of Protection

Arc flash continues to receive attention in our industry with both codes and standards requirements complemented by a decent assembly of technologies that help to reduce the amount of energy available in the electrical distribution system. When employing incident energy technologies, we must understand what is and is not provided with regard to incident energy reduction, ensuring those who must perform energized work understand the abilities of these technologies. Let’s explore a few of these technologies to better understand what is and is not provided for the worker.
Tsafety-pic.jpgwo parameters that play a significant role in the amount of incident energy at any given piece of equipment in the power distribution system include current and time. Technologies that act to reduce incident energy work to reduce both parameters in some way, shape, or form. The technologies on the market today are quite effective, but they do come with limitations that must be understood for their proper application and proper leveraging when justified energized work is performed. Let’s focus on the following:
  1. Zone selective interlocking (ZSI)
  2. Energy-reducing maintenance switch
  3. Energy-reducing active arc flash mitigation system


A circuit breaker solution employing ZSI provides a method to reduce fault clearing times should a fault occur within the zone of protection. The zone of protection is between connected upstream and downstream pair of circuit breakers. Many in the industry mistakenly believe that ZSI also provides selective coordination. Many also mistakenly believe that ZSI affords incident energy reduction downstream of a pair of devices that employ this technology.

Two circuit breakers that are zone selectively interlocked will be of an electronic trip unit type that must be programmed with appropriate settings of delay that achieve the level of selective coordination desired for the application. Where in the power distribution system these pair of devices reside will determine the degrees of delayed response times for both the downstream and upstream devices in the pair. The pair of devices are most often both located within the same enclosure, be it a panelboard, switchboard, or similar piece of equipment, and close to where the utility provides power. Two devices that employ ZSI utilize a communication voltage signal between the trip units for the upstream circuit breaker and downstream circuit breaker. There are three zones that we must review to understand the response of the circuit breakers in question should a fault occur in any one of these three zones:

  1. Line size of the upstream device: Should a fault occur on the line side of the upstream device, on the main lugs of the circuit breaker, or further upstream, none of the devices that are ZSI’d together will see the fault, so the clearing time is solely determined by the next upstream overcurrent protective device.
  2. Load size of the upstream breaker and line side of the downstream breaker: In this zone, the downstream device does not see the fault but the upstream device does. The upstream device will ignore its settings of delayed operation and trip without an intentional delay. The clearing time of that device is typically in the 0.07 second time frame.
  3. Load side of the downstream device: For a fault that is downstream of the last device in the ZSI system, at the load lug terminals of the circuit breaker, or further downstream, the fault is outside of the zone of protection offered by ZSI. The clearing time is determined by