Chapter Corner

In the Zone of Protection

Posted in: Safety Corner, August 2017

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 the downstream overcurrent protective device, which could be upwards of 30 cycles or as fast as 2 cycles depending upon the type of circuit breaker, the programmed delay of the trip unit, and the magnitude of arcing current. 
It would not be abnormal to have a service switchboard employing ZSI feeding a panelboard directly next to it that is not employing ZSI and find the upstream main switchboard has a lower incident energy than the immediate downstream panelboard. Workers working in a panelboard that employs ZSI must also realize that coming in contact with and causing a fault on the load size of the feeder breakers or line size of the main will not result in the lower incident energy values as determined by the clearing time of the ZSI system.
A circuit breaker equipped with an Arcflash Reduction Maintenance System provides a method to reduce fault clearing times should a fault occur while working on energized circuits downstream of the device with this technology employed.
An arc reduction maintenance system can be turned on and off automatically or manually to reduce arc flash energy in a similar manner to when working de-energized. In the on position, it reduces the pickup time and clearing time of a circuit breaker that may have been intentionally delayed for selective coordination purposes. In the off position, the system responds in a manner in which it has been programmed to meet selective coordination requirements.
There are two zones 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 device: Should a fault occur on the line side of the circuit breaker that employs the arc reduction maintenance switch technology – on the main lugs of the circuit breaker – the circuit breaker will not see the fault, so the clearing time is solely determined by the next upstream device.
  2. Load side of the device: For a fault that is downstream of the device employing the arc reduction maintenance system, at the load lug terminals of the circuit breaker, or further downstream into the power distribution system, the fault is inside the zone of protection offered by the technology. The circuit breaker will trip without an intentional delay with a clearing time as fast or faster than 2 cycles. The limitation of the device will depend upon the value of arcing current.
This technology provides protection within the enclosure and downstream of the enclosure within which it is employed. With regard to the previous example used, a service switchboard employing the arc reduction technology on the main of that switchboard feeding a panelboard directly next to it that is not employing an arc reduction technology will be afforded incident energy reduction by the main circuit breaker in the upstream panelboard. When working in a panelboard that employs the arc reduction maintenance switch or on downstream equipment, the switch should be treated the same way that upstream circuit breakers are treated when working de-energized. The procedures are well documented as part of NFPA 70E.
Equipment employing an Energy Reducing Active arc flash mitigation system provides a method to reduce fault clearing times. This system monitors equipment parameters that may include current, light, heat, or others to identify an arc flash event within the equipment. If an arc flash event occurs, the arc is diverted via various types of technology while clearing the circuit, eliminating the faulted condition and de-energizing the system.
There are two 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. Inside the box: The technologies employed rely upon sensors placed within the enclosure. The system will respond only to events that occur within the enclosure.
  2. Outside the box: The technologies employed rely upon sensors placed within the enclosure. The system will not respond to events that occur outside of the enclosure, such as in downstream or upstream equipment.
It would not be abnormal to have a service switchboard employing an active arc flash mitigation system feeding a panelboard directly next to it that is not employing incident energy reduction technologies and find the upstream main switchboard has a lower incident energy than the immediate downstream panelboard.


No matter what technologies are employed within a power distribution system each individual equipment being worked on must be treated independently. Calculating incident energy and proper labeling at each piece of equipment is important for safety. Understanding the clearing times of upstream overcurrent protective devices is critical as well.

As always, keep safety at the top of your list and ensure you and those around you live to see another day.

Thomas Domitrovich, P.E. is VP of Technical Sales for Eaton’s Bussmann business within the Circuit Protection Division of Eaton Corporation. Thomas is based out of St. Louis MO and has more than 25 years of experience as an Electrical Engineer. He is a LEED Accredited Professional and a licensed Professional Engineer in the state of Pennsylvania. Thomas is active in various trade organizations including the Independent Electrical Contractors (IEC), International Association of Electrical Inspectors (IAEI), Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA) and the National Fire Protection Association (NFPA). Thomas is Principle member on Code Making Panel 2 for the National Electrical Code (NFPA 70) and an Alternate member on NFPA 73 for electrical inspections of existing dwelling units both representing NEMA.