Chapter Corner

Technologies Advancing Incident Energy Reduction

Posted in: Safety Corner, July/August 2018

Technological advancements have changed the landscape when it comes to incident energy and arc flash problems in the industry. In the 1970s, the industry had ground-fault protection equipment (GFPE), the latest in electrical safety technology for the times. Since then, the integration of electronics and overcurrent protective devices (OCPD) has brought forth even more capabilities from which both owners and electrical workers can benefit. The arc quenching technologies on the market today, also referred to as active arc flash mitigation systems, have taken arc flash reduction to a whole new level. It is important for us to be aware of technological advancements to ensure the solutions employed on projects are the latest available for the safety of electrical workers and the reliability of the electrical distirbution system. These technologies can not only save lives but also a lot of money for owners.
 
Time and Current
 
The amount of incident energy at any given point in the power distribution system is a matter of the amount of available fault current and the amount of time that fault current is permitted to flow. The control of the amount of time that current is permitted to flow within a circuit has historically been left to the performance of the overcurrent protective device alone, whether it be a fuse or a circuit breaker. Overcurrent protective devices can only detect current for interruption. Some arc flash reduction systems come in the form of relays that trigger the opening of an upstream overcurrent protective device based upon current flow. The bottom line is the faster the opening time, the lower the incident energy.
 
Quenching Technologies vs. Others
 
Recently, technology has entered the market that does not rely upon the clearing time of the upstream overcurrent protective device to limit incident energy. These systems are called quenching systems, as they create a lower impedance current path, located within a controlled compartment, causing the arcing fault to transfer its energy to this new lower impedance current path. An arc flash relay that looks at current and light is used to detect the ignition of an arcing fault using two main parameters instead of only one. Upon detection of an arcing fault, the arc flash relay simultaneously sends a trip signal to the main circuit breaker and a trigger signal to the arc fault quenching equipment. The arcing fault quenching equipment commutates the arcing fault in sub-cycle times, some systems in less than three milliseconds. This arc transfer time is an order of magnitude faster than the clearing time of a power circuit breaker and thus results in an order of magnitude reduction in incident energy.
 
When the latest evolution of arc quenching systems is used, known as current limiting arc fault quenching systems, the technology causes the arcing fault to transfer to a controlled compartment and significantly reduces the peak fault current, at least 25% less. Peak current reductions translate into lower electromechanical forces and therefore less stress on the power distribution system that has to carry the fault current. Since electromechanical forces are related by the square of the current, current limiting arc fault quenching systems can reduce system stress by at least 44% during a quenching operation. Less stress on the power distribution system means less wear and tear and can translate into longer life of the equipment.
 
Characteristics of an Arc Flash 
 
An arc flash event is comprised of standard characteristics that make them detectable by sensing equipment. To understand how the arc quenching technologies function, we must understand the most common characteristics of arcing faults: high current and intense light. If current is monitored without monitoring light, the relay technology may be prone to falsely identifying a short circuit fault as an arcing fault, resulting in unwanted tripping and a cascading event as an upstream device may operate before a downstream OCPD has time to clear the fault. When light sensor technology is used without monitoring current, the relay may be prone to false tripping from ambient light, camera flashes, or other sources of light. The right mix is the monitoring of current and light. When both of these parameters can be detected, the relay has the ability to correctly identify arcing faults within the equipment.
 
However, the normal operation of lowvoltage power circuit breakers poses a problem with this method of arc flash detection because they can produce light when interrupting a downstream fault. An air circuit breaker creates temporary arcing in open air between the parting contacts when interrupting high currents. This arcing creates a bright flash of light that is released from the breaker’s arc chutes. If the breaker is performing as designed and clearing a fault external to the power distribution equipment, the arc flash detection relay may trip by sensing light emitted from the breaker’s arc chutes and high current from the external fault, causing errant operation of the arc flash mitigation system. Advanced arc quenching technologies are designed to know the difference between the light produced from an arcing circuit breaker contact and from an arc flash event inside the protected equipment. The best solution to prevent falsely identifying a low-voltage power circuit breaker (PCB) opening operation as an arc flash event is by interlocking the PCB’s opening operation with the arc detection relay. The PCB generates an output signal to the arc detection relay before the PCB’s primary contacts open, and the relay blocks the trip signal long enough for the PCB to finish its opening operation.
 
With this technique for protecting against nuisance tripping, the arc detection relay system can be left on continuously, and all portions of the equipment are protected, including the breaker compartments. Paired with an arc quenching device, this approach ensures maximum, continuous and complete equipment and personnel protection all the time, not just during maintenance operations. 
 
Doors Open
 
There are two main reasons why you would want to reduce incident energy. The first is for personnel protection. We can’t completely eliminate incident energy from an energized system, but we can make it such that the incident energy is within limits for appropriately rated PPE. The second reason to reduce incident energy is for equipment protection and reduced downtime. When an arc flash event results in little to no damage to the equipment, returning to production is simple and relatively quick. When an arc flash event results in a lot of incident energy, equipment damage could be so extensive that prolonged outages are required to repair or replace the equipment.
 
Traditional arc-resistant equipment, equipment built with a highly reinforced enclosure to contain and redirect arc energy, does not actually reduce the incident energy. Such equipment is designed to protect personnel working in the vicinity but will not protect the equipment itself from internal damage in the event of an arc flash. Furthermore, arc-resistant equipment only provides protection when all doors are closed and properly latched and all panels are bolted in place. If work is being performed with doors open, there can be no expectation of arc-resistant protection. On the other hand, the latest arc quenching technologies can provide arc-resistant protection even when equipment doors are open or panels are removed. This is an important benefit over and above any other means of dealing with incident energy.
 
Closing Remarks
 
We are living in an ever-changing electrical industry. Advancements in technology can save lives and property and help the bottom line. Current limiting arc quenching systems are one of the most promising of these new technologies that intend to address these mounting concerns and changing priorities with a revolutionary new approach.
 
Adams Baker is the Product Manager for Eaton’s low-voltage switchgear product line. He has a BS in Mechanical Engineering from Dartmouth College and a Master of Engineering Management. He has been involved with industrial new product development for over a decade in a variety of capacities including design engineering, manufacturing, sales and marketing.