Arc flash events at any facility can be devastating. Consider that an arc flash generates temperatures over 35,000° F and projectile-producing pressures equivalent to 700 mph. The explosion is able to throw a person across the room can be as loud as a jet engine, reaching noise levels of 140 dB or higher.
Despite this, protecting against these dangerous events involved compromise in the past. That is, maximizing protection against arc flashes could increase the number of nuisance service outages, but reducing these unnecessary disruptions could place equipment and personnel safety at risk. Fortunately, today’s electrical equipment designed for containing, mitigating and preventing arc flash events have progressed significantly. Now, compromise can be a thing of the past.
Typically, containing an arc flash involves installing electrical equipment that features physical barriers, such as reinforced metal containers or vaults with heavy doors that remain sealed nearly all of the time. Additionally, ductwork, flues and chimneys are installed to carry the high-temperature gases that an arc flash will produce away from equipment and workers in the area. Unfortunately, these containment techniques are only effective as long as the equipment is tightly sealed. Arc flash incidents can occur during regular inspection and maintenance procedures when the equipment is not properly de-energized prior to opening the panels.
In this regard, arc containment techniques are passive as they only react to an arc flash when it occurs. They neither affect the scale, size or duration of an arc, nor do they help eliminate at least one of the causes of arc flash events, namely, failures in the bus or circuit breakers, drives, transformers or other equipment. That would require active arc resistance, or what is commonly referred to as arc flash mitigation.
Through several technological advancements, arc flash mitigation techniques are able to reduce the size and physical force of an arc flash. These techniques can limit the scale and duration of an arc flash by quickly reducing or eliminating the energy that an arc feeds off of. Plus, mitigation methods react rapidly (some equipment can react in a matter of milliseconds) so an arc flash can be quenched well before it is out of control. In addition, arc containment and mitigation methods are not mutually exclusive of each other. They are often deployed together in the same system, although mitigation technologies can reduce the size and extent of the physical containment features in the system.
Several different arc flash mitigation technologies contribute to the effectiveness of an arc flash mitigation technologies like GE’s ArcWatch*. For example, zone selective interlocking (ZSI) is a concept whereby components such as circuit breakers in an electrical distribution system communicate with each other in order to enable hierarchical coordination. The intent of this coordination is to compartmentalize a service interruption to the smallest possible zone in the overall distribution system and still protect the rest of the system.
Additional innovations have enhanced the capabilities of ZSI even further. One example is a concept known as instantaneous zone selective interlocking (I-ZSI), which has a much faster reaction time than traditional ZSI. Circuit breakers are practically able to coordinate in real-time. The algorithms that enable the much faster I-ZSI make use of several advanced analytic techniques, including wave form recognition (WFR). Rather than just monitoring for peaks or spikes in the current as it passes through a circuit breaker, WFR performs a more thorough analysis by plotting and taking into consideration the implications of a power curve over time. As a result, circuit breakers can have more sensitive settings to increase the system’s protection against arc flash events without degrading the ability of the circuit breakers in the system to selectively coordinate with each other.
Another arc flash mitigation technique involves lowering the energy available in a section of the distribution system while technicians are working on energized equipment in the area. By doing so, the amount of energy available to an arc flash, should one occur, is less than it normally would be. This is accomplished by increasing the current sensitivity of circuit breakers so that they trip sooner. This reduces the amount of energy an arc flash would feed off of by shortening the trip time. An example of this capability is the reduced energy let through (RELT) feature in many of GE’s electrical equipment and circuit breaker offerings.
This lower power mode can be activated manually by technicians through a switch of some sort located on the overcurrent device itself or, if greater safety is required, the switch may be located some distance from the device so that the worker is outside the arc flash zone when the low power mode is activated. If the circuit breaker is equipped with serial communications, the lower power mode could be engaged through the facility’s power monitoring or supervisory control and data acquisition (SCADA) system. Motion sensors that detect personnel in the area could also switch the circuit breakers to a lower power mode for the safety of the workers and automatically return the circuit breakers to their normal mode when the workers have left the area.
Of course, once power components like circuit breakers are able to communicate with themselves and with other types of devices such as sensors, the electrical distribution system begins to resemble a sophisticated Industrial Internet of Things (IIoT), where system-wide software solutions, such as GE’s Envisage*, can be adopted to achieve a number of long term benefits, including better control of arc flash incidents.
The innovative technologies and methodologies that have been incorporated into electrical equipment in recent years have been significant. And the improvements considerable. Now, compromising arc flash protection in favor of power supply reliability, or vice versa, is a thing of the past. The two are no longer mutually exclusive. And the vast potential of the IIoT promises even greater benefits moving forward.
* Trademark of General Electric Company