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Electrical workplace safety has always been a critical issue for both manufacturers and users of electrical equipment. It is only recently, however, that protection from internal arc flash faults has been receiving the attention it deserves.
Although the probability of an internal arc fault is low, the impact of an arc flash event can be devastating. In this article, we’ll look at the causes and effects of internal arcs, measures that can be taken to protect against them, and the standards relevant to verifying that protection.
An electrical arc, or arc discharge, is an electrical breakdown of a gas (in most cases air) that produces a prolonged, self-sustaining electrical discharge. An electrical arc that occurs within a switchgear cabinet is typically referred to as an internal arc.
Internal arcs can be caused by human error, mechanical faults, or environmental conditions.
Human errors that can lead to an arc are caused by:
Arcs due to mechanical faults can arise from:
Environmental factors that can lead to internal arcs include:
In fact, it is estimated that some 70% of arcs arise from human error.
When internal arcs occur within electrical switchgear, they can release huge amounts of energy as both force and heat. They generate extremely high temperatures—as high as 19,000° C—which can instantly vaporize copper and other electrode materials. This vaporized metal is emitted into the surrounding atmosphere, first as plasma, then as metal oxides as it cools.
An internal arc can also cause a rapid rise in pressure—up to several tons per cubic meter in just a few milliseconds (in other words, an explosion)—that can destroy or severely damage the components and wiring within the panel, as well as the panel itself. Such explosions invariably result in lengthy and costly downtime for both the system itself and the manufacturing or processing plant is supplies.
And of course, the gravest hazard of such an explosion is the danger it presents to human life. The extreme heat and force of an internal arc can cause serious injury and even death to operators or bystanders, even when wearing protection cloths, in the vicinity of the affected panel at the time the arc occurs.
Means of protection against damage and injury from internal arcs can be divided into two basic approaches: prevention and mitigation.
Prevention methods, as the name implies, aim at preventing an arc before it can occur. Prevention methods include insulating live parts and placing barriers between them. In low voltage switchgear, however, full insulation or isolation can be difficult to achieve. Another prevention method is to sense the conditions of impending arcs and generate warnings, so the system can be temporarily shut down before an arc can occur.
Mitigation methods are those aimed at lessening the impact of an arc in progress. These are usually classified as either passive or active.
Passive mitigation systems are designed to limit the effects of a full-blown arc flash incident. Such methods include, venting the arc plasma and pressure away from equipment and personnel via arc flaps or chimneys.
An active internal arc mitigation system (IAMS)—like Eaton’s ARCON® Arc Fault Protection System system—detects events that indicate the onset of an arc within the system, like a rapid current rise or a bright flash of light within the switchgear cabinet. The system reacts within milliseconds, triggering an arc quenching device which creates a short-circuit in parallel to the location of the fault. The short circuit drastically reduce the voltage to nearly null required for the arc fault, thus extinguishing it. The system then interrupts the supply of power to the low voltage system by tripping the root breaker along the incoming path.
Low voltage systems and controlgear assemblies are verified according to IEC 61439-1/2. IEC 61439 prescribes mandatory requirements for LV switchgear design verification. It does not, however, address arc flash. To close that gap, the latest editions of IEC 61439 Parts 1 and 2 (released in 2020) include links to standards that do address internal arc mitigation, prevention and their verification. Those standards are IEC TR 61641 and IEC TS 63107.
IEC TR 61641 is an addendum to IEC 61439 that includes arc testing procedures for enclosed LV switchgear and controlgear assemblies. It deals with verifying a low voltage system against internal arcing, focusing on personal safety (Criteria 1-5 = arcing class A) and on the effects of the arc on the system in closed conditions (Criteria 1-6 = arcing class B; Criteria 1-7 = arcing class C).
As a technical report (TR), IEC TR 61641 is non-binding and, thus, advisory only. It describes a method of verifying the system by use of different protection methods, such as barriers, reinforcement of panels, arc flaps, arc chimneys, and even full insulation of live parts (arc ignition protected zones). It covers only normal (doors closed) operation, not maintenance situations.
One set of protection methods for which IEC TR 61641 does not specifically account is active arc mitigation systems. Fortunately, that topic has now been addressed with the release of IEC TS 63107.
First issued in May 2020, IEC TS 63107 is a technical specification that describes the important steps for integrating and verifying an active arc mitigation system—such as Eaton’s ARCON® system—into a low voltage network. This new specification covers:
IEC TS 63107 also includes other mandatory requirements related to arc mitigation devices, their integration, and their operation.
Eaton’s IEC Low Voltage Systems offer a variety of arc protection solutions that have been verified to both IEC TR 61641 and IEC TS 63107.
To learn more about internal arcs, arc energy calculation, arc risk evaluation, and related verification, as well as additional methods to increase the safety of your low voltage power distribution system, download Eaton’s new Consulting Application Guide.
