3 Approaches to reducing the danger and damage of arc flash

There are a number of ways to help reduce the damage and danger of arc-flash events. There’s also a way simply to help prevent them. Learn more about three common methods of arc-flash mitigation. 

Arc flash is one of the most devastating industrial accidents you can experience. In a literal flash, your switchgear could be destroyed, downstream equipment badly damaged, processes shut down for days or weeks, and employees badly injured or dead.

In some facilities or applications, the potential risk from arc flash is very low; and traditional (no arc-flash protection) switchgear can be used with little risk. In most applications, though, relying on equipment that includes features to help reduce arc-flash dangers makes more sense. These features provide prudent “insurance” against the destructive outcomes of an arc-flash incident.

That destructive potential is devestating. The energy released during an arc flash in an 11kV switchgear can be equivalent to the energy needed to start six space shuttles. The temperature may rise as high as 20,000 C, five times hotter than the surface of the sun and able to vaporize metal.

Arc faults may be rare but are the most severe fault that can occur within a switchgear system. There are a multitude of potential causes, including human error (the most common), technical/equipment failures, and environmental causes.

You can choose from a number of approaches to reduce this risk of arc-flash damage and injury. Three of the most common are described below.

Passive internal arc protection

With passive internal arc protection, the arc fault is interrupted after it occurs by a conventional protective relay. The average time between arc flash and relay trip is from 100 to 1,000 milliseconds (ms). It happens within the literal “blink of an eye,” which has been measured at 100 to 400 ms.

Although it lasts only an instant, an arc-flash event will almost certainly result in sufficient damage that the switchgear will require repair and parts replacement. There is also likely to be a significant disruption of the production processes that depend on the switchgear for their power.

Switchgear with passive internal arc protection incorporates ductwork of some type to provide an “escape route” for the high-pressure, high- temperature, and potentially toxic gases. Some switchgear includes a duct that vents to an outside area, a solution usually used with smaller switchgear rooms. Where switchgear is in a larger room or located away from exterior walls, the ducts vent into the switchgear installation room.

Both of these venting strategies reduce or eliminate ejection of the hot gas out the front of the switchgear, helping to reduce potential injuries. It also dissipates blast pressure, helping to diminish internal damage to the switchgear.

Active internal arc breaking

In switchgear equipped with active internal arc breaking, the protective circuit operates independently of the protective relays. Typical arc- breaking time is ~60 – 80 ms, which is the sum of the time required to detect the arc flash, for the circuit breaker to operate, and the circuit breaker operating time.

The arc flash can be detected very quickly using sensing techniques that send a signal to an upstream breaker to interrupt the circuit. Two common arc-flash detection methods are current-sensing devices and/ or optical light sensors that “see” the flash. The response time is faster than passive internal arc protection, but still not fast enough to prevent potentially significant damage and injury.

Spring-charged-mechanism breakers are known to increase their opening times as the breakers age, resulting in longer clearing times. Slower tripping times relates directly to increased damage from an arc flash. This is not an issue with magnetically-actuated breakers which do not slow with age.

Active arc elimination

Both of the approaches described above help reduce the potential damage and injury from arc flash. But it’s important to note that they both respond to the flash after it occurs. The third approach is active arc elimination, which, as its name indicates, is designed to respond so quickly that the arc is very nearly eliminated.

Like the preceding approaches, it technically also reacts to an arc flash once it occurs, but responds so quickly that there is usually minimal damage to the switchgear and little or no chance of injury to personnel. It achieves a significantly faster response than other methods, typically less than 1.5 ms but no more than 4 ms. Instead of tripping an upstream breaker, it does this by using an ultra-fast earthing switch (UFES) that initiates a 3-phase short-circuit to earth in the event of an arc fault.

Working in conjunction with rapid and reliable detection of the fault through various sensing methods, UFES helps ensure that an arc fault is extinguished almost immediately after it occurs. This means a drastic reduction of heat and pressure. Getting the switchgear back online typically requires only a wipe down of the interior and replacement of the primary switching elements, all of which can be done in a matter of a few hours.

Summary

If there’s a fire in your home, it’s comforting to have a detection and alarm system that immediately summons the fire department so they will arrive in a matter of minutes. But it would be even more comforting to have a system that prevents the fire in the first place. That is a comparable scenario to what sets active arc-elimination systems apart from other approaches to managing arc flash.

Active arc elimination is arguably the most effective method of arc-flash reduction, but there is a slight cost premium compared to other protective methods. For switchgear that lacks any arc-flash protective design elements, active arc elimination represents a small investment relative to the cost of the switchgear it protects. In evaluating the cost benefit analysis, it’s also important to consider the additional costs due to disruption of production or processes following an arc-flash event.

To help deliver the highest levels of protection for your people and switchgear, and the lowest risk of disruption to your daily operations, active arc elimination via the ultra-fast earthing switch is the right choice for your switchgear.

ABB Electrification Business