Mitigating arc flash events in process heating applications

Medium or high voltage equipment for industrial applications always carries certain safety risks. Arc flash events are a prime example of a rare, but potentially fatal, situation that can be mitigated with the right technology. As electric process heaters become a popular alternative for larger applications that have historically favoured gas-fired heaters, it’s vital to consider how they incorporate arc flash mitigation features. Here, Dennis Long, chief system designer for energy and environmental technologies business unit at industrial heating technology manufacturer Watlow, shares insight into why arc flashes happen and how manufacturers can mitigate their risk.

Concerns about decarbonisation, automation and safety have driven many organisations to replace gas fired heaters with larger electrical alternatives. As medium voltage process heaters are relatively novel to many applications, they represent a potential new source of risk that manufacturers must recognise.

Potential risks
An arc flash is an electrical explosion that occurs when there is a short circuit in a system, which can be caused by a build-up of corrosion or conductive dust. If the voltage is high enough, and if there is a path to ground or a lower voltage, the resistance of the air is overcome and results in an arc.

Arc flash events can result in significant damage. As the energy release increases, the risk of fire and injury rises with it. If the energy release is high enough, molten conductor metal and high-pressure plasma energy that can escape from the cabinet, posing a risk to anyone in the vicinity.

The potential arc flash energy is determined by several factors including equipment voltage, available current and the duration of the event. While it may be practical to reduce the potential arc flash energy while limiting voltage or current, overall project cost can make this difficult. Although arc flash incidents are rare, their potential for damage, injury and death makes them a great concern. Some estimates put the incidence of arc flash events between five to ten per day worldwide.

Reducing the effects
There are three main strategies for minimising the effects of arc flashes, including increasing the distance from the potential source of an event, reducing the available fault current and decreasing the duration of the event. All strategies can be combined to ensure maximum safety, but this is not practical when considering overall project cost. That said, the duration of the event is the most viable influencer to reduce damage and has the largest impact on the total amount of energy released.

There are two key approaches to compare: arc-resistant cabinets and arc mitigation technologies. Arc-resistant cabinets aim to reduce exposure to arc events by encasing the system in a metal-clad cabinet with a venting system. The heated gas and pressure is redirected through a duct, reducing the energy that could potentially explode. However, a drawback lies in the fact that the cabinet must be closed for the arc-resistant cabinet to work, as many arc events occur during maintenance, when the doors are open.

Mitigation technologies
Instead of redirecting the energy from the event, arc mitigation seeks to reduce the energy of the event itself by limiting its duration. This is done by detecting the arc flash early and automatically tripping the appropriate circuit. This can be done via sensing current, referred to as current arc mitigation, or sensing light, known as optical arc mitigation.

In optical arc mitigation, the light emitted by the arc within the enclosure builds quickly, which can be detected by a photoelectric receptor, even in the early stages of the event. When detected, the signal is then sent to a protective relay, which trips the breaker automatically without the need for human intervention.

One of the main advantages of this approach is that it is independent of the actual magnitude of the arcing fault current. This allows the system to detect arcing in an early stage of its development and trigger the break sooner, which limits the duration of the event and the total energy produced.

On the other hand, current arc mitigation uses current transducers to sense an increase in current produced by the arc. If the transducers are not sized correctly, they may not shut the system down or may be unable to clear the event.

Arc mitigation technologies also reduce damage to equipment, as it can function even when the doors are open and maintenance is being performed. For example, in the Watlow POWERSAFE™ thermal system, sensors can be placed within the thermal controller, SCR node single contact or node, which are protected by a feeder. When the sensor senses an arc flash event in any compartment, the feeder shuts down the lineup to limit damage caused by the arc.

As medium voltage process heaters become more popular, they must be designed with safety in mind. Arc flash mitigation technologies represent the best approach here, as they decrease the duration of arc flash events and hence the energy released, helping to minimise the risk and damage involved.