Protection Coordination Study

To begin the process of arc flash mitigation using engineering controls, the first thing to do is a protection coordination study. A protection coordination study is an essential part of arc flash mitigation because it lets you know how your protective devices will react to a fault. The most important part of the protection coordination study tells you how long a fault will be present before a device is tripped: arc flash incident energy is reduced by reducing the amount of time the fault is active. The protection coordination study will allow you to determine whether or not you need to change the the settings of your circuit breakers and relays. This is assuming that your circuit breakers and relays are digital and adjustable. If they are not, you might want to look at substituting them for newer parts.

Virtual Main System

I was going to try to explain a virtual main system, but the folks at Schneider-Electric have summarized it much better than I can, so I'll just quote them:

"A virtual Main consists of a digital relay and CT's added to the LV side of a substation transformer and wired to a Fault Breaking overcurrent device (Vacuum C/B) with the ability to use a "Maintenance Switch" to set a lower fault level trip or to use Short time Zone interlocking with the Secondary feeders to allow for faster tripping of the "virtual" main. "

All they're really saying here is that you have current transformers on the low voltage side of your transformer, and when the CT's recognize a fault they are wired to trip a breaker on the high voltage side of the transformer, protecting the entire circuit. You can find more information about maintenance switches and zone interlocking in our article on substitution.

Optical Relaying

Optical relays are a relatively new concept. Instead of tripping solely based on overcurrent, optical relays sense the light generated by an arc fault and use that as another trigger for tripping. An optical relay requires both overcurrent and the light generated by an arc fault to trip: the relay will normally not trip if no overcurrent is detected (but it will trip if there is high overcurrent but not light). These optical relays can have tripping times as low as 2.5 ms!

System Grounding

High resistance grounding limits single-line-to-ground fault currents to very low levels. These low current levels translate into low incident energy levels and therefore reduce the risk of an arc flash hazard occurring. From the IEEE Red Book (Std 141-1993, pg 367), "There is no arc flash hazard, as there is with a solidly grounded system, since the fault current is limited to approximately 5 A). You have to be careful if using this method of arc flash mitigation: high resistance grounding doesn't mitigate arc flash in the case of line-to-line of 3-phase-to-ground faults. Some more information about how high resistance grounding pertains to arc flash can be found here.

Location Design

Location, location, location. The location of equipment is very important. An arc flash causes rapid increases in temperature and as a result, rapid expansion of the materials that the electricity is flowing through. The explosive action of metals being vaporized and rapidly expanding is called an arc blast. Any equipment that has a high risk for arc flash should be placed in your facility in such a way that the effects of the arc flash incident are not magnified by location. For example, if an arc flash takes place in a small area, there is much less space for the energy to dissipate, and the effects of the arc flash will be concentrated. This is something that is more difficult to rectify after a facility has already been constructed, but is something to be considered during the initial design stages of any new building used to house electrical equipment.

So there you have it, five different techniques to consider when looking at arc flash mitigation from an engineering controls: protection coordination studies, virtual mains, optical relays, high resistance grounding, and location. This is by no means an exhaustive list, and an important thing to remember is that your arc flash mitigation techniques will be unique to your facility by necessity. As always, thanks for reading!