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Arc Flash Mitigation Techniques – Engineering

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arc flash engineering controls featured imageIf you can’t eliminate the hazard completely, and you can’t substitute the problem areas of your system with more appropriate methods, then engineering controls are your next best bet for arc flash hazard mitigation. There are many ways that engineering design can mitigate arc flash. You could even argue that every method of arc flash mitigation falls under the category of engineering controls. Today I’ll talk about some of the big ones.

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

A conventional mechanical relay might not be enough to protect against arc flash

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

Where you put your equipment is important.

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!
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Protection Coordination Study

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In power system studies – the critical four I said:

The purpose of the protection coordination study is to verify that the various protection devices in your system, relays, breakers, fuses, etc. are coordinated correctly and are sized appropriately for the equipment that they are protecting.

which is very true. To expand on that, a protection coordination study really has two purposes. First it ensures that the electrical equipment is properly protected from over-currents and overloads. Secondly the study determines that the selective coordination is employed such that the system reacts as intended and in a predicable manner. You should (almost) always sacrifice equipment protection for selective coordination.  The cases where the opposite is true is a topic for a future article.

TCCs are the bread and butter for a Protection Coordination Study
TCC from an ETAP example file

A protection coordination study will consist of time-current curves (TCC) of all the electrical equipment, protective devices and large motors. Above is a sample TCC showing a circuit for a 50hp motor, it shows the breaker (CB16) that protects the motor from over-currents, the over-load protection (OL_H1), and the fuse (Fuse3) that feeds the MCC.  The dashed line at the far left shows the motor starting and running curve, the next shaded area is CB16 and OL_H1.  ETAP models them as a system and only shows the parts of the curves that are relevant. At the far right is the fuse.
There is no overlap between the motor curve and the motor protection (CB16 and OL_H1) so therefore the motor will operate as intended, and there is no overlap between the feeder fuse and the motor protection, so if there is a fault on the motor circuit the main fuse won’t activate.  If there was an overlap, then there is a chance of the fuse operating on a motor fault, removing a larger portion of the system from service, and possibly making it difficult to troubleshoot the issue to get the system back into operation.

Why do I need a protection coordination study?

You will need a current protection coordination study at your facility to ensure that your system reacts to a fault in a predictable manner and to know how long a fault will be present before the protection takes action. This time is critical information for an incident energy study, and will greatly affect the severity of an arc fault.

When should a protection coordination study be completed?

A protection coordination study will be completed during the design phase of any facility, however after that it will be reviewed on a periodic basis, but at least every 5 years. As with all power system studies, if there have been changes within the system, you should verify that the protection is still adequate.
Another time to revisit the protection coordination study is when equipment is experiencing nuisance tripping: this can be caused by abnormal currents in the equipment, or a faulty relay.

How do I get one done?

I always recommend that the protection coordination study be completed with software, unlike the short circuit study. It is extremely hard to print curves to scale for overlaying, and to try multiple options when an issue presents itself. However, it is relatively easy to add different devices to the circuit and verify protection and selective coordination when using power system modelling software like ETAP, SKM Powertools or Easypower.
To determine the best way to get a protection coordination study completed check out this post from a couple of weeks ago. In it I explain the various methods that you can use to complete a power system study, which can work for any individual study.
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