Arc Fault

What is Arc Flash

Cole Ferguson · Aug 10, 2016 · Leave a Comment

There are really only two types of electrical hazards: shock and arc flash. CSA Z462 gives a short definition for arc flash:

Arc flash hazard – a dangerous condition associated with the possible release of energy caused by an electrical arc. Arc flash takes two forms:
- Arc blast refers to the pressure wave created during an arc flash incident. This pressure wave can throw molten metal at you at very high speeds in addition to causing physical harm like concussions.
- Arc burn refers to the incredibly high temperature in the area around an arc flash. This temperature can be hotter than the surface of the sun and can cause debilitating burn injuries if you aren’t wearing appropriate PPE.

CSA Z462 mentions that under normal operating conditions, most equipment is not likely to cause an arc flash hazard. It then points to Table 4A for examples of activities with potential for causing an arc flash hazard. Because of the severity of arc flash incidents, it is very important that you understand what it is and how it can harm you.

Arc Flash Causes

An electrical arc occurs when current passes through the air from one conductor to another. A lightning strike could be considered an arc from the clouds to the ground. An arc fault occurs when you have unwanted arcing in your electrical system. This could be because of a breakdown in wire resistance, for example, due to heat. Arc faults can also occur during switching: current cannot immediately drop to zero, so some of the current arcs across the gap as a switch is opened. Undesired arc faults can damage your electrical system.

Arc flash is a severe case of arc fault. In high voltage systems, when an arc occurs it usually burns out and destroys the physical conductors, so air is the only conductor left. Air is normally an excellent insulator, but will break down to plasma when the ratio of voltage to arc length is relatively large.

Arc Flash Effects

A high voltage arc in a small space will have a very large electric field and will cause the air to break down in to plasma. This plasma will use up all of the energy available to it before it dissipates. The plasma will create temperatures as hot as 35,000 degrees Fahrenheit (Clark, n.d.) which is hotter than the surface of the sun!
When an arc flash occurs, temperatures can climb so high that it can burn the skin right off of you! Not only that, but these temperatures tend to vaporize any nearby metals (such as the ones used to make the conductors themselves). When copper (a common wire material) is vaporized, it suddenly expands to 67, 000 times its original volume. (Ray A. Jones, 2000)

Here’s a quick example using one foot of AWG 10 wire. AWG 10 wire has a conductor diameter of 0.1019 inches. (Wire Gauge and Current Limits Including Skin Depth and Strength, n.d.) A one foot long piece of AWG 10 wire has a volume of 1.80271*10^-5 cubic feet ((0.1019²/4) * 12 inches * 1 cubic foot / 1728 cubic inches). That seems pretty small. But when it gets vaporized during an arc flash, it suddenly takes up a space of 1.21 cubic feet! Now consider that a cable can be made up of multiple wires: A typical arc flash packs the same explosive power as dynamite (caused by the superheated metal as it expands into vapor). This explosive action is known as arc blast. If the force of the arc blast doesn’t kill you, you are at serious risk for broken bones and organ damage, especially if the explosion knocks you into a hard surface. It’s even more dangerous if you’re working at a high elevation: the arc blast could knock you off a solid foothold and put you at serious risk for fall injury.
Not only does the arc blast cause a pressure wave that can throw you around with a large amount of force, but it will also throw around any metal that didn’t get vaporized during the arc flash. Usually so hot that it becomes molten metal, this can burn through clothing as well as any equipment nearby and can cause serious harm to anything it touches. The explosion and high temperatures could also cause damage to or destroy nearby electrical equipment that is not part of the initial arc flash incident.
Overall, arc flash is a great risk to both people and equipment, and can result in very costly injuries to people or damage to equipment. The risk of an arc flash incident can be reduced by appropriate electrical safety program usage. In order to account for arc flash in an electrical safety program, an arc flash study is required.
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Arc Flash Mitigation Techniques – Substitution

Cole Ferguson · Jul 25, 2016 · Leave a Comment

If you can’t eliminate an arc flash hazard, the next best arc flash mitigation technique is substitution. Unfortunately there are not a lot of ways to perform substitution to mitigate arc flash: electrical energy cannot simply be replaced by some other form of energy to provide power to equipment.

Substitution of Electrical Equipment

You can, however, replace equipment. After performing an arc flash study, you may discover that one of the reasons for high incident energy is that your equipment is old and out of date. If equipment is out of date, it must be replaced: replacement equipment could be selected for a number of reasons, including:

Selecting breakers and fuses with faster tripping times than those currently installed. As technology improves, newer protective devices generally trip faster than older ones. A faster tripping time leads to reduced arc flash incident energy. ” Per the equations in IEEE Std. 1584-2002, arc flash incident energy varies linearly with time. If the duration of the arcing fault doubles, the available energy doubles; halve the duration and you cut the energy in half.”
Arc resistant switchgear as a substitute for currently installed switchgear. Arc resistant switchgear contains the energy of an arc flash inside the equipment and directs it away from personnel.

Zone Selective Interlocking

Another way to reduce arc flash incident energy is by taking advantage of Zone Selective Interlocking (ZSI). ZSI allows your breakers to communicate with each other to provide the fastest tripping time possible. You can read more about ZSI here. An important thing to note is that in order to take full advantage of ZSI, you need to make sure your devices are coordinated.

Maintenance Switch

You can also mitigate arc flash hazards by adding an arc flash reduction maintenance switch to your system. The maintenance switch reduces incident energy levels on equipment downstream from the maintenance switch. The maintenance switch has its own analog circuit which is designed to trip faster than digital circuit breakers. More information can be found here.
To summarize, there isn’t a whole lot of substitution that can be done when dealing with electrical power systems. However, you can still replace out of date equipment with arc resistant ones. You can also add equipment that can perform zone selective interlocking, and can add equipment with a maintenance switch to your system.
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Protection Coordination Study

Jeff MacKinnon, P.Eng · Jul 23, 2015 · Leave a Comment

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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