You've had an arc flash study done and installed labels on all your equipment. You've given training to your electrical workers on how to interpret those labels, but you've still got one problem… some of the levels are very high. In fact, some of them are so high that the workers are uncomfortable even going near the staff… now what?
Now, you need to figure out how to reduce arc flash incident energy levels… and in this article, we are going to show many of the ways you can mitigate arc flash hazards levels.
Table of contents
- What is arc flash incident energy anyway?
- What are the factors that determine the levels?
- How can we reduce incident energy levels?
- Increase the working distance
- Reduce fault clearing time
- Reduce available fault current
What is arc flash incident energy anyway?
Basically, the incident energy is the measurement of how hot the arc flash will get.
It's usually measured by the amount of energy (in calories) that you would expect to receive on every square centimetre of skin that is exposed to the arc flash (at a certain working distance)… cal/cm2.
Honestly, it's not the easiest thing to wrap your head around so I like to think of it as very close relative to temperature.
And just while we are on the topic… it takes 1.2 cal/cm2 for the onset of second-degree burns.
So, that's a good number to keep in mind while you read.
What are the factors that determine the levels?
There are three determining factors that account for the magnitude of an arc flash.
Distance, time and current.
When we talk distance, we are talking about the distance between a person's torso and the hottest point of the arc flash.
The time we are factoring into the equation is the amount of time it takes for the arc to initiate and then extinguish. It literally happens (and is over) in the blink of an eye… but that's all it takes to have devastating results.
Current is a factor of electrical system. Basically, we want to know how much current can the system supply to the point of fault during the duration of the arc flash incident.
How can we reduce the incident energy levels?
There are actually a number of ways we can lower the arc flash results, but each of them are going to hit at least one of the three elements we just discussed.
- We can increase the distance the person is standing from the potential arc source.
- We can decrease the amount of time that the arc flash will exist for.
- We can reduce the available fault current available to feed the arc flash (but this one typically has some drawbacks).
Increase the working distance
This is usually the easiest one to implement as long as you can think of a way to do it. It won't require changing the system design or installing any permanent equipment (possibly).
One thing to consider is that using these methods will not decrease the incident energy value displayed on your arc flash labels. The reason is that these are only temporary methods which may not always be possible (even on the same equipment). But it will reduce the incident energy you would be exposed to (because you are standing further away).
As a general rule of thumb you can reduce the incident energy by 1/2 every large step backwards you take… but don't quote me on that… I just wanted to give you an idea of how much increasing distance does work.
A quick tip: if you can get outside of the arc flash boundary then you know for sure that you are working from a distance where the incident energy is less than 1.2cal/cm2.
One of the most hazardous jobs an electrician can do is racking of a circuit breaker (on a live bus) using a traditional hand-held racking device, crouched down on one knee well within the arc flash boundary.
A remote racking device is a tool which replaces the traditional system with something that can be operated from a distance.
Check out this video to see how one works.
Remote switching is very similar to remote racking but this is used to operate circuit breakers from the open to the closed position or closed to open instead of racking them out onto the floor.
The most well-known type of remote switching device in the industry is by CBSArcSafe and interestingly named the “chicken switch”. This device can be installed to the front of the switchgear over the operating mechanism. The worker can stand back outside the arc flash boundary and safely operate the breaker.
The cream of the crop is what’s called on-board racking and as far as I can tell is only offered by Powell Industries.
On-board racking comes built into the switchgear so it offers the maximum amount of protection and ease of use. Operators can rack in and out breakers without ever visiting the electrical room to install a remote racking system as seem above.
Here is a link to Powell’s website: https://www.powellind.com/ProductsServices/Pages/OnBoard-Racking.aspx
Use live-line tools
Live-line tools get the worker further away from the equipment than if they were only to use their hands to do the work.
Things like installing temporary protective grounds and testing for the absence of voltage (on high-voltage systems) should always be done using live-line tools.
One thing to note here is that using live-line tools typically will not get you out of the arc flash boundary or even out of the working distance… what I mean by this is when you look at the incident energy level on the arc flash label you also need to take note of the working distance. HV and MV equipment should be calculated at 36 inches, which is the typical distance one would be standing while using a live-line tool.
So, this means that most times, using live-line tools is an absolute must… but not necessarily reducing the level of exposure to the arc flash hazard.
Typically, switchgear is not designed to withstand an arc flash. The doors and covers could (and most often will) come flying off exposing the worker to not only the arc flash hazard but also a metal projectile.
This is where arc-resistant switchgear comes in. This switchgear is designed to withstand the blast of an arc flash and direct the heat energy and fire ball away from the worker.
There are two types of arc-resistant switchgear. Type 1 is only resistant from the front and type 2 is resistant from the front, sides and rear of the equipment.
From a risk standpoint you would need to use arc resistant switchgear when racking the breakers on a live bus… there are not many other tasks where I see it as valuable. You might consider using remote racking instead… but I’ll leave it up to you.
Quick side note: when you are considering if the doors are open or doors are closed during an arc flash hazard assessment you are trying to determine the chances that an arc flash will occur… not if the door will protect you or not. If doors are closed the risk of an arc flash is significantly lower.
Here is a link to Eaton's arc-resistant switchgear to give you an idea of what is out there.
Reduce fault clearing time
Figuring out how to make protective devices operate faster seems to be the favorite among engineers… and there are lots of opinions on how to do it.
I'll go over some of the most common ways to reduce the fault clearing time and give my two cents (for what it’s worth).
Current limiting fuses
Without getting super technical a current limiting fuse will dramatically reduce the energy delivered to an arc fault because it must clear the short circuit current is less than one-half cycle in its current limiting range.
Basically, it’s a really fast fuse.
Through careful planning and engineering, you can replace certain fuses in your electrical distribution system with current limiting fuses and it will have a huge effect on your arc flash incident energy levels.
Here is a white paper from Mersen if you’d like to learn more.
High-speed relays & sensors
If fuses aren’t your thing (and you have breakers as your main distribution protection devices) then using an “arc flash relay” with some combination of light and current sensing is your best option.
The way these systems work is by detecting both the presence of light and an increase in current, then sending the signal to a super fast acting relay which tells the breaker to trip. When an arc flash happens you’ll get both of these phenomenons and they will occur instantaneously. Light travels pretty quickly (as we all know) so you’re really limited how fast the system will work as a whole.
Here is a video from Littlefuse that shows just how the system works.
This method uses current to detect a fault… but it’s a little different than measuring the current passing through a single point (which then you are waiting for the relay to realize it is a fault and not just a big motor starting).
The way differential protection works is by using a simple calculation that current in equals current out.
Let’s say you have a switchgear line-up with the main breaker feeding a bus that then has six feeder breakers feeding power to the plant. If you were to have an arc flash inside this piece of switchgear then that means you would have a certain amount of current that is coming through the main breaker that is not going out one of the six feeder breakers.
Differential protection basically looks at the current flowing through the main breaker and compares it to the sum of the current flowing through the six feeders… any difference and the system trips.
Schweitzer Engineering Laboratories is probably the favorite among engineers for solutions using differential protection (feeder, bus, and transformer).
A maintenance switch is a toggle switch that is connected to your low voltage draw-out switchgear.
Before an electrical worker goes to perform work on a particular motor control center or switchgear line up he or she flips the “maintenance switch”. Once engaged, the instantaneous trip setting of the breaker will switch to a lower (but preprogrammed) value… thus lowering the energy level.
One thing to note with this solution is that you will require a dual arc flash label… one label when the system is in regular mode and one label when in maintenance mode.
You can’t always rely that the switch will be in the “maintenance mode” when the worker is performing a relatively high-risk task on or near the equipment.
Here is a white paper from Square-D that goes over maintenance switches.
Reduce available fault-current
High resistant grounding
When an arc flash occurs, chances are (and I’m not sure what percentage but it’s high) that the arc started with a phase-to-ground short circuit as opposed to a phase-to-phase short circuit.
If you have high resistance grounding on your system then you effectively eliminate the risk of a full-blown arc flash from happening. The phase-to-ground fault will sit at low amperage (5 or 10 amps) and never cascade into an arc flash explosion.
Here is how it works:
This greatly reduces the risk of an arc flash.
The one thing to keep in mind is that it will not lower the incident energy levels that are on your labels. The label will still need to account for the worst-case scenario… but the nice thing is high resistance grounding makes the worst-case scenario significantly less likely to ever happen.
There are a ton of options out there for lowering your arc flash levels… almost too many.
My advice, get an engineer who understands the difference between hazard and risk to help you decide on what methods of arc flash mitigation would be best suited for you.
If you're still wondering how to reduce arc flash incident energy in your specific situation please reach out to me by email at firstname.lastname@example.org.