# How to calculate arc flash boundary

If you’re looking for the quickest and easiest way how to calculate the arc flash boundary then you’ve come to the right place!

There are a number of ways to do it, all of which come with their own quirks and nuances but in this article, I’m going to assume a couple of things:

- You aren’t planning on doing an arc flash study
- You are likely going to be using the category (or table) method to determine the arc flash PPE.
- You’ve realized this doesn’t give you the arc flash boundary… so you want a quick way to figure it out.

If that sounds like you then keep reading!

## What is the arc flash boundary?

Originally this was referred to as the arc flash protection boundary… until someone realized it didn’t offer much protection.

The reason?

This boundary (or invisible barrier) is the distance away from a potential arc flash hazard that someone would receive 2nd-degree burns if exposed to an arc flash.

The reason this was selected is because that is what is considered the survivable level… not the most comfortable level.

So, instead of changing how far away this distance really is the term protection was dropped and now we have the arc flash boundary.

### Why do I need it and what do I do with it?

During any electrical job (that will expose people to an arc flash) the worker must set up a barricade at or beyond the boundary.

You can accomplish this with red or yellow tape, cones, pylons, a stand-by attendant… really any of the traditional ways one would alert someone of a hazard that is not always present.

### OK… How do I calculate it?

Like I mentioned before, there are a number of ways to calculate the boundary but the one I’ve chosen as a go-to is the Ralph Lee calculation method (here is a link to his white paper).

Here’s what the equation looks like…

Now, if you’re not a math wiz don’t worry, stay with me… I’ll walk you through this.

Dc is what we are trying to determine… the arc flash boundary.

4.92 is just a constant… Ralph Lee figured it out, you don’t have to.

MVA is from the transformer that is feeding the piece of equipment you are working on. Go find the transformer and look at the nameplate. MVA should be there… or KVA (divide KVA/1000 to get MVA).

*note:* if the MVA is less than 0.75 you’ll need to multiply it by 1.25 in order for the calculation to work.

t is the time that the arc flash is supposed to last. This one can be a bit tricky but you can follow these rules of thumb for the most part:

- current limiting fuse t = 0.004 seconds
- 5kV to 15kV circuit breaker t= 0.1 seconds

### An example arc flash boundary calculation

Let’s say I’m racking out a 5kV circuit feeder breaker that is fed by another 5kV circuit breaker (which I’ll refer to as the main breaker). Above that is a transformer with an MVA equal to 10.

So if an arc flash were to occur when I was racking out the feeder breaker then I’m relying on the main breaker to trip. We know from our rule of thumb that this will take 0.1 seconds.

How far away do I need to place my barricade?

Let’s look at the equation.

My barricades need to be set up at 2.22 meters or about 7 1/2 feet.

Remember, this is only to keep others safe. Anyone who is working within the arc flash boundary will still need to be wearing arc flash PPE.

### I hope this helps!

This should give you a handle on how to calculate arc flash boundaries. It’s not perfect, and remember that there is some limitation to this method, but when you have nothing else it will certainly do.

If you feel like after reading this article that maybe you’d be better off with an arc flash study then please let us know if we can help!

### Looking To Learn More About Arc Flash Studies?

Download our free Definitive Guide to Arc Flash Studies to help get a more detailed understanding of the potential arc flash hazard at your facility.

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