Arc Flash Boundary - Eight Rules of Thumb

Arc Flash Boundary

 

With electrical equipment there are two main hazards to consider, shock and arc flash. 

We’re going to look at the arc flash hazard. Essentially the closer you are to the arc the hotter it gets. 

For someone working on or near exposed parts that have an arc flash hazard, the important thing to know is how bad it could be…this is called the ‘Incident Energy’. For electricians, this means wearing the right PPE when they get up to work. For everyone else that means keeping a safe distance.  

We’ll look at how that safe distance is communicated, where it comes from, what it means, and who’s responsible for it.  

For more on Arc Flash ‘Working Distance’ and Shock Approach Boundaries, see this post, Arc Flash Boundary: Working Distance & Approach Distance Explained 
 

Typically, the arc flash boundary is the largest boundary, but in some cases the shock hazard limited approach boundary can be larger. The largest boundary sets the approach limit and determines where a barricade should be set up to alert others. 

 

 

What is an arc flash boundary?

 

In the safety world, a boundary is an approach limit to a hazard. 

With an arc flash boundary that hazard is the onset of a 2^nd degree burn.

So still not super safe, but not life threatening.

Arc flash PPE protects against the short burst of intense radiant heat that is released from an arc up close. Without the right protection equipment covering your entire body, the safe distance you must be is the arc flash boundary.

It’s calculated as the distance from the arc source for the hazardous energy released to drop to 1.2cal/cm2. 

For a more look at the some of the calculations, see this blog, What is the Arc Flash Boundary? - Arc Flash Boundary Calculation

If you're considering an arc flash study, feel free to click the link below. We can start by reviewing your single-line diagram to determine the best path forward.

At Leaf, we utilize a four-phase approach designed to ensure both worker safety and full regulatory compliance. The study is a critical component of this process, and we’d be happy to walk you through how it fits into a safety strategy. 

 

 

Where does the Arc Flash Boundary come from? 

 

Practically:  It’s determined mostly by the size of the arc flash. The bigger the arc flash, the further it goes. This distance can be found on an arc flash hazard label which is applied to each piece of equipment included in an arc flash hazard study. The boundary is just a number on the label until an electrician opens something up exposing live conductors and does a task that could potentially start an arc flash. Since the risk of arc flash isn’t always present the alerting methods are temporary. An electrician determines the hazard from an arc flash label and sets up a zone around the work location to alert people of the safe distance.  

 

 
For more information on what an arc flash hazard study includes see this, Arc Flash Study Services.

Technically: It’s the approach limit from an arc source at which the incident energy equals 1.2cal/cm2. Incident energy is a measure of radiant heat released for a given surface are. It’s caused by an electrical arc sustained in plasma, after the current flow has been initiated and is sustained across the conductor gap. The heat radiated from this arc is what burns you, but as radiant heat decreases by the inverse square law as it travels (see image below). 

The intensity is inversely proportional to the distance squared. 

  

If each ray (red lines) in the graphic above provides 1.2cal/cm2, the arc flash boundary would be set at the distance where only one ray goes through an area (A) within a limited amount of time (2 seconds). 

 

 

Arc Flash Boundary Calculator

 

This is an embedded interactive graph for understanding where the arc flash energy is related to distance. Explore the importance of distance with arc flash hazards. Change the size of the arc flash by adjusting the time and current.

Based on Doughty et. al paper (1998). Reference Z462 Annex D.3. For updated methods, see IEEE 1584-2018.
 

Full Interactive Calculator Arc Flash Boundary Calculator - Doughty Method | Desmos

 

 

 

 

Current, Time, Distance: To know exactly how big a hazard you might be dealing with you need an Arc Flash Hazard Study. We assess each high potential hazard location that is worked on at your site. For each location the worst case arc flash is determined, given the range of available fault current, protective device operating times, and equipment types and working distances.

 

 
When should you be worried about the boundary?

 

Thankfully, the likelihood of any piece of electrical equipment randomly starting an arc flash is very low. Walking by a piece of equipment does not increase the likelihood of an arc flash occurring. 

Electrical conductors are protected by the equipment enclosures to prevent any accidental contact. An arc flash is typically initiated by work being done on or near the exposed conductors. 

If everything inside the front cover is a ‘dead front’ and finger-safe (meaning, there’s no way to contact any exposed conductors) the arc hazard likelihood might not be increased by their work.  

However, if you see someone working on something with the doors open, be cautious, and try to ask if they consider there to be an arc flash hazard present.

If the work requires wearing full arc flash PPE, there is always an associated arc flash boundary, and the area should be zoned off to help alter others! 

 

 
What type of work makes the boundary active/in effect or engaged? 

 

Once the conductors are exposed the likelihood of an arc flash occurring goes up since any task, or moving parts and some types of tools could initiate an arc. 

This means the hazards on the label and arc flash boundary may apply. 

Here’s an example: 

Opening hinged door(s) or cover(s) or removal of bolted covers (to expose bare, energized electrical conductors and circuit parts).

Arc Flash increased Likelihood of Occurrence: YES

 

However, there are a few observation style tasks that once the doors are open don’t increase the likelihood if you stay far enough back. 

Performing infrared thermography and other non-contact inspections outside the restricted approach boundary. 

Arc Flash increased Likelihood of Occurrence: NO

 

There are also exceptions for abnormal or faulty equipment, as well as certain types of jobs that have higher risks. 
 
Table 2 in CSA Z462: Workplace Electrical Safety has lots of examples of the type of situations where task increased the likelihood of an arc flash occurring.  

Here’s a section from the middle of the table, where the likelihood is always YES: 
 

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How can you alert others when the boundary is in effect?

 

Essentially, anytime someone needs to wear arc flash PPE, there should be a barricade up to alert bystanders of where the hazardous zone is. 

If you are the qualified electrical worker and person in charge of the work being done, part of your Job Safety Plan should include setting up a barricade. 

Even a job where everything is locked out will still require dressing up to the required PPE while testing for dead. An arc flash hazard may also exist when you’re powering something back up for the first time after newly completed work. So, a means to alert bystanders should be in place before anything is exposed, and until everything is and running smoothly. 

Putting up some warning tape clearly shows which area is restricted while you are working. 

In some cases, with large arc flash boundaries in smaller electrical rooms, it might make more sense to have someone guard the door until it’s safe to enter.  

If a piece of equipment is being worked on and has the barricade in place, only those working on or qualified should enter the zoned off area, and everyone inside must have the appropriately rated PPE when required! 

To be qualified to enter the arc flash boundary, you’ll need to some electrical hazard awareness training. Once the equipment is verified to be in an electrically safe working condition, PPE can be removed, and equipment work continued. 

 

Markings, Tape and other measures

 

Since an arc flash hazard isn’t always in effect it makes sense that the alerting for the arc flash boundary is also temporary. 

Putting up a temporary barricade is the simplest method of restricting access to unqualified persons to the hazardous area. Using a few safety posts with warning tape zoning out the area will communicate this clearly. 

Sometimes electrical equipment on the worksite floor may have permanent barricades and lines painted around it. These aren’t typically for the arc flash boundary. They are usually to comply with electrical installation codes and to help protect the electrical equipment from physical damage or show that the area isn’t to be used for storage. 

There may also need to be some communication and coordination ahead of a job if the area being barricaded needs to regularly accessed to continue regular operations. 

And finally, don’t forget to make sure you have these materials available for your electrical team or contractors when the time comes. 

For more information on other requirements for equipment in an electrical room, Industrial Electrical Room Safety: Requirements Guide
 

 
Distances and what they mean, how precise are they?

The arc flash boundary distance is calculated and printed on equipment hazard labels. This number is usually accurate to the inch, but you don’t necessarily need to get out your measuring tape, unless you’ve been waiting for the opportunity! 

Adding a few feet, or simply blocking access to the whole room, isn’t likely to disrupt anyone’s route.  
If your arc flash study is out of date or hasn’t been done, a safe rule of thumb can be applied using the boundaries from Table V1 of CSA Z462 Appendix V. You can find more information on this here, What Arc Flash PPE is Required if There Is No Label 

It provides a good rule of thumb for low voltage equipment (<=600V). Essentially if the equipment has an upstream device that is equal to or smaller than a 
•    45 KVA transformer, 
•    200A fuse, or 
•    80A breaker 

the arc flash boundary won’t be larger than 2 meters (about 7 feet). This is for Category 2 equipment (8 cal/cm2). 

For Category 4 equipment (40 cal/cm2) the upper limit on an arc flash boundary is at 4.5 meters (about 15 feet). 

These rules of thumb might work for areas where you can afford to tape off large areas or infrequently worked on equipment where you don’t mind wearing a lot of extra PPE. 

However, if you need more flexibility to keep operations running in a busier or fuller site floor or MCC electrical room, consider looking into an arc flash hazard study or Download our free Definitive Guide to Arc Flash Studies Below. 

Don’t have time to read our free resources? Request a Free 15 minute call with an Electrical Safety Expert today by emailing training@leafelectricalsafety.com or by booking here -> Book Now