Arc Flash Boundary

[bdm5066]

I'm struggling with the concept of the "Arc Flash Boundary" as defined in NFPA 70E. Does the boundary only apply if live conductors are exposed?

For example, if I'm walking by an energized MCC (not interacting with it in any way), do I need to stay out of the arc flash boundary (assuming I'm not wearing appropriate PPE)? This assumes all doors are closed and no live conductors are exposed.

Thanks in advance for the insight.

 

[EC Dan]

The boundary only applies if there is a likelihood of an arc flash occurrence, and you can use table 130.5(C) to estimate the likelihood of occurrence for certain activities. If there are no other risk control methods besides PPE, then the incident energy and arc flash boundary must be determined to determine the appropriate rating for PPE.

 

[David Castor]

Using your example, NFPA 70E does not require PPE in this situation because there are no exposed live parts and no interaction with the equipment.

But some situations can require PPE even if there are no exposed live parts. Racking in a circuit breaker would be one example of this. Also, PPE is recommended when energizing equipment for the first time or after maintenance even if no exposed live parts..

 

[Jraef]

But yes, the term "Arc Flash Boundary" only refers to EXPOSED conductive parts. If the doors are closed and properly secured, that concept no longer applies (assuming compliant equipment in original factory condition with proper maintenance etc. etc.)

Keep in mind though that if an MCC is in a common walkway, and an electrician must open any door to check something, the Arc Flash Boundary is now valid, so it means setting up barriers to keep non-qualified / non-PPE suited people away from the AFB. For this reason it's no longer a good practice to put MCCs and Switchgear in areas that are frequented by unqualified people. I know in a lot of older facilities they put them wherever they fit and in one place I was just at, when they screen off the MCCs to work on them they end up blocking the personnel walkway and making people walk in the fork lift traffic area, which means having to cone off and e-route the fork lifts. It's a big circus production now every time they need to work on the MCCs.

 

[David Castor]

I'm going to disagree with Jraef on this. Exposed live parts are a sufficient condition to have an arc flash hazard, but not a necessary condition. If someone is "interacting with the equipment in such a manner that could create an electric arc" then an arc flash hazard could exist and this requires an arc-flash boundary. Racking in a circuit breaker is a prime example of this.

Decades ago, we used to design water treatment plants with the MCCs as part of the control room. This was not a great approach for arc-flash safety as it turned out.

 

[Jraef]

I'm going to disagree with Jraef on this. Exposed live parts are a sufficient condition to have an arc flash hazard, but not a necessary condition. If someone is "interacting with the equipment in such a manner that could create an electric arc" then an arc flash hazard could exist and this requires an arc-flash boundary. Racking in a circuit breaker is a prime example of this.

Decades ago, we used to design water treatment plants with the MCCs as part of the control room. This was not a great approach for arc-flash safety as it turned out.

Yes, but...

For example, if I'm walking by an energized MCC (not interacting with it in any way),