Is anyone aware of a need to remote rack equipment calculated between 25 and 40 calories? It is my understanding that for that range, properly rated PPE protects sufficiently. It is not necessary to rack remotely. At energies above 40 cal, the dangers are a percussive nature and cannot be protected with PPE. I'd like to know your thoughts. Thanks in advance!

Please remember, in the hierarchy of risk controls, PPE is the last option. Remote racking would be a wiser choice.

Hi Josiah. Just a couple of items here. It sounds like you may be caught in the so called "40 cal" myth. You may want to investigate into some of Jim's articles and white papers on this subject. Also your statement "It is my understanding that for that range, properly rated PPE protects sufficiently". With properly rated clothing and PPE protects against any incident enrgy level, it is not just subject to the 25- 40 cal/cm2 range

If you need to rack a breaker, consider investing in some kind of a remote racking system. These put the operator well away from the breaker being racked. They're not cheap but this is not something to take lightly. You've seen the video of the person racking in a large breaker and the arc flash? If not, go to youtube and do a search. The video should convince you.

The racking mechanism in switchgear is pretty complex. They have to be maintained to function properly. In my early days as a Field Engineer, I saw many issues. They are fragile and break sometimes causing the breaker to be racked in mis-aligned. Also, switchgear have fairly high arc flash energy.

Also know that PPE does not prevent all injuries from arc flash. We are trying to protect the torso area with all of your critical organs. Usually, your hands are extended outwards when racking a breaker. This means that your hands and arms will receive more energy if they are located closer than the working distance used to calculate the AF energy. AF PPE is help you survive an AF accident, not prevent all injury and burns. As mentioned above in the last posting, AF is the lowest in the hierarchy of protection.

Thank you all for your replies. Our existing practice is to use protective devices, fiber optic pickups, and maintenance switches to bring incident energy down to 25 cal. I understand that it is always safer to remote rack but considering the practicality when there are hundreds of breakers throughout the plant that would need modifications for new racking equipment, we have to weigh options and implement on a priority basis.

I'm not sure what the "40 cal myth" is. It sounds like Leonard might be telling me percussive forces capable of injury do indeed occur between 25 and 40 cal. This is the heart of my question. If this is true, then why don't we label cubicles above 25 cal as Dangerous and instruct not to work on live? It seems to me that the industry has decided that 40 is the boundary and not 25. Thanks again.

The myth is that above 40 Cal/cm2, the arc blast becomes the dominant hazard. Meaning the blast would do more injury than if the electrician wearing AF ppe is exposed to an AF. Many (including our company) print Red Danger labels instead of Orange Warning labels for locations above 40 Cal/cm2.

It turns out that arc blast is a function of the amount of fault current, enclosure size, and type of door. With the door off, there is not much of a pressure wave. Hugh Hoagland of e-Hazard has done research into this area and has several articles about it.

Posting a red label stating"Dangerous, remote rack only" or (Dangerous, no safe PPE exists" is what we do, too. (if we can't get it below 25 by using the options I stated earlier)

First, if you are using a maintenance switch and only dropping incident energy to 25 calories... I would think you should be able to get it much lower. In LV, with a maintenance switch there is no reason to be > 8.

I think you will find, for various reasons, most people will advice using the remote racking capability first, PPE 2nd.

The force created by an AF event is not the same as calories. Force tends to be higher in MV than LV, tends to be higher with higher fault current, may be higher depending on the size and geometry of equipment around the arc, and may depend on how the arc establishes itself and what materials are being ablated. I.e. there are a LOT of variable and it is very chaotic. WHAT does protect you against blast and heat is distance, hence the remote racking!

40 cal was adopted as a stand-in for dangerous forces, but the reality is that it is not a good stand in, hence it is decreasingly used for that purpose.