# Arc flash opinions wanted



## Thedroid (Dec 6, 2010)

The site is a mine, and not under 70E. Wearing PPE is the responsibility of the individual.

I'm not familiar with the calculations, but I think that the main secondary would be the highest cal/cm potential.


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## gesparky221 (Nov 30, 2007)

The task for racking out MV breakers at our plant is listed on our study as ED, extreme danger. The instuctor that led our arc flash training basically told us you would need a switching hood, and proper rating of the clothing which I think mine is a 4. But then he tells us if a flash occurs at that level you will look good for the funeral. The blast effects will kill you but you won't get burned. We always shut all the loads down, and open and rack out the mains before opening air break switches.


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## Zog (Apr 15, 2009)

Thedroid said:


> I have my own on the subject, but want to here others. The load center unit substations consist of 4160/480 1500 KVA. The main secondary breakers are 1600A GE AK 1-50 and the feeder breakers are 600A GE AK 1-25. The breakers are being pulled for inspection, testing, and maintenance, and are not going into the same holes. The main secondaries are heavy, and not easily handled, and although all cubicles are being cleaned, the breakers live in a dusty environment, and have for over 50 years.
> 
> The main secondary can be opened in order to remove the feeder breakers without risk, but removing the main secondary requires a different approach. The incoming line is fed by a MV circuit breaker into the primary section air interrupter switch. These switches are very rarely exercised, and the switching procedure would have to be studies before any attempt to open these switches. The MV switchgear is also 50 yrs old, but in good condition, and easy to open and rack out. The breakers are not going back into the same compartments, and can be difficult to install, especially on the one AKD-5 load center. The older AKD switchgear do not present as much troubles.
> 
> What is the least dangerous approach to handling this job?


 Remote switching and racking



Thedroid said:


> Where is the highest arc flash potential?


 that would require an arc flash study to determine, but typically the 480V main. 



Thedroid said:


> Should the primary air interrupter switches be switched with the power on?


 Depends what they are rated for, typically for this vingae gear you will have a key interlock in the MV switch that requires the LV main to be opened and racked out to be able to get the key to unlock the MV switch. 



Thedroid said:


> Does racking in/out the MV breaker present more of a arc flash than the main secondary breaker?


 Again, need an arc flash study, but typically the MV side will have a lower arc flash hazard. 



Thedroid said:


> I feel the main secondary breakers should not be racked in or out with the power on. Period. The temporary loss of power does not outweigh the risk, and this type of maintenance would normally only happen during a production outage. I don't like the idea of opening the air interrupter switch either. They could benefit from some maintenance, but are not a problem as long as they're left alone. I feel that racking out the MV breaker is the safest way of isolating the breaker.


 Likely you are right, but need a study to know for sure. At least use the tables.


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## mrmike (Dec 10, 2010)

It was always my opinion to shut down all loads that are hooked to any breaker or switchgear before disconnecting or "opening" it up. 
The load is going to be Disabled anyway so Why interupt it under load??? I had an electrical engineer/foreman tell me that a certain 13,800 volt pole mounted switch was rated to be disconnected under heavy load "like 2000 amps" I don't remember the exact figure. Well I questioned him- why do it under load when we don't have to??? Everything will be going down anyway. 
Well, guess what we did anyway & it arced big time & it the knife stuck a little because of the load & smoked & arced big time !! After that he changed his mind & the procedure was to shut all equiptment down first.!! It is going to be going down anyway!! So to put my 2 cents into this question- the power "voltage" can be on but load "amps should be off.....................that is where the Arc, Flash etc comes from.........................


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## Big John (May 23, 2010)

I'll add a couple cents:

I absolutely agree with "no racking while energized" policy. Racking an energized breaker is probably one of the more dangerous jobs an electrician will ever encounter. 

Regarding your interrupter switch, we have a bunch of those that we often switch energized with no problem. Just be aware it's likely not a load break switch, so you do want to be a certain you are switching it under zero load. 

From what you say your MV gear is in good shape, and you're unsure about your air-break switch. If it were me, I would open the circuit with the MV breaker while wearing appropriate PPE. Then I would open my air-break switch and do visual confirmation that each phase is disconnected. The reason I wouldn't use these as a primary disconnect (even with zero load) is just because you are unsure of the switch operation. 

Then I would be entirely comfortable racking my LV gear.

Personally, I would use this outage as an opportunity to run the AB switch. It's there precisely for this reason, and if you can't use it, there's no point in having it. I would be prepaired to do tune and lube on it while the breakers were being worked on. Then make that part of your regular maintenance in the future so you know it will function reliably.

You have three other options: 
- Rack the MV breaker live. Which is a really crappy option.
- Remote rack the MV breaker. Much better option.
-Work behind the MV breaker while racked in and locked open.

I would not be happy with either one. The only way I would even consider the second option is if you could safely get appropriately rated grounds on the load side of the MV breaker.

So, to sum up all that nonsense: I'd either get a remote racker, or I'd bite the bullet and operate your air-breaks.

-John


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## Thedroid (Dec 6, 2010)

Well the job went well, with no issues. 54 LVPCB's racked out, cleaned up, contact resistance checked, and primary injection tested with no accidents. I racked out 11 MVCB's in order to do the job. I still don't feel comfortable messing with the main secondaries live, especially after having trouble racking in several breakers. Id like to take the opportunity this shutdown to look a little closer at the air break switches, and become more familiar with them. Racking the MV gear is scary even with the ppe on.


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