# arc flash with no frequency



## Wirenuting (Sep 12, 2010)

nolabama said:


> Say I open a peckerhead of a motor that has been given voltage but no frequency is the hazard the same? Example 350vac 0Hz


350vac at 0hz?
No such animal except for during that .001 second of time.
You must have been quick to catch that "AC" reading.


----------



## Big John (May 23, 2010)

Wirenuting said:


> 350vac at 0hz?
> No such animal except for during that .001 second of time....


 If a drive is putting that out, is it putting out straight DC?

If so, the flash hazard may even be worse. DC is much better at sustaining an arc than AC, but last I heard 70E was still working on effective flash calculations for DC.

-John


----------



## Wirenuting (Sep 12, 2010)

Big John said:


> If a drive is putting that out, is it putting out straight DC?
> 
> If so, the flash hazard may even be worse. DC is much better at sustaining an arc than AC, but last I heard 70E was still working on effective flash calculations for DC.
> 
> -John


I agree with you about it being DC.
But he didn't mention a drive.


I don't know about a DC arc flash. I thought that you needed the frequency fluctuation to achieve the arc flash & sustained explosion. 
Having strait DC would create a welding effect I would think. 
It's something I never thought about.


----------



## nolabama (Oct 3, 2007)

Yes its is drive related. Drive has start command but not run.


----------



## Wirenuting (Sep 12, 2010)

nolabama said:


> Yes its is drive related. Drive has start command but not run.


Ok, then maybe Zog can answer the Flash question. 

edit: looks like Zog did give EvilBunny some info last year on a DC arc.

http://www.electriciantalk.com/f2/dc-arc-flash-hazard-20774/


----------



## Big John (May 23, 2010)

Wirenuting said:


> ...I don't know about a DC arc flash. I thought that you needed the frequency fluctuation to achieve the arc flash & sustained explosion.
> Having strait DC would create a welding effect I would think....


 The only reason I say this is think of contact ratings: A switch that can safely interrupt hundreds of volts AC, may only be rated for 50 volts DC.

My understanding is that the zero crossing of AC is what helps make it self-extinguishing. Maybe a DC arc is harder to strike, but I'm pretty sure it's harder to extinguish, as well.

-John


----------



## nolabama (Oct 3, 2007)

That seems to deal more with the limited power a battery may contain. I think a stalled drive would have infinetly more power. But the drive should open in event of a fault? Or is this handled upstream?


----------

