It is currently Thu Jun 04, 2020 12:54 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: All faults are Arc Flash event?
PostPosted: Wed Apr 01, 2015 6:23 am 

Joined: Wed Apr 06, 2011 2:20 pm
Posts: 16
Are all faults( Short circuit or ground falt ) Arc Flash event?


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: All faults are Arc Flash event?
PostPosted: Wed Apr 01, 2015 8:15 am 
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 514
Location: Wisconsin
abk453 wrote:
Are all faults( Short circuit or ground falt ) Arc Flash event?

No.

A solid 'bolted' connection (e.g. wiring error) may result in a fault, but there may not be any arcing.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: All faults are Arc Flash event?
PostPosted: Thu Apr 02, 2015 4:11 pm 
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Not all arcing faults result in an "arc flash" event, never mind a "significant" arc flash. At low voltages/available fault currents, you may get sparks but if the air path is not heated enough to cause it to conduct the arc may self-extinguish after 1-2 cycles. In DC arcs this is a little more obvious because at first you get "glow" then "sparks" then "arc". In the "spark" and "glow" regions the current is relatively low and the arcing voltage is significant. Eventually the arcing voltage reaches almost a constant and the current becomes limited only by the available current. This s a power arc and what is responsible for an arc flash. In AC the same thing is happening except that the arc extinguishes and restrikes 120 times per second, and in a 3-phase system arcs jump around a lot since only 1 phase at a time passes through zero current.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: All faults are Arc Flash event?
PostPosted: Mon Apr 06, 2015 2:03 pm 

Joined: Fri Aug 26, 2011 7:38 am
Posts: 32
Location: Baltimore, MD
I have proof that you can have faults without flashes.

I once had a project where a large high school had an existing 3000-ampere 120/208-volt wye service, and they were building an addition on the other side of the building. I drew up plans showing a new feeder from the main switchboard supplying a delta-delta transformer, 208-volt delta primary, 480-volt delta secondary. The transformer secondary had the B phase intentionally bonded to ground. From there a 480-volt feeder went the length and width of the building to the addition's electrical room, where we had a transformer go from 480-volt delta to 120/208-volt wye (with the secondary's neutral intentionally bonded to building steel, water service, a ground rod, and the equipment grounding conductor - NOT the grounded phase conductor! - of the 480-volt feeder).

I got the call from the electricians. As soon as they energized the first transformer, the new circuit breaker in the switchboard tripped. Clearly my design was flawed- or so they thought. I knew better but drove down for a first hand look. They had intentionally bonded the B phase to ground- the B phase of the 208-volt side- the primary. The contractor knew intellectually that the 480-volt size was the secondary but whoever wired it did it the way they always did it.

I explained that they had created a bolted single-phase fault, and that the circuit breaker had clearly demonstrated it was capable of interrupting that fault current! I recommended (but didn't have the authority) that they have the breaker tested before using it again; I don't know if they did. I do know that they disconnected the 208-volt B phase from ground and bonded the 480-volt B-phase to ground, and that I never heard of any more trouble from the installation.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 7 hours


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883