Arc Flash

[neil seidner]

Does anyone know what the difference is between an "Arc Flash" and a "Flash Over"?

[bigrockk]

I look at it this way, An Arc flash is caused by a short circuit, or contacts / switch gear opening under load etc.

Flashover usually occurs internally in motors, generators or alternators etc (mostly d.c. machines in my experience) due to internal faults, dirt, moisture or severe operating conditions. Once the "flashing begins the air becomes ionized and the flashover is perpetuated due to the ionization.
I am sure there are many more examples of Arc flash and Flashovers but these are a few examples I have come across in the past.

Below are a couple of excerpts from articles I found on the "web".
The links at the bottom of the page will lead you to the full articles.
Hope this helps!

An electric arc flash can occur if a conductive object gets too close to a high-amp current source or by equipment failure (for instance, while opening or closing disconnects). The arc can heat the air to temperatures as high as 35,000 F, and vaporize metal in the equipment. The arc flash can cause severe skin burns by direct heat exposure and by igniting clothing.

Flashover - an unintended electric discharge (as over or around an insulator) electrical conduction through a gas in an applied electric field

Corona is a serious issue in metal clad switchgear because of its highly destructive nature?it is the cause of most flashovers in the equipment. The fundamental cause is an electrical breakdown of air brought on mostly by an insulating deficiency within the switchgear. Corona results from the ionizing of gases because of this high electrical stress. The stress is often brought on by contamination and poor insulation values.

http://www.mt-online.com/articles/0703flash.cfm

http://www.cdc.gov/elcosh/docs/d0500...0/d000580.html

<font color="#a62a2a" size="1">[ May 15, 2004 11:33 PM: Message edited by: bigrockk ]</font>

[neil seidner]

Thanks Big Rockk. I understand it a little better now. I am trying to figure out if a fault that occured earlier this week at work was an Arc Flash or a Flash Over.

The fault occurrred in a 600V starter on the line side terminals. All 3 terminals had burn marks on them. All 3 fuses in the disconnect feeding the starter failed. The contacts in the starter showed no signs of damage. Dust (containing metal) from a grinder probably worked its way into the enclosure and settled near or on the terminals, causing the fault.

<font color="#a62a2a" size="1">[ May 16, 2004 10:57 AM: Message edited by: ccrash ]</font>

[bigrockk]

Well my guess would be to call it a flashover.

My assumption would be that the "grinding dust" caused "tracking" between the line terminals and this lead to a flashover causing a short circuit condition, which in turn took out the fuses.
Were you able to clean up the starter or was it destroyed?

[neil seidner]

Thanks Big Rockk. What you are describing, in terms of tracking causing a flashover, seems to make sense. The starter was able to be cleaned up and reused. The millwright agreed to build something so that the dust would be directed away from the starter.

P.S. Go Flames Go!!!

[twh]

While we're on the topic, a co-worker installed a fuse in a live circuit and the arc "flashed" to the other lines. Thought it was worth mentioning.

bigrockk, where in Saskatchewan are you?

[bigrockk]

Yikes! Hope your co-worker didn't receive any burns.

TWH;
I am just a little ways north of Saskatoon off of Hwy.11...Just moved here a little while ago from Calgary (Go Flames Go!!!)
Love the area,its great!
How about you TWH...close by?

<font color="#a62a2a" size="1">[ May 17, 2004 01:08 PM: Message edited by: bigrockk ]</font>

[twh]

I'm in Regina.

Welcome to Sask.

tim

[watts45]

I was wondering if we will have to use arc flash suits or simular personal protection equipment as a part of our daily job servicing 600 volt switch gear. Guess that goes for 208 volt services too.

[twh]

Won't be a concern, if you already shave your eyebrows.