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 Post subject: Switchgear Shrouds/Barriers
PostPosted: Wed Jun 29, 2011 4:35 pm 

Joined: Wed Jun 29, 2011 3:33 pm
Posts: 14
I put the following question to Jim Babcock at SKM early 2010, with his response below. From people's experience, is this still the concensus in the industry, i.e. that shrouds/barriers are assumed to not be there?

What is the consensus on Shrouding and barriers between phases for Arc Flash studies? If these are in place, do you say that an arc flash cannot occur? If only barriers, do you use the “taut string” distance? Do you simply assume they aren’t there (in case they are damaged/installed incorrectly/not rated adequately etc).

Answer:
The use use of barriers between phases increases the gap and thus the risk of flash and lower calculated incident energies. But there is no concensus as there has been little testing using barriers, taping, etc. Some have expressed the concern about joints in the bus work being completely barriered. I know some of the engineering service companies have taped buses and joint to effectively increase that distance with good results. In this country, it is assumed that barrieers are not there.


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PostPosted: Thu Jul 07, 2011 2:52 am 

Joined: Tue Apr 12, 2011 6:11 am
Posts: 17
Location: Oregon, USA
To avoid any possible confusion - it would seem Mr. Babcock would have meant his reply to read :

"The use of barriers between phases increases the gap and thus reduces the risk of flash and lower calculated incident energies."

I cannot say what the consensus might be, but if it helps I will observe that barriers are often easily removed, and are sometimes necessary to remove as they can either hinder or obstruct certain work operations in some cases. It may not always be an ideal practice, but that's life. At the same time, a missing shroud or barrier may be an allowing factor for an arc to occur at all.

If assuming no barriers is usually conservative, my impression is that it makes sense to do so.


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PostPosted: Mon Jan 09, 2012 5:27 pm 

Joined: Fri Jan 06, 2012 12:46 pm
Posts: 3
Putting a shroud around the feeder protection device in the panel may very well mitigate an arc flash from that device if it closed, but it does not remove the possibility of an arc flash occuring at another device.
The can around the main breaker only protects you from the main breaker and the arc produced if it trips under load, but there will be a branch involved some where and that device will probably not be shrouded, because it is too small.
I am of the mindset to actually have two adjoined but somewhat electrically isolated panels. One for the control using only 24 VDC class 2 power supplies and the other containing the line voltage and any higher than 50 VAC/VDC secondary lines. The reason because most of the trouble shooting will occur on the low voltage side, if and when a line voltage device has to be tested energized then you have to don the PPE and do it.
You can do a good job with PLC I/O selection and solid state relays coupled with careful wiring layout isolating the line from the control.


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PostPosted: Tue Jan 10, 2012 8:05 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
What is missing here is the usual problem...the risk assessment portion.

Even if you calculate the potential arc flash, the second part of the exercise would be to look at the tasks involved and the probability that it would occur, and whether or not this is an acceptable risk.

Most people are just assuming that it WILL occur.

An example of why this might happen is when in the 2012 NFPA 70E, the notes in more than one place say that the Technical Committee feels that given proper maintenance and operation, the risk for 600 V or less MCC's is not likely.

Also in the 2012 70E, they totally revised the risk assessment in the appendix. BUT, unfortunately it's even worse than the old version...it's not a complete risk assessment method on par with say those that are ANSI standards.


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