It is currently Thu Feb 22, 2018 9:14 am



Post new topic Reply to topic

A 480V receptacle or panel is feed from a MCB located in an MCC. The MCB is usually in the range of
Calculate the arc-flash level at the panel/receptacle.
The instantaneous MCB would reduce the available fault to level to 0.
Use the same fault level as at the MCC, ignoring the feeder impedance and the protective device.
None of the above.
You may select up to 4 options

View results
Author Message
ekstra   ara
 Post subject: Remote panels/device
PostPosted: Tue Jul 02, 2013 8:06 am 
Offline
Sparks Level
User avatar

Joined: Fri Nov 09, 2007 3:27 pm
Posts: 54
Location: Texas
A 480V receptacle or panel is feed from a MCB located in an MCC. The MCB is usually in the range of 100A and appropriate wire size of about 150' length is the average feeder size.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Jul 05, 2013 6:45 am 
Offline
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
Two different choices given in one answer, may skew the results you are looking for.

Receptacle - no analysis required.
Panel - analysis/label most likely required.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jul 08, 2013 12:45 pm 
Offline
Sparks Level
User avatar

Joined: Fri Nov 09, 2007 3:27 pm
Posts: 54
Location: Texas
JBD wrote:
Two different choices given in one answer, may skew the results you are looking for.

Receptacle - no analysis required.
Panel - analysis/label most likely required.


Why would you say "no analysis required" for the receptacles? Isn't inserting the plug creates a potential fault?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jul 09, 2013 5:29 am 
Offline
Plasma Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 711
Location: Rutland, VT
A couple of things come to mind with receptacles:
1. There are no moving parts to fail during operation such as a switch or circuit breaker
2. The receptacle is well insulated and protected so that inserting a plug will not short out
3. Typically the receptacles are small (30/60/100A) 3 phase, and therefore the breakers protecting them are small...little fault current, fast acting.
4. One is not usually plugging in with the load set to be energized. There is usually a ON/OFF switch on the load being plugged in.

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jul 09, 2013 7:12 am 
Offline
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
LaszloZW wrote:
Why would you say "no analysis required" for the receptacles? Isn't inserting the plug creates a potential fault?

Creating an arc is not the same as creating an arcing fault.

My company's opinion is that there is not enough data on failures to indicate that properly maintained 'typical cords and plugs' pose a statistically significant risk. Of course, your company's risk analysis may use different criteria and therefore have different results.

Are you looking for an answer that is correct 99.99% or 99.99999% of the time?
How frequently are these plugs 'used', 2X per hour or 2X per year?
Are the plugs being used with equipment that is frequently rewired or otherwise likely to contain shorted conductors?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jul 09, 2013 8:51 am 
Offline
Sparks Level
User avatar

Joined: Fri Nov 09, 2007 3:27 pm
Posts: 54
Location: Texas
JBD wrote:
Creating an arc is not the same as creating an arcing fault.

My company's opinion is that there is not enough data on failures to indicate that properly maintained 'typical cords and plugs' pose a statistically significant risk. Of course, your company's risk analysis may use different criteria and therefore have different results.

Are you looking for an answer that is correct 99.99% or 99.99999% of the time?
How frequently are these plugs 'used', 2X per hour or 2X per year?
Are the plugs being used with equipment that is frequently rewired or otherwise likely to contain shorted conductors?


One of the most common failure mode of withdrawable circuit breakers and MCC buckets occurs at the stabs. Plug-receptacle combinations utilize the same technology to make and break connections.

How often MCC's and withdrawable circuit breakers are inserted/withdrawn? I would say with far less frequency than plugs are inserted. Again we're talking about 480V rated plugs, not 120V office receptacles where arc flash hazard is meaningless.

In addition, some of these receptacles do ahve an internal switching mechanism that supposed to be load rated, but not rated for short circuit interruption. Their function is to switch on the load AFTER the stabs are mated. The potential failure of those is at LEAST as high as a circuit breakers, so if we require arc-flash PPE to operate an MCB in an MCC, I don't see why we wouldn't require the same at the receptacles. If anything there is more reason to do so there.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jul 09, 2013 8:58 am 
Offline
Sparks Level
User avatar

Joined: Fri Nov 09, 2007 3:27 pm
Posts: 54
Location: Texas
wbd wrote:
A couple of things come to mind with receptacles:
1. There are no moving parts to fail during operation such as a switch or circuit breaker
2. The receptacle is well insulated and protected so that inserting a plug will not short out
3. Typically the receptacles are small (30/60/100A) 3 phase, and therefore the breakers protecting them are small...little fault current, fast acting.
4. One is not usually plugging in with the load set to be energized. There is usually a ON/OFF switch on the load being plugged in.


1. One of the most common failure of MCC buckets is the misaligned stabs.
2. Misaligned, bent stabs.
3. Establish the fault level and if it is 0, then fine, but don;t just say: Oh, its litte fault current......
4. Last famous worlds: 'usually'. Accidents and incidents are not routine occurences either. What happens when you plug in a shorted equipment?!


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jul 09, 2013 1:12 pm 
Offline
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
LaszloZW wrote:
One of the most common failure mode of withdrawable circuit breakers and MCC buckets occurs at the stabs. Plug-receptacle combinations utilize the same technology to make and break connections.

How often MCC's and withdrawable circuit breakers are inserted/withdrawn? I would say with far less frequency than plugs are inserted. Again we're talking about 480V rated plugs, not 120V office receptacles where arc flash hazard is meaningless.

In addition, some of these receptacles do ahve an internal switching mechanism that supposed to be load rated, but not rated for short circuit interruption. Their function is to switch on the load AFTER the stabs are mated. The potential failure of those is at LEAST as high as a circuit breakers, so if we require arc-flash PPE to operate an MCB in an MCC, I don't see why we wouldn't require the same at the receptacles. If anything there is more reason to do so there.


A plug may have exposed blades however the receptacle/cap does not, this in marked contrast to the relatively, if not entirely, exposed connectors/stabs on typical drawout breakers and MCC buckets.

Both too many and too few operations can affect the likely hood of failure. A motor control center bucket, or a circuit breaker, that has not been withdrawn from its cubicle in several years has a high probability of 'sticking' or 'jamming'.

Internally switched plugs typically have their contacts completely isolated from each other unlike the contacts inside of a circuit breaker, resulting in a much less likely L-L short circuit during normal operation.

I believe that when a proper risk assessment has been performed the results for 'plugs' would support Informational Note 2, to 130.7(A), that normal operation is not likely to be a hazard.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Sep 16, 2013 8:20 am 
Offline
Sparks Level

Joined: Thu Oct 18, 2007 7:10 pm
Posts: 250
Location: NW USA
I thought 480V receptacles were not operable energized(???). For this reason it seems most pin and sleeve receptacles have an adjacent disconnect switch. I am aware of only one brand name of load break rated 480V receptacles, that is "Meltric" and I believe such receptacles have an internal, spring-loaded, load-break, arc-extinguishing mechanism that opens as the plug is operated. As such the plugs would seem to not need a label as there is nothing going on that is likely to disturb an energized exposed conductor (the exposed conductors are not energized at time of disturbance). I would not wish to operate a 480V plug without such protections.

The question becomes a bit more pertinent in my mind if one considers instead of a plug, a 480V drawout fuse holder. These are sometimes used as disconnect switches, have no apparent arc extinguishing mechanism and don't typically work in concert with another enclosed switch. I do not prefer these on 480V installations, but they do exist.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Sep 16, 2013 8:22 am 
Offline
Sparks Level

Joined: Thu Oct 18, 2007 7:10 pm
Posts: 250
Location: NW USA
As the sub panel in original question is likely to require work when energized, and as the arc exposure will be affected either higher or lower by the resistance of the feeder cable, this would require a separate calculation and label, in my opinion.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Sep 17, 2013 4:17 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1878
Location: North Carolina
All of this depends on the plug. A standard NEMA 60 or 100 A plug does not specify a disconnect. Most welders and fans might have an on/off switch that can be accidentally left on and it is common to mount the plug on a disconnect. But this is definitely not a requirement and I've seen plenty that have provisions but leave it up to the operator. I was accused in one plant of a lot of things because the operator plugged in a 20 hp fan without turning off the disconnect and the plug arced and exploded for the third time in about 6 months, despite getting away with this practice "for years", and after repeated coaching sessions. Instead of eliminating the problem (the operator) we applied intelligence by requiring electricians to connect/disconnect these loads.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 3 guests


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:  
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883