It is currently Sat Nov 18, 2017 10:08 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: I'm confused
PostPosted: Thu Mar 10, 2011 8:41 am 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
We have a panel that is listed a HRC3. As I am planning a small job for this panel I notice a good portion of the breakers are the ones pictured. Obviously they have a 10k rating. Is it possible to have an HRC3 panel with 10k breakers installed and everything be fine? Thanks for the input.


You do not have the required permissions to view the files attached to this post.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 9:10 am 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
It is possible because time to clear is the biggest factor in the IE equation. So you may be under 10K fault, but taking seconds to clear and generating a large IE. You need to find out your RMS sysmetrical fault value to know if 10K breakers are sufficient.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 9:29 am 
Offline

Joined: Wed Nov 12, 2008 10:26 am
Posts: 46
Location: CA
The HRC3 applies to the incoming line and not the load side of the smaller CBs.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 10:38 am 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
So if

the fault level only applies to the line side of the breaker why do I have to be in CAT. 3 clothing to operate the breaker? Now I'm even more confused... :D :D


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 11:04 am 
Offline

Joined: Wed Nov 12, 2008 10:26 am
Posts: 46
Location: CA
If one of those 60A CBs blows-up while you're operating it, the maximum IE is related to the availble energy at the line-side, in-coming side, of the breaker panel.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 11:37 am 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
So

why don't the breakers need to be rated at a higher fault level? My dumb self sees this as an underrated situation.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 1:17 pm 
Offline
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
I think you are confusing fault current, AIC, and Ei. The difference between the line and load side of that breaker is not current, it is the duration an arc would be present based on the clearing time of the upstream breaker for the line side and the clearing time of that breaker for the load side.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 10, 2011 4:47 pm 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
I guess I'm seeing

it from Zeroseq's point of view. If the available fault current exists when operating the breaker how does it not exist if I hook up a load to the load side of the breaker and have a fault?

If I am understanding you all correctly (obviously not) I am somehow magically loosing some available current. Is is because the 10k is RMS? Thanks for the input.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 3:42 am 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
Possibly less confused

Ok so I got some sleep and I think I now understand. You could only have say 8k of available fault current but because the device up stream and long conductors cause the fault current duration to be to long you then end up in a higher HRC.

Am I even getting close to understanding????? Thanks again. I do appreciate the help.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 6:35 am 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
Yes. You are mixing up several distinct and seperate electrical parameters.

Fault current has many components but the simplest to understand is the bolted fault. Imangine someone took a copper buss and bolted it across all three phases where the incoming cables enter the panel. Then they turned on the power. V=IR, and because the voltage is constant, and R is near zero, I will be very high. If we looked at the steady state value of I, we will call that Fault Current RMS symetrical. The breakers in the panel need to have an AIC rating equal or greater to this value.

What is AIC do for you? Remember in high school the right hand rule of thumb for magnetizm - you put your thumb in the direction of current flow and your fingers wrap in the direction of the magnetic field. Only in AC power, the current direction is changing 120 times per second (there is one up and one down direction in each sinewave). With high current you get strong magnetic fields. These fields are inside the breaker, and they are pushing and pulling the seperate phase components to and from each other. If that 'push' or even the continuous working of those components causes them to break, or to pull fasteners out that hold them in place, they could touch each other. If they touch, its a short circuit and the high current melts the metals and the gases released caused the breaker to exploded. What the AIC rating does is test the breaker to make sure it can withstand that intensity of a magnetic field, without the components breaking and without the breaker exploding.

IE is the measure of heat intensity due to an arc, where something crosses phased conductors and causes that melting of components and the rapid release of gas. We won't call that an 'explosion' because it melting and gas generation take place in open air. There is no enclosing device to break and give way under the blast pressure. To arrive at a value for IE, fault current is one of the variables in the equation, but its only one, there are many more such as the spacing between the component phases, the distance your chest is from the blast point, whether the blast is in a box or open air, and one of the most important - the time it takes for upstream overcurrent devices to open and clear the arc. Time to clear, has the greatest effect of all the values. So even if you have low fault currents, you could have high IE values because the time to clear is very long.

As to operating the breaker, you are off track here depending on what you are doing. The HRC level on the sticker is for performing live work, that means you are removing the covers and working inside the panel while it is energized. If you are just operating the breaker dead front, meaning the covers are on and you can not see any live parts inside the panel - then you don't need any HRC or at least much less than the label. Why both, because 70E is about working on live parts, switching breakers with cover on is not live work. 70E doesn't apply to non-live work except for large energy switching like switchgear. NFPA generates confusing in that if you follow their table matrix for what HRC you need versus function, they call out HRC 0 for operating breakers with covers on. NFPA does not explain the discrepancy. My personal assumption, is that the HRC is only meant for people who operate breakers and 'also' perform live work like electricians. Having just installed a new breaker and button up the covers, there is a higher risk when the breaker is thrown on for the first time. Others will disagree.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 6:48 am 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
Yes. You are mixing up several distinct and seperate electrical parameters.

Fault current has many components but the simplest to understand is the bolted fault. Imangine someone took a copper buss and bolted it across all three phases where the incoming cables enter the panel. Then they turned on the power. V=IR, and because the voltage is constant, and R is near zero, I will be very high. If we looked at the steady state value of I, we will call that Fault Current RMS symetrical. The breakers in the panel need to have an AIC rating equal or greater to this value.

What is AIC do for you? Remember in high school the right hand rule of thumb for magnetizm - you put your thumb in the direction of current flow and your fingers wrap in the direction of the magnetic field. Only in AC power, the current direction is changing 120 times per second (there is one up and one down direction in each sinewave). With high current you get strong magnetic fields. These fields are inside the breaker, and they are pushing and pulling the seperate phase components to and from each other. If that 'push' or even the continuous working of those components causes them to break, or to pull fasteners out that hold them in place, they could touch each other. If they touch, its a short circuit and the high current melts the metals and the gases released caused the breaker to exploded. What the AIC rating does is test the breaker to make sure it can withstand that intensity of a magnetic field, without the components breaking and without the breaker exploding.

IE is the measure of heat intensity due to an arc, where something crosses phased conductors and causes that melting of components and the rapid release of gas. We won't call that an 'explosion' because it melting and gas generation take place in open air. There is no enclosing device to break and give way under the blast pressure. To arrive at a value for IE, fault current is one of the variables in the equation, but its only one, there are many more such as the spacing between the component phases, the distance your chest is from the blast point, whether the blast is in a box or open air, and one of the most important - the time it takes for upstream overcurrent devices to open and clear the arc. Time to clear, has the greatest effect of all the values. So even if you have low fault currents, you could have high IE values because the time to clear is very long.

As to operating the breaker, you are off track here depending on what you are doing. The HRC level on the sticker is for performing live work, that means you are removing the covers and working inside the panel while it is energized. If you are just operating the breaker dead front, meaning the covers are on and you can not see any live parts inside the panel - then you don't need any HRC or at least much less than the label. Why both, because 70E is about working on live parts, switching breakers with cover on is not live work. 70E doesn't apply to non-live work except for large energy switching like switchgear. NFPA generates confusing in that if you follow their table matrix for what HRC you need versus function, they call out HRC 0 for operating breakers with covers on. NFPA does not explain the discrepancy. My personal assumption, is that the HRC is only meant for people who operate breakers and 'also' perform live work like electricians. Having just installed a new breaker and button up the covers, there is a higher risk when the breaker is thrown on for the first time. Others will disagree.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 11:17 am 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
I follow you until

your last paragraph. If the energy is there and could displace itself while I operate the breaker I need the PPE. If I am confused and you are correct my life got a whole lot easier.

I understand what I was missing out on from before but now I need to re-confirm my beliefs about PPE for the breaker operation. You are obviously way ahead of me on all of this but I thought the PPE was still needed to operate the switch.

HELP!!!!!!! and Thanks


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 11:26 am 
Offline
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
haze10 wrote:
As to operating the breaker, you are off track here depending on what you are doing. The HRC level on the sticker is for performing live work, that means you are removing the covers and working inside the panel while it is energized. If you are just operating the breaker dead front, meaning the covers are on and you can not see any live parts inside the panel - then you don't need any HRC or at least much less than the label.


Have to disagree with you Haze, unless the equipment is arc rated it is not designed or tested to contain an arc flash. If you read the Definition of "Arc flash hazard" in section 100 it makes that very clear.

haze10 wrote:
Why both, because 70E is about working on live parts, switching breakers with cover on is not live work. 70E doesn't apply to non-live work except for large energy switching like switchgear.


I think you are confused. Not "live work" I can see, meaning you would not need an EEWP, but that does not mean there are no PPE requirements.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 12:26 pm 
Offline
Sparks Level

Joined: Wed May 13, 2009 3:19 pm
Posts: 56
Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.

I would think operating a small molded case breaker would constitute as “normal operating conditions”.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 12:37 pm 
Offline
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 57
Properly maintained

means what. Most of my stuff is 40+ years old and is still providing juice to the load but I can't say it is working or maintained. Thanks again for the input.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 3:02 pm 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
Confused,
Unfortunately this makes your education even more difficult. But what you see here are differing opinions for arc flash regulations. Some, like me, believe that 70E only applies to live work, or the switching of high energy. Others, believe it applies to all switching even with dead fronts. Most of those people stop at the 120V circuit breaker level. The real hard core point to light switches and plugging in the coffee pot. In order for you to make an intelligent decision you have to read 70E, Art 130 and make your own conclusions.

The use of HRC clothing is outline in Art 130. The only reference for the use of HRC clothing in the text is when switching high energy sources, and in the Matrix Table. Many of us, but not all, say high energy is switchgear. You are free to establish that point at 400A circuit breakers or even 20A circuit breakers. If you follow the Matrix Table, then switching dead front is HRC 0. If this is an industrial site and you follow the table for other work, you should follow it for this work. If the janitor switches off the office lights at the light panel, and thats something you want to do, then your inhouse policy should justify the discrepancy, or your policy excludes 20A 1Pole 120V breakers.

Your question started out as to why you have 10KAIC and HRC3. If you want to discuss operations I'd start another thread with the specifics of what you want to do, and whether you are following the matrix method or the calculated IE method.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 11, 2011 9:58 pm 
Offline
Plasma Level

Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
You do make a good point, as a switchgear guy when I think of operating breakers I am thinking high energy systems. Providing replacement equipment to gear that has a hole blown through the metal enclosure is a common thing for me to see. So I will never, ever, rely on containment of an arc by an enclosure. Most people rarely, if ever, get to witness that sort of aftermath.

I think nearly all people agree switchgear operations, covers on or off, pposes a serious arc flash hazard. I also think most everyone agrees that turning on a light switch poses no hazard. So there is a line somewhere, and where that line resides is very difficult to determine.

However, when you do an arc flash study you have identified a specific hazard level, and since non arc rated equipment is not tested or designed with arc containment in mind that hazard needs to be observed and employees need the proper protection.

Then you bring "risk" into the equation. Will that enclosure contain an arc? Maybe, maybe not, but better chance it does than if it were exposed. I don;t think anyone will figure out an actual calulation for "risk", there are too many factors.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 17 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


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