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 Post subject: Siemens Switchboards
PostPosted: Mon Mar 12, 2012 11:11 am 
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Hello,

I'm looking for some feed back concerning my switchboards. I'm dealing with Arc Flash and how it applies to my situtation. My low voltage switchboard has arc flash labels on it's breaker compartments. There's my question, the Siemens breakers protrude through the switchboard doors by about 2 inches. How does this affect the ratings or distances of arc flash when in operation. Is there a hazard when the door/covers are in place, when the breaker itself extends beyond the door.

Thanks for any advice..
Steve


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PostPosted: Mon Mar 12, 2012 12:40 pm 
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Have you done an actual arc flash study or have you just used the HRC's from the task tables for your labels?


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PostPosted: Mon Mar 12, 2012 5:47 pm 
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First, when you said "low" voltage, there is a difference between 208 V and 240 V panels and higher voltages according to IEEE 1584 in that the risk is low enough that one does not do the calculation.

Second, the working distances that are stated in IEEE 1584 and most widely used are taken from the buses which are typically all the way at the back of the cabinets to the face/chest area with doors open (typical working positions). In many cases especially with outdoor gear, the actual distances need to be taken into consideration.

Third, the arc flash calculation usually considers worst case. In a panelboard scenario, that would be the arc flash incident energy potential assuming that an arcing fault develops across the source power cabling such as if the insulation was damaged and arced at a nipple coming into the panel. Once you get past the primary breaker in the panel (if you have one), the incident energy is typically much lower (unless the primary breaker trip setting is less than the arcing current).

Fourth, the typical working distances given in IEEE 1584 (the most widely used numbers) are taken from the bus bars that are typically located at the back of the enclosure to the face/chest area assuming that someone (an electrician) is working on the equipment, and assuming that the fault location is vertical, and that it is faulting within an open enclosure. Of course if any one of these assumptions is violated, the results would change. In some cases such as working distance it is possible to relatively easily modify the parameters to use more realistic assumptions. For instance where I work, we use hot stick only work on many overhead line operations so we modelled the incident energy with the increased working distance in mind. Others are not so easy. For instance it is known that "doors closed" would necessarily produce lower incident energy ratings. But there's no data indicating what the rating should be.


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PostPosted: Tue Mar 13, 2012 5:49 am 
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Thank-you gentlemen,

The switchboard voltage is 460. There is 3 ship service generators tied to the switchboard plus a motor generator set. Each supply has it's own breaker compartment with the breakers protruding through the door. The labels placed by siemens state extreme danger. Is there a hazard, when in operation,just by walking past the switchboard ? Has anyone dealt with this before ? Do I have to install covers over the protruding ends of the breakers ?


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PostPosted: Tue Mar 13, 2012 9:38 am 
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Is there a hazard just walking by? Yes, but very slight. The only time the PPE requirements are in play is when you are "interacting" with the equipment in such a way an arc flash could occur. Operating or racking are 2 examples.

Installing covers would not reduce the danger for all intensive purposes. Your switchgear would need to be "Arc rated" for that to make a difference.


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PostPosted: Wed Mar 14, 2012 6:24 am 
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Zog wrote:
Is there a hazard just walking by? Yes, but very slight. The only time the PPE requirements are in play is when you are "interacting" with the equipment in such a way an arc flash could occur. Operating or racking are 2 examples.

Installing covers would not reduce the danger for all intensive purposes. Your switchgear would need to be "Arc rated" for that to make a difference.


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PostPosted: Wed Mar 14, 2012 6:28 am 
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Sorry new to this. Thanks for your advice. when you say operating does this refer to the door being closed and the breaker is closed by a remote pushbutton.


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PostPosted: Wed Mar 14, 2012 9:09 am 
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That is the big debate, you will find many discussions on that topic on this forum. The 70E added some clarification (Which I feel just made more confusion) by saying equipment that is properly maintained per industry standards and is in good condition is not likely to be an arc flash hazard. Now the question is what is properly maintained? Power plants typically do a complete overhaul of thier breakers every 10 years as EPRI and most OEM's recommend, but industrial facilities rarely do this and are lucky if they even do routine testing so what is properly maintained? As a person that does overhauls on breakers we find damaged or worn parts in nearly every breaker that comes through the door so my interpretation of properly maintained is likely different from many out there. The majority of these power plants that do this still use remote racking and switching devices to operate the breakers from outside the arc flash boundary.

Also keep in mind that only arc rated switchgear is designed to contain an arc flash, typical enclosures were never designed to contain the arc flash so no one can say for sure if it will be contained or not, I see failures all the time where the arc is not contained so again, my views on this may differ from others.


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PostPosted: Wed Mar 14, 2012 5:44 pm 
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There have been multiple statements made in the ROP's (responses to proposals for changes) where the Committee has stated that simply walking by does not present a hazard.

In the definitions section under "Arc Flash Hazard" the notes say to use the tables to determine what the committee feels is hazardous and what isn't, but they don't elaborate any further. This means that either any activity in the tables is considered hazardous or not. Since one of the activities is reading a meter while operating a meter stick (H/RC 0) it would seem that "nonhazardous" activities are included in the table. Based on this I contend that any time the Committee puts a "0" in the tables for H/RC when other entries in the table are >0, these are clearly the activities that the 70E Technical Committee considers not normally hazardous, ASSUMING that recommended maintenance is being performed and there is no indication of abnormal operation such as right after a breaker faults.

In the specific case of operating equipment, IEEE 493 has extensive (though fairly old) data on expected failure rates for breakers. With a little work and some very mild assumptions you can actually come up with a reasonably conservative estimate of failure rates of the breaker during operation. It appears that the acceptable risk probability for the technical committee is around 10^-5 failures/year based on what this seems to suggest. For panelboards and others using molded case or insulated case breakers, the probability of failure is pretty close to or even lower than this. For drawout breakers, it's marginal (very close to 10^-5) and only wearing appropriate PPE and doing the proper maintenance is going to keep you alive.

This is based on data from 1970's. The failure rate of the trip units has roughly doubled with solid state units in the 90's and is now almost an order of magnitude better with microprocessor units with self-checking functions. Mechanical maintenance with modern lubricants and the slow improvement in mechanisms (reduction in wear parts and failure modes) has also improved dramatically. However that being said, I have not seen actual data nor a MIL handbook style calculation of expected failure rates with the modernized breakers. The relay failure rate data is pretty good but with the breaker mechanisms themselves there isn't much out there.


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PostPosted: Mon Apr 02, 2012 10:18 am 
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Thank-you gentlemen for your thoughts on the matter.

Steve


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