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 Post subject: Working Distance Determination
PostPosted: Thu Aug 16, 2012 4:12 pm 
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Joined: Wed Aug 15, 2012 12:30 pm
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Hey guys just registered here but have been reading through the forums. I have a question regarding working distance and what is the reference used? For example we have a 15kV breaker that needs to be racked in. Is the distance measure from the stabs at the back of the breaker or the closest point on the breaker itself that will be energzied such as the top end of the vacuum bottles?

Also on a side note I have read several posts on doors closed/open. Does anyone here allow for a downgrade in PPE while racking operations due to the door being closed? If there is already a thread on the subject could you give a link I tried searching and did not come up with anything here.

One other item- what is the highest calculation that you have seen? I just started a new job and a facility that I have not worked in before. They have some stickers on some 480 gear that over 5000 cal. An no that is not a typo.


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PostPosted: Thu Aug 16, 2012 5:06 pm 
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Probably the most dangerous thing you can do is racking a breaker, on to/off of energized bus. I would not recommend any downgrading of PPE.


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PostPosted: Fri Aug 17, 2012 4:17 am 
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Quote:
Hey guys just registered here but have been reading through the forums. I have a question regarding working distance and what is the reference used? For example we have a 15kV breaker that needs to be racked in. Is the distance measure from the stabs at the back of the breaker or the closest point on the breaker itself that will be energzied such as the top end of the vacuum bottles?


The 1584 Committee used the distance between the chest area and the bus bars at the back of the enclosure for the table. For 15 kV I believe (don't have it in front of me) this is 36". Generally everyone just uses the tables and does not actually physically measure it except if they have an unusual circumstance such as if you are operating a pole mounted interruptor with a hot stick in which case the 36" value is vastly lower than the real world case.

Quote:
Also on a side note I have read several posts on doors closed/open. Does anyone here allow for a downgrade in PPE while racking operations due to the door being closed? If there is already a thread on the subject could you give a link I tried searching and did not come up with anything here.


Rack mounted breakers have the highest rate of failure according to IEEE 493 data among other sources. This is pretty obvious simply because there are vastly more moving parts in rack mount equipment than in bolted ones. CIGRE has performed a study and found that arcing faults in this type of breaker happen 80% of the time at the stabs. IEEE 1584 (the most popular source for incident energy calculations) and 70E among others clearly indicate that although doors provide SOME protection to thermal energy, the reduction is variable and varies by design, size, and manufacturer to the point where they simply err on the conservative side and ignore it. Generally doors also turn into projectiles in many cases. Finally with regards to reducing PPE, are you nuts? This is absolutely the most patently stupid idea anywhere in 70E. The incident energy does not decrease due to the task...the likelihood of an arc flash (and thus the RISK) decreases. So either you are wearing adequate PPE as calculated or you aren't. The tables should say X or ZERO, not X, X-1, X-2, etc.

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One other item- what is the highest calculation that you have seen? I just started a new job and a facility that I have not worked in before. They have some stickers on some 480 gear that over 5000 cal. An no that is not a typo.


This is 99% likely to be the engineers screw up. Many engineers have made two serious mistakes in the past. First, they used an equation that is very simple developed early on that treats voltage and current the same. It generates values that are way above anything ever actually measured. This leads to an answer but it is usually wildly incorrect.

Second, they use their old short circuit calculations and simply ignore short circuit decreases due to cabling. For short circuit calculation purposes this leads to safe (conservative) values. However for incident energy it doesn't. The short circuit is used to determine how fast the breaker opens. Lower currents generally lead to much longer breaker or fuse opening times, and higher overall incident energy.

Using 70E, anything over 40 cal/cm^2 is stated as the condition where due to the concussive force of an arc blast, you won't survive it anyway. There is ongoing research on this but for right now the assumption used is that over 40 cal/cm^2, no amount of PPE is going to protect you anyways. So it really doesn't matter if the incident energy is 41 cal/cm^2 or 5000 cal/cm^2 because it is treated identically for PPE purposes.

For the record a few areas in the plant I work at have incident energy values into the hundreds. The working distances are listed so large that basically, it encompasses the entire plant site. This is obviously not physically possible but points out the fact that when it comes to this particular equipment, working live is simply not an option.


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PostPosted: Fri Aug 17, 2012 7:23 am 
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Thanks guys. Paul I agree with you- they are nuts!! The first time i watched them rack a breaker without any PPE at all I was appalled. They are not only downgrading but if the enclosure door is shut with the two door bolts screwed in are not wearing any PPE at all let alone downgrading by one level. So they are bascially taking a Level 4 requirement and then because the door is closed saying they don't need anything.

They are under the impression that with the doors closed they are protected. This apparently came from a previous engineer that no longer works here. These are not blast rated gear. I was told that with the doors closed the calculation is different because the pressure is distributed equally on all sides of the enclosure rather than being directed outward through the opening. I looked in 1584 and 70E and could find nothing that allowed for this. I am at this time doing my best to get them to modify their practices to be in compliance with 70E as far a PPE goes. I used to work for a company that does SC and Arch Flash studies as part of their business though I have only been in the field getting the data for them and have not been involved in doing the calculations.

The other interesting thing that is done is that the labels are or for some odd distances. Sometimes we will have two labels that give two different distances. For instance we might have one for 36" and one for 42". It appears to me that this is being done to make it acceptable to do certain things with less PPE but I do not know that for sure.


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PostPosted: Fri Aug 17, 2012 5:01 pm 
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Glider61 wrote:
They are under the impression that with the doors closed they are protected. This apparently came from a previous engineer that no longer works here. These are not blast rated gear. I was told that with the doors closed the calculation is different because the pressure is distributed equally on all sides of the enclosure rather than being directed outward through the opening. I looked in 1584 and 70E and could find nothing that allowed for this. I am at this time doing my best to get them to modify their practices to be in compliance with 70E as far a PPE goes. I used to work for a company that does SC and Arch Flash studies as part of their business though I have only been in the field getting the data for them and have not been involved in doing the calculations.


Here's some help, though not much. First, a web site that has an arc blast calculator:
http://www.arcadvisor.com/faq/arcblast.html

You can also read Ralph's paper here:
http://www.electrician2.com/html/elec_pressure.pdf

It is pretty well known that Ralph Lee's numbers are completely off and that the pressure is not that high. However this is all that is available out there. Now the argument that it's "equal on all sides" is true. And the output above is given in PSI. So take your door dimensions and calculate the pressure in pounds on the door hinge and bolts is equal to door area (LxW) times the arc blast pressure (given above). You can do a pretty good job of estimating the fasteners by looking at the maximum limit of bolts that are equivalent diameters to the fasteners on the door including the latch. Be sure to add it all up because we wouldn't want to underestimate given the fact that this stuff is supposed to be retaining hundreds or thousands of pounds, right? The door doesn't stand a chance. More interestingly the pressure wave occurs within milliseconds. This is so fast that metals don't do what you expect...they do not undergo plastic deformation and finally fail by ripping. You don't find fasteners that "tear"...they are sheared off due to the resulting inelastic deformation. The door goes flying straight out. I don't recall where but there is a Youtube video demonstrating this quite nicely as it rips a manequin arm completely off.

Quote:
The other interesting thing that is done is that the labels are or for some odd distances. Sometimes we will have two labels that give two different distances. For instance we might have one for 36" and one for 42". It appears to me that this is being done to make it acceptable to do certain things with less PPE but I do not know that for sure.


I PREFER to use standard distances. However if you insist, you can look at your body position very carefully while attempting to rack or open/close the breaker you referred to relative to the stabs at the back of the enclosure and try to figure out the minimum working distance. It will likely be more with the door closed. If I were to do that, I would very clearly make sure the label explains which tasks it applies to. Another good example is using a hook stick. I'm not even sure that I put a lot of stock in 70E's working distance numbers but haven't taken the time to look at this.


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PostPosted: Mon Aug 20, 2012 9:09 am 
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Just to clarify the "typical" Working Distances are from IEEE P1584 and the software uses they has defaults.

I agree that many of the engineering incident energy analysis studies are wrong as the engineer doesn't understand all of the "details" required to consider including the 2 Second Rule. As well the software unfortunately includes default label templates that are also incorrect and I have approached ETAP, SKM and Easypower and they don't care and don't have competence either.

Don't let the software dictate your label or the results of the calculations. Ensure you as the owner control the default settings in the software.

We need to be careful with arc blast pressure.

Use the typical Working Distances and if your equipment is unique and work task unique then review the Working Distance. Be aware that a rack in and out tool actually increases the Working Distance. You will find that engineers doing incident energy analysis studies may be very conservative at the expense of their clients by using 18" for all work tasks, don't let this happen.

Also watch out for labels designed by the lawyer, not the engineer or you. One large equipment manufacturer doing studies has a label designed by their lawyer that exposes their client to OH&S risk, it serves to cover the $*$& of the large equipment manufacturer and not facilitate providing arc flash and shock hazard information to the electrician.

The risk related to the work task has be be evaluated, we need to control the likelyhood of an arcing fault occurring. The Electrical Safety Program you develop with outline how to apply all of the available preventive and protective control measures. For the USA refer to ANSI Z10.

Regards;
Terry Becker, P.Eng.
[url="http://www.esps.ca"]www.esps.ca[/url]


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