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 Post subject: Standard working distances for activities
PostPosted: Tue Jun 05, 2012 7:02 pm 
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Hi, I'm currently working on an arc flash study, using the SKM PTW software. My colleague gave me an arc label as an example - it's originating from Digsilent, as he works heavily with PowerFactory software. I wanted to generate a similar label to that, which lists several activities and gives PPE categories for both door opened and closed for thoose activities. Is there a standard to determine typical working distances to be considered for activities like switching and racking on metal clad enclosures, electrical work, visual inspection, cover removal and so on? These terms and so the related distances must be coming from somewhere.
I am from Australia, and arc flash calculations are just recently getting required by certain clients.


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PostPosted: Wed Jun 06, 2012 4:04 am 
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Hi Andras,
It sounds like you might be attempting to mix 2 different methods. PPE selection for doors open / closed is based more on using NFPA's Hazard/Risk tables where the category depends on the task and risk associated with it. When performing arc flash calculations, the hazard is more absolute. i.e. calculations are performed based on specific conditions and the result is the prospective incident energy independent of doors open vs. doors closed.

As far as the working distance, the 2002 edition of IEEE 1584 suggests the following:
18 inches for low voltage distribution equipment
24 inches for low voltage power equipment i.e. LV switchgear
36 inches for medium voltage equipment i.e. switchgear.

You can adjust this as you feel necessary. One problem is with training workers to understand the importance of attempting to maintain this distance when performing live work. Easier said than done! There was a survey "question of the week" asked on this forum recently about maintaining the working distance and as expected, it is difficult to do.

There can be many mistakes with regard to the working distance as well and the article (link below) that I wrote for "Electrical Contractor Magazine" below might help identify a few of these.

[url='http://www.ecmag.com/index.cfm?fa=article&articleID=13527']Working Distance Article[/url]

I hope this helps a bit. There is a lot of good talent willing to help out at this forum so I'm sure others may jump in here too.

Best Wishes,

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Wed Jun 06, 2012 3:13 pm 
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brainfiller wrote:
Hi Andras,
It sounds like you might be attempting to mix 2 different methods. PPE selection for doors open / closed is based more on using NFPA's Hazard/Risk tables where the category depends on the task and risk associated with it. When performing arc flash calculations, the hazard is more absolute. i.e. calculations are performed based on specific conditions and the result is the prospective incident energy independent of doors open vs. doors closed.


Yes, you're right. That was exactly my feeling when found Table 130.7(C)(9) in NFPA 70E. Thanks for driving me the right direction.

Regards,
Andras Csaszar


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PostPosted: Tue Jun 12, 2012 5:14 am 

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Andras,

I was the person that created the label you posted. This was part of a presentation at the DIgSILENT user conference in Melbourne 2011.

I can explain more about the practices applied in this style of label as used in Australia. This label format was driven by end-users, in this case Australian resource companies that wanted to do more than state the arc hazard alone. They wanted inform their electrical workers of the appropriate PPE for various activities, after further consideration of the risk & consequence. Company policy drove definition of the rules that are applied with consideration of switchgear condition and operational assessments. Equally, the activities nominated on these labels are company-defined terms and not anecdotal. For example, Electrical Work includes testing-for-dead but not normal elecitrcal work that would apply once an isolation permit is raised. 'Switching' is the changes state of any isolator or CB and also, fuse replacement.

As for the working distance, it is not stated on this label because that that Company's policy document nominate standard distances to apply for different voltages and board types. The report that is produced for each labels (i.e. from the Calc Ref) has the full details of the calculation inputs, which in addition to all the IEEE 1584 equation inputs, also considered multiple protection clearing stages, effect of motor contribution into the arc , circuit breaker opening time etc.

The Incident Energy and Arc Flash Boundary values stated on the label apply to the Doors OPENED case at the nominated working distance (by the Company Policy). All the PPE nominated for the defined activities employs the company risk assesment policy. For example, any activity performance externally to a HV switchboard that is IAC (Internally Arc Classified under IEC 62271) rated is CAT 0 providing the board is within technical limits (i.e. clears arc faults within the IAC clearing time on the nameplate) and the covers are closed and intact. Having said that, a company may put a floor on the PPE level for HV switching and require minimum CAT 2 (as per this example). Other rules might be that all visiual inspections with Doors Closed is CAT 0 (or daily work wear level) because it is acceptable risk to simply be looking at a board, notwithstanding policy that you minimise your time in substations.

I am sure that many people here will disagree with this approach, but the reason working distance was not stated on the label that input was is treated as having an ‘implied precision’. By this, I mean does different electrical worker really need to worry about their precise distance variations of 400mm, 450mm, 550mm, 600mm as they interact with equipment. Unless they are standing back like a soldier upright, it is otherwise a case of arms and hands are in close, head next, then torso, then hips. Then boom, they are reacting and moving – so everything is already changing all the time anyway.

Remember IEEE 1584 is a curve fit equation from a normalisation of multiple test record data that is interpolated and extrapolated to fill the data set range of valid voltage and fault levels. In simple terms, Incident Energy is then obtained by nominating the various input factors to extract a ‘valid’ result out this data set. There are errors in every direction, not least of which are assumptions the arc is homogenous in all directions and for the duration of the clearing time.

Still, it is the best equation model available at present but given all the normalisation, some people consider it accept to lock down some of the inputs for consistancy in terms of understanding relative performance across their asset base. Busbar gap can be treated the same i.e. open a switchgear panel with some bus work and tell me which one of the many different busbar gaps is the right one to put into the equation.

This approach has gained momentum among electrical asset owners in Australia and I can provide more information if you wish to see the direction of local standards.

Here in Australia, we have been tryig to marry our IEC-harmonised approach to equipment, our local approach to work practices and the IEEE equations for Incident Energy. NFPA 70E has no place or mandate in the Australian workplace however, in the same way that AS/NZS 3000 has no place or mandate in the USA workplace. I say this from statutory/regulatory view but of course, we should all share the best ideas and practices globally.

This is why this forum is great.


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PostPosted: Tue Jun 12, 2012 5:36 am 
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Location: Netherlands
Note that the colors on those labels are not according to ISO 3864, which is something that you may want/need to follow. Usually this boils down to two different signs on the label: a warning for the danger of arc flash (yellow triangle) and mandatory use of PPE (blue circle).

I can upload an example if you want.


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PostPosted: Tue Jun 12, 2012 6:29 am 

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Excelllant point Joost. You have highlighted the important point that across the world there are many differences. Certainly, all labels must confirm to local regulations, as they contain what is considerd statutory safety information.

In Australia, where this label example was used in the mining industry, it complies to local requirements. T

The "Warning" yellow triange is to the ISO convention but the Red/Black is Australian convention for Danger, including electrical. For the "Mandatory" signs referring to required PPE (i.e. hardhat, hearing protection, gloves, face shield etc) these are usually very large area signs. Substations entry points have these as applicable to the particular installation. For electrical PPE, substational also have the labels/signs detailing what CAT 0,1,2,3,4,5 PPE profiles cover.

We do use the full range of ISO signage here in Australia
- "Warning" Black on Yellow triange.
- "Mandatory" White on Blue circle
- "Prohibited" Red Circle on White with crossbar
- "Safe" White on Green Rectangle
- "Fire Safety" White on Red Rectangle

Peter Willis
DIgSILENT


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PostPosted: Tue Jun 12, 2012 7:27 am 
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Interesting to know, the Red/Black Danger symbol is something I've see on US symbols and that's what triggered me.

As for the OP: I find the labelling part of SKM Powertools to be atrocious, especially design and coherent label numbering. You can export the arc flash result tables to Excel and with a little fiddling format them for a mail merge in Word. It takes some time to set up the first time but saves so much hassle in the long run that I haven't looked back.


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PostPosted: Tue Jun 12, 2012 8:37 am 
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jvrielink wrote:
Interesting to know, the Red/Black Danger symbol is something I've see on US symbols and that's what triggered me.

As for the OP: I find the labelling part of SKM Powertools to be atrocious, especially design and coherent label numbering. You can export the arc flash result tables to Excel and with a little fiddling format them for a mail merge in Word. It takes some time to set up the first time but saves so much hassle in the long run that I haven't looked back.


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PostPosted: Tue Jun 12, 2012 8:44 am 
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"I find the labelling part of SKM Powertools to be atrocious, especially design and coherent label numbering."
I think the actual label generation part of SKM is very flexible. However the HMI to use it is best described as " atrocious" indeed.
I could easialy use some other words too.... I am not sure why you think you need the labels "numbered" but there is a specific field for that.

We use the spreadsheet inteface too, from the "worst case" scenario output, with a little post processing. Instead of trying to mail merge anything, we use an outside printing service, to provide a MUCH more durable label than any thing we could print.


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PostPosted: Wed Jun 13, 2012 8:20 pm 
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I have also tried using that feature, but as it has been said, there wasn't too much pleasure in it...
I actually liked that I could include any kind of warning label as a picture; however, the number of freely definable text areas were not enough when tried to build up to the same label as Digsilent's.

I found label numbers when came up to the point to chose one to print. Is that the same number Joost has mentioned?


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PostPosted: Wed Jun 13, 2012 8:46 pm 
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AusPowEng wrote:

For example, any activity performance externally to a HV switchboard that is IAC (Internally Arc Classified under IEC 62271) rated is CAT 0 providing the board is within technical limits (i.e. clears arc faults within the IAC clearing time on the nameplate) and the covers are closed and intact. Having said that, a company may put a floor on the PPE level for HV switching and require minimum CAT 2 (as per this example). Other rules might be that all visiual inspections with Doors Closed is CAT 0 (or daily work wear level) because it is acceptable risk to simply be looking at a board, notwithstanding policy that you minimise your time in substations.


That particular label came up with Category 2 PPE level for two activities performed with door closed: switching and cover removal. The later makes sense, although I would rather put it into the doors opened category, but switching with doors closed should remain in Category 0 PPE once the switchgear has been arc rated. Any explanation on that?


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PostPosted: Mon Jun 25, 2012 7:22 am 

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Sorry for the delay, I haven't logged in for a while due to EOFY work deadlines. The particular label you have from my presentation was an of one Form 3/4 LV board but the context of the example was about adopting suitable local labelling formats and to consider options with listed activities to assist electrical workers.

This particular case reflects one Company's rules. The "Doors Open" case is the inherent hazard. They don't use CAT 3, so the 17.91 Cal/cm^2 goes to CAT 4. If the switchgear is not a design with AS/NZS 3439.1 Appendix ZD Arc Performance Test report, then 'Doors Closed' switching drops to CAT 2 prividing door fixings are intact and that the door has no vents. If the board design did meet Appendix ZD and was within technical limits (i.e. cleared arc faults with the specified time), then the 'Doors Closed' switching drops to CAT 0.

I would agree that Cover Removal should have no discount - there was going to be distinction made between the extremes of a hinged cover, a cover with locating pins and a cover with nothing. This never progressed far though, as it was getting too complicated.


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