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 Post subject: Labeling Arc-Resistant Switchgear
PostPosted: Fri Nov 02, 2018 1:00 am 

Joined: Thu Nov 01, 2018 6:25 am
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Hi everyone! Since in the Table 130.7(C)(15)(a) of the NFPA 70E standard indicates that the Arc-Flash PPE Category and Arc-Flash Boundary do not apply for arc-resistant switchgear with doors closed, how should I label my arc-resistant switchgear?


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 Post subject: Re: Labeling Arc-Resistant Switchgear
PostPosted: Sun Nov 04, 2018 6:50 pm 
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There are two conditions: doors open and doors closed. With doors open, the same conditions that would apply to non-arc resistant gear apply. With doors closed and latched (bolted) shut, the arc resistant gear test effectively makes it equivalent to <1.2 cal/cm2.

This brings up the same issue when you have multiple scenarios to consider. Some other common ones are:
1. Gear that is operated from a distance either because it is pipe operated (typical for gang operated overhead switches) or hot stick operated, or a mimic panel is used (remote trip/close), or a remote racking device is used. All of those drastically change the arc flash rating by placing the operator significantly farther away.
2. Normal operation of equipment that is property maintained at which point there is very little likelihood of an arc flash in the first place and PPE is not required according to the tables.
3. One or more generators or other alternative loads or say main-tie-main schemes with the tie and one or both ties closed which changes the available fault current and thus the incident energy, sometimes drastically.
4. Various diversity scenarios which with large amounts of motor loads can significantly change incident energy.

NEC defines where labels are required but does not define content. 70E defines content but only gives requirements for a single value. Even "simple" studies frequently have multiple scenarios such as a scenario where the plant is assumed to have just recovered from a power outage where almost no loads are online generating very low incident energy values so those involving emergency backup generators. The problem though is what value or values should go on the label? Should it be the highest value particularly if it is something very theoretical that would never or almost never be the normal plant practice even though this might qualify as "conservative", or should it be the most likely condition? Or should multiple results be listed and if so, how complex should they get before the label really no longer serves any useful function? All valid questions but there is no guidance from engineering standards.

Personally I would opt for the "normal" (single value) label and include the fact that arc flash PPE is not required as part of training/work instructions. You may or may not want to put extra wording on the label just as those with emergency backup generators may want to put something on the label that states that under "backup" conditons ignore the label and get the results from the report or some other secondary document.


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 Post subject: Re: Labeling Arc-Resistant Switchgear
PostPosted: Mon Nov 05, 2018 10:17 am 

Joined: Mon Dec 07, 2015 9:45 am
Posts: 33
Location: Massachusetts
I'll summarize my thoughts, which I believe PaulEngr's post agrees with in content as well, as:

Apply a single label for the worst case incident energy someone working in, on, or around the gear could be exposed to. Have your study reflect the various operating conditions and interaction scenarios that the workers in, on, or around the equipment could be involved with, and they can use the information in the report to determine what level of PPE to wear for the task they are performing in the operating conditions they will be performing them.

This leaves no ambiguity as to what should be worn if the worker does not take the time to look at the study and determine if a lesser level of PPE can be work for the task.


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 Post subject: Re: Labeling Arc-Resistant Switchgear
PostPosted: Tue Nov 06, 2018 2:50 pm 
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SteveA wrote:
I'll summarize my thoughts, which I believe PaulEngr's post agrees with in content as well...
This leaves no ambiguity as to what should be worn if the worker does not take the time to look at the study and determine if a lesser level of PPE can be work for the task.


Not quite. The concept is valid but there is a practical issue with it and the reason that I recommend "normal" or "worst normal" rather than worst case. If you are dealing with a plant with only radial feeds and no emergency backup generator, then your approach works because worst case is the normal case without for instance maintenance switches turned on. The problem comes in with a plant that has large emergency backup generators (particularly whole-plant generators) or a significant cogen capability or double ended switchgear. In those circumstances, "worst" case is either relatively rare (e.g. running on generator power) or might never have actually occurred during the plant's operational history such as running a study with all mains and all ties closed on double ended switchgear. So the "worst case" is the rare case and plant personnel are basically instructed to ignore the labels at all times and get the information needed to do their jobs some other way because the label serves no practical purpose.

See where I'm going here? The label gets ignored and the secondary information may be confusing, hearsay, wrong, or subject to various "lookup errors". The label may as well be the generic kind "Warning! Arc flash hazard present" since it is not used in the first place. I'm not advocating for or against the use of labels or an alternative to a label system here but pointing out that the goal here should be to make things as simple to understand as possible to avoid mistakes and errors as much as possible, particularly when as you said the correct action is both task and condition specific.

The conditions that I'm describing may seem unusual to you but I can assure you that in the paper/wood products industry, glass plants, water industry, power generation, iron/steel, refineries, and chemical plants, these scenarios are the norm rather than the exception. Thus the approach needs to be one of deciding what particular scenario goes on the stickers and what additional content can or should be identified in conjunction with procedures and practices developed to support the label system.


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