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 Post subject: NFPA70E-2012 Annex D Table D.7 and IEEE1584-2002
PostPosted: Fri Oct 05, 2012 2:29 pm 

Joined: Sun Apr 24, 2011 5:00 pm
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Location: Stratham, West Australia
Are there any differences between NFPA70E-2012 Annex D Table D.7 and IEEE1584-2002 in the method of calculating the incident energy and arc-flash boundary used to specify PPE to NFPA70E-2012 Tabel 130.7(C)(16) ?


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PostPosted: Sat Oct 06, 2012 5:53 am 
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mwjnewman wrote:
Are there any differences between NFPA70E-2012 Annex D Table D.7 and IEEE1584-2002 in the method of calculating the incident energy and arc-flash boundary used to specify PPE to NFPA70E-2012 Tabel 130.7(C)(16) ?


Annex vs. 1584: No, no differences.

Table 130.7(C)(16) vs. 1584: Yes, major difference. The table starts with the IEEE 1584 values. They are then modified according to a risk assessment performed by the 70E Technical Committee. The original 1584 values show up as the largest H/RC values given in each section of the table.

As I've more or less outlined elsewhere, I do not agree with the 70E Technical Committee's methodology for two reaons. First, they don't publish the reasoning behind the "1/2/3 category decreases" which is important. Second, the hazard is not decreasing in value...the likelihood is. So the values should either be the original IEEE 1584 calculated value or "acceptable risk" (no PPE required).


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PostPosted: Sat Oct 06, 2012 3:17 pm 

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Thanks Paul. Would you agree with the following :

Disadvantages of the NFPA70E Table 130.7( C )( 15 ) (a) Hazard Risk Category ( HRC ) Table

1. Although the above look-up table provides estimates of the Hazard Risk Category (HRC) , it cannot be used without confirmation that the available short-circuit currents and protective device operate times are below those specified in Table 130.7(C)(15)(a). [font=Calibri]Manual calculation of arcing current and corresponding protection device tripping time would take considerable effort.[/font]

2. NFPA70E-2012 Clause 130.7 ( 15 ) permits the HRC to be reduced based on different work activities having varying risks of an arc flash incident. The look-up table allows a 1, 2, or 3 reduction in HRC number based on the work activity which may be considered to have a lower risk of an arc-flash fault occurring, however the arc-flash will have the same hazard.

The preferred method of performing an arc-flash hazard assessment is the IEEE1584 method which is based on the latest research in developing the IEEE 1584-2002 Standard - IEEE Guide for Performing Arc-Flash Hazard Calculations.
No reduction of HRC is made based on the work activity.

A reduction of HRC to “acceptable risk” ( no PPE required ) should only be permitted if all switchgear doors remain closed and the switchgear has been tested and certified to withstand an internal arcing fault of specified magnitude and duration.




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PostPosted: Wed Oct 10, 2012 4:15 pm 
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mwjnewman wrote:
Thanks Paul. Would you agree with the following :

Disadvantages of the NFPA70E Table 130.7( C )( 15 ) (a) Hazard Risk Category ( HRC ) Table

1. Although the above look-up table provides estimates of the Hazard Risk Category (HRC) , it cannot be used without confirmation that the available short-circuit currents and protective device operate times are below those specified in Table 130.7(C)(15)(a). [font=Calibri]Manual calculation of arcing current and corresponding protection device tripping time would take considerable effort.[/font]

2. NFPA70E-2012 Clause 130.7 ( 15 ) permits the HRC to be reduced based on different work activities having varying risks of an arc flash incident. The look-up table allows a 1, 2, or 3 reduction in HRC number based on the work activity which may be considered to have a lower risk of an arc-flash fault occurring, however the arc-flash will have the same hazard.

The preferred method of performing an arc-flash hazard assessment is the IEEE1584 method which is based on the latest research in developing the IEEE 1584-2002 Standard - IEEE Guide for Performing Arc-Flash Hazard Calculations.


So far so good.

Quote:

No reduction of HRC is made based on the work activity.


This is true because IEEE 1584 is concerned with determining the potential hazard, not the risk. Risks are determined using a variety of methods but are outside the purview of IEEE 1584. ANSI has at least two or three that I'm aware of. ISO has 2 or 3 as well. PMMA has one. The list of standards is quite extensive. The reason for so many methods is that each one has peculiarities specific to the process or industry it is designed to support.

NFPA 70E specifically requires that either the tables are used OR a risk assessment is performed. This is different from a hazard assessment which is only one part of a complete risk assessment. To be fully compliant with NFPA 70E, the risk needs to be evaluated.

Quote:
A reduction of HRC to “acceptable risk” ( no PPE required ) should only be permitted if all switchgear doors remain closed and the switchgear has been tested and certified to withstand an internal arcing fault of specified magnitude and duration.


No, strongly disagree. For instance, are you going to test an office light switch? There is a test for "arc resistant switchgear" but it is only a standard for medium voltage gear, not 480 V gear. Are you going to extend this to anything other than medium voltage gear? What about a 120 V lighting panel? Further, are you going to require a secretary to wear FR clothing to start up the coffee maker in the morning? What about walking through the switchgear room, perhaps even taking readings off displays or vacuuming the floor? Do these involve an arc flash risk (not hazard)? How about operating cutout fuses with a hot stick? How about opening or closing a DE-ENERGIZED disconnect switch? What about operating a molded case circuit breaker of "any" size? Does size even make a difference? How about an electrician taking voltage measurements on a PLC if everything in the panel is 24 VDC? Does it make a difference if the control voltages are 120 VAC? Is operating a circuit breaker or pretty much any other switching activity an acceptable risk? How about operating a start/stop button while standing directly in front the the contactor (note: IEEE 493 failure rates suggests that the risk of a failure is orders of magnitude more likely than switching circuits with no load on them)? What about racking a breaker in/out of a cell? Is there an available fault voltage/current combination below which you hit the 1.2 cal/cm^2 threshold anyways at a given cutoff at which point you may as well forget about it? Does it make a difference if you are using tools that are insulated to prevent arcing faults by insulating against both phase-to-ground and phase-to-phase faults?

You just can't make blanket statements like this without taking into consideration both the task and the design of the equipment.

Instead I suggest you draft your own version of the 70E tables. It should have 4 columns: "task", "shock protection required", "arc flash PPE required", and "EEWP required". Each column is yes, no, or a specific requirement (such as 1.2 cal/cm^2 PPE). You can use the tables in 70E and/or NESC as a starting point but I've found plenty of other situations that need their own group, often for clarification for the items that constantly come up. For instance...operating a fuse cutout with a hot stick 2 feet from the end of the stick. Let's suppose that the working distance is then 8 feet (8 foot hot stick) and the arc flahs hazard boundary is <8 feet. So the table could state no for shock protection (no gloves needed as per IEEE 516 for insulated tool work), 1.2 cal/cm^2 for arc flash PPE, and no for EEWP (it's a switching activity). Towards the top of the task list by the way you may want to include a line referring to equipment which has faulted...in which case we drop any pretenses that the equipment is in good condition and operating normally...so arc flash PPE is definitely required even for tasks that would not normally require it as per 70E.


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