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 Post subject: Changes to calculated and labeled arc flash hazard stickers
PostPosted: Sat Feb 14, 2015 4:31 pm 

Joined: Sat Feb 14, 2015 4:19 pm
Posts: 2
I am looking for references in NFPA 70E that identify when the hazard level (Incident energy) for arc flash and electric shock boundaries can change from those posted on the stickers? Thank you in advance for any help.


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 Post subject: Re: Changes to calculated and labeled arc flash hazard stick
PostPosted: Sun Feb 15, 2015 8:31 am 
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Joined: Wed May 07, 2008 5:00 pm
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Location: Rutland, VT
I am not entirely clear on what you are asking. If the incident energy and arc flash boundaries that are on the labels are from a study, then unless there is significant changes to the system that the study was based on, then there should be know change. The study should be reviewed every 5 years for system changes that may affect the results of the study.

If the labels were generated by use of NFPA 70E tables, then there may be changes based on which revision was used or if there were changes to the short circuit and clearing times of the system.

Hope that helps.

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 Post subject: Re: Changes to calculated and labeled arc flash hazard stick
PostPosted: Mon Feb 16, 2015 6:55 am 

Joined: Mon Jun 09, 2014 9:19 am
Posts: 43
There are times when the energy available could change- When you are on generator power vs normal power, the utility company changes a fuse during a power outage that has a higher rating than when the study was performed, and when equipment fails to operate. I am sure there are more but these are some examples of what we have experienced.

You are suppossed to perform you analysis at the best and worse case scenarios for the facilities. Best case would in most cases be when you are on normal power and with good information from the utility company. Typically worse case is on emergency power running a generator. You need to apply the labels to your equipment as a worse case that way you don't need to worry about employees wearing PPE that isn't rated for the hazard.

If you have a good relationship they will inform you when they change a fuse size, but that is if you are lucky! When you do your review every 5 years you will know what they did with fuses after you reach out to them. You can't do much about what they do. We have asked several of our utility companies for our stations to lower fuse rating sizes and they laugh at us! So just make sure you post labels for worse case to protect your employees.


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 Post subject: Re: Changes to calculated and labeled arc flash hazard stick
PostPosted: Mon Feb 16, 2015 9:13 am 

Joined: Sat Feb 14, 2015 4:19 pm
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Thank you for the replies. I was looking for specifics that are not indicated on the label, or the study itself for distances that the scenarios were calculated. specifically distance that the energy level was calculated. I have heard that these distances are at arms length? Is there also any reference to "imminent failure" of equipment that can be reasonably suspect for a higher level? The clearing times for the overcurrent protective devices were not shown on the study and subsequent labels were installed based on information that was not provided to us?


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 Post subject: Re: Changes to calculated and labeled arc flash hazard stick
PostPosted: Mon Feb 16, 2015 9:49 pm 
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Location: North Carolina
Distances using the IEEE and 70E methods are based on the distance from the bus at the back of the enclosure to the face/chest, or for open air conditions from the hands to the face/chest. Ergonomic distances suggest 15“ from normal hand position to body and adjusting upward for face/chest increases this to 18“. With deeper gear (e.g. switchgear) the distance inside the gear increases. IEEE 1584 allows other values to be used. For instance at my site we connect jumpers to substations from a bucket truck with a hot stick and operate cutouts and overhead switches from the ground which are at least 12 feet away. So the standard distance of 18“ would not apply.

If equipment is showing signs of imminent failure, this changes whether or not PPE is required as per the 2015 edition but both 70E and IEEE continue to assume that breakers and fuses still work, which drives opening time. Don't forget that if done properly calculating incident energy uses worst case conditions. So it would look at the line side of a circuit breaker for instance, not the load side. So failure of a breaker in a given enclosure does not affect incident energy. It would be affected by failure of the next protective device upstream.

As a thought experiment I took the model for a large chemical plant and did an analysis. About 25% of buses were over 40 cal/cm2, and about 25% were under 1.2 cal/cm2. If I increased the trip times on the buses protected by breakers only to 2 seconds (roughly approximating failure to trip for all breakers), another 25% of all buses went from under 40 cal/cm2 to over 40 cal/cm2. Thus demonstrating that just wearing a 40 cal suit for everything and eschewing proper maintenance is dangerously naive at best. 70E gives no guidance for not maintaining equipment.

The only reason the values change is changes to protective devices or changes to the system. For instance using different fuses because the store room was out of stock or increasing breaker settings because it is tripping may change the result. Having large motors on/offline, switching to a backup generator, or closing bus ties on double ended switchgear can also affect the value. If the study is done properly, system changes are accounted for and the value represents a worst case value. Older (prior to 2015) editions of 70E gave different values (H/RC) for different tasks but the incident energy did not chnge and Dupont cataloged cases of injuries using those tables (50% failure rate), so I don't agree with the premise that was used for decreased PPE.


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