Meaningful Arc Flash Calculations Possible for 208/240 8KVA?

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Anitech Systems

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LA, CA
We have someone that wants us to put NFPA 70E Arc Flash info on some control cabinets we made / are making.

The Enclosure is fed from a single phase 2P+G 208/240VAC, 6400W/8000VA UPS.

The Enclosure has a 40A 208/240VAC Disconnect Breaker that feeds several 480W 24VDC 20A Power Supplies.

The enclosure is labeled with 10K SCCR (in some cases 5K),
based on the branch circuit side device specifications.

The UPS manufacturer doesn't have SCCR, Bolted MVA, Transformer Impedance,
... information available for their UPS smaller than about 15KA.

A 50A Single Phase Breaker feeds the UPS input disconnect (2P+G).

The 480 to 240/208 transformer that feeds the UPS input disconnect information is not available to us,
and is considered by the end user to be out of our scope, so we don't need to know;
{... and the transformer hasn't been selected yet in most cases, by the time they want the ARC Flash info from us}.


The Shock Protection Boundary from Table 130.3C: 50 V?300 V Limited Approach Boundary 3.5ft,
Restricted Approach Boundary Avoid contact, Prohibited Approach Boundary Avoid contact seems fine.

However, as far as the information I have available, and using formula D.2(e),
with 8KVA and 6 cycle clearing time, e.g.: POWER((53*(0.008*0.125)*0.1),0.5)
I get less than 0.9in for the ARC Flash Boundary?

Providing the above information, can anyone direct us to any reference information (or even software products)
that would allow us to make any kind of meaningful e.g. NFPA 70E 130.5 / Annex D Arc Flash Hazard Analysis Calculations?
 

jim dungar

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Location
Wisconsin
Occupation
PE (Retired) - Power Systems
There is no industry standard method for calculating arc fault incident energy for single phase circuits.
There are several different methods, but no clear favorite.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Isn't there a general rule that for the secondary side of transformers less than 75KVA, arc flashes generally aren't self sustaining, and that the arc flash hazard category can generally be considered 0.

I know you can't predict what size transformer will be supplying these, but on the secondary side of your UPS, with only 8KVA available, it sounds like the hazard would be pretty small.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
AFAIK, there's no kVA cutoff/exemption right now, so arc flash levels are being calculated per the 1584 equations for all transformers encountered in an arc-flash project.
I suggest to the OP to follow 1584.1-2013, (3. Arc-flash study general guidelines) for the single-phase analysis, that advises to: assume it has three-phase service
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Isn't there a general rule that for the secondary side of transformers less than 75KVA, arc flashes generally aren't self sustaining, and that the arc flash hazard category can generally be considered 0.

This is, actually, part of the IEEE1584 standard.
When this standard was first published, there was little testing (i.e. only 1) done for 208V systems, so it appears the committee made a recommendation based on 'gut feel' an anecdotal evidence.

There is now quite a bit of discussion going on about the results of more recent and complete testing concerning these circuits. The consensus seems to be that the next edition will be based on available fault current and voltage.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
AFAIK, there's no kVA cutoff/exemption right now, so arc flash levels are being calculated per the 1584 equations for all transformers encountered in an arc-flash project.
I suggest to the OP to follow 1584.1-2013, (3. Arc-flash study general guidelines) for the single-phase analysis, that advises to: assume it has three-phase service


Currently, all of my customers use the NFPA 70E informational note and follow the IEEE 1584 standard, which contains the 'kVA cutoff' suggestion.

I would not be surprised to find about 50% of my customers routinely using the NFPA70E task tables for '120V single phase' circuits.
 

ron

Senior Member
The IEEE 1584 Annex note (not part of the body of the document) is for 125kVA and below, I believe.

The let-through of an 8kVA transformer will be really small. If you guess at the available fault current (which of course you wouldn't know for a generic product installation), it is just as inaccurate as not knowing at all.

I think the fact that there is an 8kVA transformer in the power path, and most do adopt the Annex note of 125kVA and less being exempt, you will likely pass the sniff test if you place PPE 0 on the box downstream of the 8kVA transformer.,
 
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