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Jensaugust12

Post subject: ARC gap IEEE1584 Posted: Sun Mar 18, 2018 5:51 am 

Joined: Wed Nov 01, 2017 6:09 am Posts: 27

Is 152mm considered to be the maximum arc gap to be used or is it normal to put in a larger gap if this is real. (The question is would the equations then be considered to be valid or out of range)


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PaulEngr

Post subject: Re: ARC gap IEEE1584 Posted: Sun Mar 18, 2018 10:22 pm 

Plasma Level 

Joined: Tue Oct 26, 2010 9:08 am Posts: 2174 Location: North Carolina

Out of range.
EPRI has done some work in terms of extending the gap length out to if I recall correctly about 12" (300 mm) and some preliminary work on even longer arc gaps but that's about it. While the joint IEEE/NFPA arc flash group is more interested in modelling conditions which are more representative of industrial equipment, EPRI is a utility consortium and thus more interested in models for outdoor overhead lines and large open enclosures such as transformer air termination cabinets that are typical of utility construction practices where space is less of a premium and sheet metal is inexpensive. The general results that they have found is that equipment specific testing is more productive than generalized equations such as IEEE 1584 empirical equations. They have amassed a significant list of equipment types and conditions. The general result from this testing is that they have been finding that the results are usually significantly lower than the calculated results from IEEE 1584 empirical equations or ArcPro. There are also some significant exceptions. For instance certain types of meter sockets produce incident energy levels WAY above their calculated values. Similar results have been produced for certain general configurations of equipment, particularly the "box barrier" model where 3 vertical bus bars terminate into a solid insulating material.
If the equipment you are considering is on "the list" you might well consider using the actual test results rather than some empirical equations that are intended for general use only.
Two problems arise as the arc gap increases to very wide gaps. The first is that the orientation of the arc and the orientation of the victim become important. All current models essentially treat the arc as a point source of radiation. As the arc gap increases this simplification breaks down. A second problem is that for instance with a horizontal arc convective air currents draw the arc up and stretch it out so that the arc length is longer than the arc gap. As gap increases the arc tends to wander a bit so the arc length is longer anyway regardless of orientation. Also with longer arcs there are distinct anode, cathode, and "arc body" regions that behave differently and need to be modelled individually.
Arc gap severely affects the incident energy in the models. If you increase it, the models tend to "blow up", too, producing unrealistic results. Although the incident energy does indeed increase dramatically with increased arc gap, with very large gaps it doesn't quite work the same way as the models. As the arc gap increases, arcs are no longer stable and may not restrike or the arc may be weak. In either case ArcPro and IEEE 1584 empirical models work under the assumption of a strong, stable arc. So far there are no models for selfextinguishment or weak arcs. Obviously this is more significant with large arc gap distances. To model these types of effects we would have to consider two more factors. Right now the model data mostly relies on either open air or relatively "open" box structures which is completely unrealistic. Actual equipment rarely has just 3 bus bars suspended in a huge amount of open space. Real electrical equipment is packed into the smallest space that it can roughly fit in and still allow room for maintenance (compact MCC enclosures exception: I don't think those are intended to be maintained!). These "obstructions" are all energy absorbers or potential energy absorbers as well as holding and containing heat and thermal mass. So actual equipment has a tendency to absorb a lot of energy given off by an arc which reduces the actual incident energy while at the same time the degree of "containment" has a tendency to hold in hot air in the local area of the arc which can aid in arc restriking. This is the reason that the revised IEEE 1584 model may push the lower cutoff (<=125 kVA transformer, 208 VAC or less) lower yet. There is precedent for this. OSHA has documented a fatality with a 240/120 V temporary construction panel. Lab results of "boxbarrier" tests have shown better arc stability. So taking this all into consideration it is clear that we have a long way to go towards modeling arc gaps longer than about 300 mm.


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Jensaugust12

Post subject: Re: ARC gap IEEE1584 Posted: Tue Apr 10, 2018 12:20 am 

Joined: Wed Nov 01, 2017 6:09 am Posts: 27

Can I rephrase my question. Is the arc gap considered a input value in IEEE1584 or is the standard values corresponding to voltage and choosen equipment always to be used?


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wbd

Post subject: Re: ARC gap IEEE1584 Posted: Tue Apr 10, 2018 5:39 am 

Plasma Level 

Joined: Wed May 07, 2008 5:00 pm Posts: 863 Location: Rutland, VT

The gap is a variable in the equations in IEEE 1584 and denoted as G. There is a reference to Table 4 which lists typical gaps for various types of equipment. The gap range for IEEE 1584 is 13mm to 152mm, so any gap value in that range can be used for G.
Clause 5.4 states that for cases where the gap is outside the range of the model, the theoretically derived Lee method can be applied.
_________________ Barry Donovan, P.E. www.workplacesafetysolutions.com


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