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 Post subject: Gap + X factor for non-bussed equipment
PostPosted: Fri Aug 24, 2012 4:22 pm 

Joined: Fri Aug 24, 2012 2:39 pm
Posts: 1
As I understand it, the 1584 equations most modeling softwares use use gap and 'x' factor values from 1584 tables 4 and 6. The tables sort equipment into 4 categories: "Open Air", "Switchgear", "MCC and panels", and "Cable" for voltages between 208 and 1000 volts.

For equipment like control panels, circuit breaker disconnects, self contained starters, or even fused disconnects how do most people categorize that equipment? What is the reasoning for your categorization?
If the terms from the tables are defined or a 70E/1584 example considers equipment similar to control panels, please point me to a document.
If multiple designations apply (control panel could be intepreted as all of them except switchgear!) do most people run multiple calculation iterations with each type?

For the same bolted fault and work distances, I think the arc fault current compares like the following "MCC and panels" > "Switchgear" > "Conductor" > "Open Air".

(We all know this comparison of current does not hold true for arc flash energy because of the time variable multiplication and different currents will have different times.)


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PostPosted: Sat Aug 25, 2012 1:49 pm 
Sparks Level

Joined: Thu Jul 07, 2011 6:03 am
Posts: 64
Location: Netherlands
If you look at IEEE 1584's equations the distinction between "MCC and panels" and "Switchgear" you see it appear twice:

1. Calculating lg Ia as + 0.00526 - 0.0304 G (lg Ibf). This results in a negative exponent, meaning a larger gap (Switchgear) will result in lower arcing current. This can either increase or decrease incident energy, depending on your protective device's time current characteristics.
2. Calculating Ei as a multiplicative factor (610^x / D^x). The distance factors for "MCC and Panels" and "Switchgear" differ slightly, resulting in 1.58 and 1.49 respectively (both at 18'' working distance). Disregarding item 1, this means higher incident energy for "MCC and Panels". Note that table 3 provides a higher typical working distance for "Switchgear" and this has a far more significant effect.

So what to make of it? Bus gaps can vary even within switchgear, especially near breaker connections. In my experience they're more often smaller than 25 mm rather than larger, but I mostly work with IEC switchgear so YMMV. The increased working distance for "Switchgear" is also something I don't particularly like. It's very effective, sure, but breakers are usually brought out to the front of switchgear and that brings energized parts closer as well. Unless a customer specifically asks for it, I resort to "MCC and panels" for all low voltage equipment.


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PostPosted: Tue Aug 28, 2012 6:28 am 

Joined: Thu Feb 04, 2010 5:00 pm
Posts: 36
Location: Camp hill Pa
Watch it, alonger Arc will produce provide and arc with lower current, however a lower Arc current will take longer to clear and there may be a higher total energy due to the longer clearing time. When I was young, a manufacturer of Spin tight explosion proof enclosures was invoved in aan accident in a refinery. It was demonstrated that tracking from pole to pole could result in a short circuit, that did not have sufficent current to trip the upstream circuit breaker, but provided enough heat to melt though the aluminium enclosures. (bakelite cases can decomposed into carbon and lose the insulating value and allow the current to travel across the case. this can happen if they are contimanated.)


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