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 Post subject: Bus Gaps vs Arc Gap vs Arc LengthPosted: Wed Mar 05, 2008 8:03 am

Joined: Wed Mar 05, 2008 7:42 am
Posts: 16
Hi everyone, I'm new in this forum. I'm hoping getting great infos on Arc Flash Analysis and also share my knowledge.

Here goes my first question.

In IEEE 1584-2002, one of the input values to perform Arc Flash Calculation is the Bus Gaps. The definition of this term is fairly clear (p.11) : Gap between conductors.

In NESC 2007, Table 410-1 (p. 247) states that their calculations are based on "Arc gaps as follow: 1 to 15 kV: 2 inch [...] See IEEE Std-4 1995.

Finally, in Electrical Safety Handbook edition 2, Flash Hazards calculations describing Heat Flux Calculations, states that an Arc Flash is proportional to Arc Length. (3.52)

Are all these values the same? What's the difference between Bus Gaps, Arc Gap and Arc Length .

Bus gaps can be found on the field or we can take default values proposed by IEEE 1584-2002. Where can we found Arc Length information?

Thanks

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 Post subject: Posted: Wed Mar 05, 2008 9:07 am
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Arc Gap, Bus Gap, and Arc Length

Electric John wrote:
Hi everyone, I'm new in this forum. I'm hoping getting great infos on Arc Flash Analysis and also share my knowledge.

Here goes my first question.

In IEEE 1584-2002, one of the input values to perform Arc Flash Calculation is the Bus Gaps. The definition of this term is fairly clear (p.11) : Gap between conductors.

In NESC 2007, Table 410-1 (p. 247) states that their calculations are based on "Arc gaps as follow: 1 to 15 kV: 2 inch [...] See IEEE Std-4 1995.

Finally, in Electrical Safety Handbook edition 2, Flash Hazards calculations describing Heat Flux Calculations, states that an Arc Flash is proportional to Arc Length. (3.52)

Are all these values the same? What's the difference between Bus Gaps, Arc Gap and Arc Length .

Bus gaps can be found on the field or we can take default values proposed by IEEE 1584-2002. Where can we found Arc Length information?

Thanks

Welcome to the Forum,
I am going to assume that, since you mentioned the NESC, you are concerned with overhead outdoor construction at 1 to 15 kV for the purposes of my response.

We use ArcPro for our calculations and "bus gap" is not an issue since two basic assumptions are that the arcs will all be single phase to ground and will not in a box.

As you stated from NESC 410-1, the "arc gap" is 2", 4", or 6" depending on the system voltage level (15, 25, or 35 kV). For utility "primary" systems, I do not get concerned with the other publications.....just the NESC. I must assume that the physics and/or other dynamics of the arc are modelled properly in the software. I expect this is the case since, as I have noted in previous posts, the energy levels of the table match the ArcPro outputs to within .1 or less.

For the low voltage portions of the systems (480 and 208), ie..padmounts, transclosures, meterbases, etc. have not decided whether to use the factors recommended with ArcPro, revert to some of the other pubs for arc in a box type calcs, or just use the NESC minimum of 4 cal.

Hope it helps,
Alan

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 Post subject: Posted: Wed Mar 05, 2008 12:50 pm

Joined: Wed Mar 05, 2008 7:42 am
Posts: 16
Arc Gap, Bus Gap, and Arc Length

Thanks for the greeting.

If Table 410-1 seem to have use the Arc Pro software in order to do their calculations, why do they mentioned the "arc gap" is 2", 4", or 6" depending on the system voltage level (15, 25, or 35 kV).? As I understand, Arc Pro doesn't use Arc Gap information since it's open air and LG fault only.

To match the table with Arc Pro, what are the inputs to enter? I'm currently using Heat Flux method and my calculations are approaching NESC Tables. I wished NESC would have published their equations used to built their table. The inputs for Heat Flux method published in Electrical Safety Handbook are:

Faulted Current
Arc Length
Working Distance
Opening Time
Arc Voltage

Thanks!

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 Post subject: Posted: Wed Mar 05, 2008 2:08 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Arc Gap, Bus Gap, and Arc Length

John,

Arcpro absolutely does us the arc gap. The inputs are system voltage, distance to worker, and arc gap to calculate the energy. You will also have to select the electrode material of copper or stainless steel. There is no aluminum selection. The S. steel yields the highest results and matches the NESC table.

Alan

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 Post subject: Posted: Thu Mar 06, 2008 7:20 am

Joined: Wed Mar 05, 2008 7:42 am
Posts: 16
Arc Gap, Bus Gap, and Arc Length

When you are specifying the Arc Gap in Arc Pro, are you talking about a bus gap (mentioned in IEEE-1584-2002) or an arc Length (mentioned Electric Safety Handbook 2002)?

What about he Faulted current and the Opening time in Arc Pro?

Electric John

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 Post subject: Posted: Thu Mar 06, 2008 10:06 am
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Arc Gap, Bus Gap, and Arc Length

Electric John wrote:
When you are specifying the Arc Gap in Arc Pro, are you talking about a bus gap (mentioned in IEEE-1584-2002) or an arc Length (mentioned Electric Safety Handbook 2002)?

What about he Faulted current and the Opening time in Arc Pro?

Electric John

I would expect that the "gap" would relate to arc length. Don't know but I also expect that they are using standard REA (or close to REA) type overhead construction with 2 to 3 feet for phase spacing. And yes you do enter the available fault current and clearing time of the upstream device. To date I have been starting at 150% of relay pickup, then using increments of roughly 1kA up to the max available fault and adding the relay + breaker time at that level to calculate the energy along different portions of the system.

Alan

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