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 Post subject: What kind of arc flash does power system software calculate?
PostPosted: Fri Feb 04, 2022 5:41 pm 

Joined: Fri Feb 04, 2022 5:32 pm
Posts: 1

I have a quick question regarding arcs for Higher voltage switchgear.

Say I have a 138 kV system and we are calculating a 138 kV breaker arc flash boundary with typical substation clearance values.
1600 mm is phase to phase spacing and 1300 mm is phase to ground spacing.

When ETAP ArcFault calculates the arc flash energy and current for a 3-phase arc-flash, what exactly is it calculating? Is it calculating the current that would pass through if there was a sustained arc where current jumped from phase A to Phase B (flash-over)? That doesn't make sense because the substation clearances are designed so that flashover is basically impossible between two buses. These phase-phase substation clearances account for the dielectric breakdown of air so that the voltage between them won't arc through the air.

I am a little confused because ETAP uses VLL*25.4 /(10kV/cm) to calculate arc gaps, but the problem is at 138 kV the phase-phase and phase-ground are so far away that the 10 kV/cm dielectric strength of air can't be broken down unless there was an insulator flashover.

Or is it calculating the arc flash energy from a sustained fault for all 3-phases? For example if you shorted all 3 phases together, you would definitely have a huge arc from the short-circuit.

Furthermore, why is it that the ETAP calculated arc-gaps are so low? A 138 kV system has 1600 mm phase-phase spacing but ETAP is telling me the phase-phase arc gap is actually 300 mm from its calculations.

Thank you for your help!

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 Post subject: Re: What kind of arc flash does power system software calcul
PostPosted: Wed Feb 09, 2022 8:19 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 584
Perhaps some ETAP users can chime in.
We are not concerned with some spontaneous phase to phase arcing event, the concern is a worker initiating an arc. This might happen if the worker were to bring a tool within that 10kV/cm zone.

I would suggest using ArcPro for this purpose. Last I knew, it was the only software capable of handling line to ground faults. It also does not default to overly conservative Lee equations for higher voltages. ArcPro was used to generate the tables used in NESC and the OSHA T&D rules.

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