Arc Flash LV center tapped single phase Transformer

[vreijb]

I am performing Arc Flash for a plant with many small 15,25,27.5,50KVA center tapped transformers feeding LV Panelboards. The result from the Arc Flash Calculations are very High and probably not practical.
Is there assumptions made in this situations similar to LV 3-Phase system Like for V<240V and Ib<2K, IE=1,2cal/cm.sq.
The fault currents on single phase are usually lower, and weather the arc will be sustainable at low KA and Volatge

Any comments on interpretation of IEEE 1584-2018 related to above is appreciated

 

[dpc]

The IEEE 1584-2018 equations still only apply to three-phase ac systems. So you can't directly apply them for single-phase. The recommendation in IEEE 1584 is to model as an equivalent three-phase system and calculate from this.

Also, IEEE 1584 still suggests using a maximum arc time of 2 seconds unless egress would be blocked or limited.

The tables in NFPA 70E can be used - this would yield 4 cal/cm2 below 250 V within the stated fault current and clearing time maximums.

4 cal/cm2 is commonly used below 250 V if the conditions are outside of the range of the IEEE 1584 equations. I would not be comfortable with 1.2 cal/cm2 assumption. But that might just be me.

Cheers,

Dave

 

[Kiribanda]

Since an arc cannot sustain at lower voltages,the model used per IEEE-1584-2002 was not valid
for voltages less than 240V. Therefore, equipment below 240 V need not be considered unless it involves at least one
125 kVA or larger transformer in its immediate power supply.But the model used in IEEE-1584-2018 is different.
It says, sustainable arcs are possible but less likely in three phase systems operating at 240 V nominal or less
with an available short circuit current less than 2000 A.That means this 2kA sc current is equivalent for a
33kVA single phase transformer connected to a 240V system.Therefore, if your transformers are single phase
and 240V,AF incident energy from them can be disregarded if their ratings are less than 33kVA.

 

[vreijb]

Thanks. I have transformers 0.6/120-240V 25KVA, 37.5KVA, 50KVA, & 75KVA.
The IE values are not practical with ETAP results. ETAP is selected for single phase calculation method. I presume they use equivalent formulas. If I use transformer secondary fuses, then I can get more logical results. The transformer impedance plays a big role in this case lowering the Current and increasing the time to clear the arc fault, but as you said the arcs will not be sustained at this level faults and voltages.
I will be more comfortable to use 4cal/cm2 as well. existing tags in previous study shows 1.2 cal/cm2

 

[dpc]

Since an arc cannot sustain at lower voltages

Since 2002, this has been shown to been not totally correct as you note later in your post. Arcs can be sustained for short periods of time under certain conditions. Even if arc is not sustained, some PPE is needed, since molten metal droplets are often created when the fault occurs. In practice, A-F labels should be provided for all panelboards and PPE should be worn for energized work.