Hi Guys,
Trying to model batteries on ETAP for a DC Arc Flash study. Unfortunately the library for batteries on ETAP is very limited and does not have the real life models that we find on site. I'm using Maximum Power Transfer method to calculate my DC IE. My question is that what parameters of the real battery should I keep similar to the ETAP model when finding an alternative ?

Initially I thought that I should pick a battery on ETAP that has similar amp hour rating, specific gravity and Vpc. But unfortunately the amp-hour rating and short circuit ratings on ETAP are never similar to the real life batteries simultaneously. One of the either is always off.

Upon more research, my opinion is that I pick batteries on ETAP that have similar short circuit ratings to my real battery since the Maximum Power Transfer method uses steady state fault current to calculate the DC arcing current, so the amp-hour rating does not play any role. But i'm not sure ? Is that a valid assumption ?

thanks !

Hi Shawn;

I don’t have ETAP and do not know its DC capabilities.

To determine arc flash incident energy you need the short circuit current for the specific model cell used in your system. This information is available from the battery manufacturer. Some battery manufacturers publish the short circuit currents on their data sheets while other make you ask for it.

Assuming most cells of a specific amp-hour rating have similar short circuit currents is a mistake. The short circuit current of a cell depends upon the design of the cell and is not directly rated to the amp-hour size.

Many stationary batteries have one or more cable connections within the battery, inter-tier or inter-row cables. The resistances of these cables reduce the battery short circuit current and thus the incident energy at the battery terminals.

Arc times for a battery will depend upon battery voltage. NFPA 70E recommends using a 2-second arc time, but that time isn’t achievable below 250VDC. Arc time has a large effect on incident energy.

I have a pretty decent library of discharge and short circuit data for stationary batteries. Let me know if you need data on a specific model cell and I'll see if I can help.

Hello Shawn and Mike

Thank you for your posts.

I have the same problem with SKM/PTW software when calculating the d.c. short-circuit current supplied from a stationary lead-acid battery.

When a battery is selected from the PTW library, the user enters the following data for an example PTW project:
Battery Subview:
Rated Voltage 250V
Number of Cells 108
Number of Positive Plates 73
Min Voltage (V/Cell) 2.13

Short Circuit (ANSI) Subview:
Open-Circuit Voltage 230.04V
Battery Size Type: 8 Hrs Rating ( Custom, Max and One Minute also selectable )
Battery rating 12A
Circuit Resistance 3.000 Ohm
Circuit Inductance 0.010 mH
Cell Interconnected Resistance 22 Ohm
Cell interconnected inductance 0.001 mH

My question for Mike is similar to Shawn’s:
How do I relate the above data to a real-life battery from Mike’s library of stationary batteries to calculate the Peak d.c. short-circuit current ?
The PTW software uses 0.5 * Peak d.c. short-circuit current and trip delay time to calculate arc flash incident energy to NFPA 70E 2021 Annex D.5.

Hi Shawn, ETAP has a simple interface that is easy to create libraries based on data sheet. You can contact ETAP helpdesk to request libraries as well. Meantime, depending upon the version you are using , the modeling of batteries is different. For maximum power method Arc flash , the program uses bus nominal voltage and the available short circuit current . For further assistance, you can reach out to ETAP helpdesk that can resolve your queries.
Thank you