We have a few different types of battery strings at our site for UPS power backup, and I am having difficulty determining the Incident Energy. (Or accepting that my calculations give a reasonable answer).

I would appreciate anyone who can help back check my calculations or guide me in the right direction.

Here are a couple examples that I want to calculate the Incident Energy:
(40) EnerSys 12HX-400FR VRLA batteries.
(60) EnerSys 4DXC-21B Flooded batteries.

For each, there is a breaker, but I am choosing to disregard it as the electricians would be working on the batteries themselves, and I feel the breaker is really only providing protection downstream. So, I am using the standard 2 second arcing duration when no breaker is present. Is that a correct assumption to make?

I am trying to use the IEEE Vol 46, No 6 equations referenced in NFPA 70E. However, I am not sure what to use for the system resistance. So I just used V/I using the nominal voltage and short circuit current of the batteries. I see there is an iterative calculation to find the resistance, but I don't understand how to do that for these batteries. Can anyone clarify what to use for the resistance, Rsys?

(40) EnerSys 12HX-400FR VRLA batteries.
From the manufacturer data sheet, I see Isc = 4225 Amps
Rsys = Vsys / Isc
12HX-400FR Rsys = 540V/4225A = 0.128 Ω

IEmaxpower = 0.005 *( V^2/ Rsys )* Tarc / D^2
12HX-400FR IE = 0.005 *( 540^2/ 0.128 )* 2 / 46^2 = 10.78 cal/cm2

So I would need to have electricians dress CAT 3, until they break the string in half, then can drop to CAT 2.

(60) EnerSys 4DXC-21B Flooded batteries.
From the manufacturer data sheet, I see Isc = 19090 Amps
Rsys = Vsys / Isc
12HX-400FR Rsys = 540V/19090A = 0.028 Ω

IEmaxpower = 0.005 *( V^2/ Rsys )* Tarc / D^2
12HX-400FR IE = 0.005 *( 540^2/ 0.028 )* 2 / 46^2 = 48.72 cal/cm2

So, this is CAT Dangerous! And per our safety policy, we can't do anything with these batteries if we are trying to comply with NFPA 70E?

I appreciate any help or guidance you provide!

If I understand what you are detailing, I would assume the breaker is in its own enclosure and removed from the batteries, you can use that for clearing time, in my opinion.

Including cable impedance where applicable will help bring down the AFIE. If you need assistance on the iterative process, Jim has a paper on the calculations.

Mike

1. CAT 3 PPE isn't required. The PPE must be rated at 11cal/cm2 or better. There's a lot of this level PPE available. This also allows use of an AR balaclava and AR face shield instead of an AR hood.
2. The resistance of the inter-jar connectors and the inter-tier cables are not accounted for in your calculation. These cables are series resistance and thus reduce the battery short circuit current. Depending upon cable sizes and lengths, this could reduce the incident energy to less than 8 cal/cm2.
3. Shifting to the 60-cell battery: Below 150VDC it is very difficult to sustain an arc. The longest arc duration anyone in a lab has achieved at this voltage is 0.715 seconds, see the Bonneville Power paper from 2017. They had to do a lot of work to achieve that arc duration. Hydro Quebec could only get 0.493 seconds in their experiments, see Paper No. ESW 2020-14 Low Voltage 1-500VDC Arc Flash Testing by Kirk Gray P.Eng, S. Robert P.Eng, & Timothy L. Gauthier. Obviously, the reduced arc time substantially reduces the incident energy.
4. The short circuit current for the 4DXC-21's includes the resistance of the intercell connection, but not the resistance of the inter-tier cable. This series resistance will make a significant reduction in the battery short circuit current.
5. Using an arc time of 0.715 seconds brings the incident energy <5cal/cm2.
6. For work on a 60-cell 4DXC-21 we require our workers to use daily wear 8cal/cm2 or better clothing, EH rated safety boots, safety glasses, and non-melting or arc rated work gloves.