Nominal vs Float DC arcflach calculations

[AppleJaxJap]

can not find guidance in 70E or IEEE.


This issue makes VERY little difference in the calculation BUT I still want the reference

UPS
40 bats @ 12V = 480Vdc But
Float voltage is 2.25V Per Cell so the actual voltage on the bus when maintenance is being done is 2.25 x 6 x 40 = 540Vdc
I use the 540Vdc in the AF calculation as it provides a more conservative result (CYA)

Where this really makes a difference is in 96Vdc inverters.
<100Vdc does not need a calculation done but if you use the float voltage you are at 108Vdc and it does.

looking for some reference IEEE or other white papers that might provide guidance on this calculation voltage should be used.

This is your life and its ending one moment at a time.

 

[mb3928]

IEEE 1375-1998 "Guide for the Protection of Stationary Battery Systems," has some relevant information. Section 6.6.2 suggests that the higher float charging voltage only increases the short circuit current by about 1%. The reason being that although the voltage is higher, the chemical capacity of the battery to supply fault current is mostly unchanged. So for your arc flash calculations you might be justified in using the nominal DC voltage, but with the fault current increased by 1%.

There is also the fault current contribution of the battery charger itself, though for modern power electronic devices I wouldn't expect that to be much more that 1.5x nominal current.

 

[AppleJaxJap]

thaks thats what i needed i will review IEEE 1375-1998
thanks again

This is your life and its ending one moment at a time.