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Units (for inputs) in IE calcs for capacitors (70E-2021)
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Author:  w5m [ Thu Aug 26, 2021 10:46 am ]
Post subject:  Units (for inputs) in IE calcs for capacitors (70E-2021)
In Annex R of NFPA 70E-2021, equations are defined for calculating Incident Energy (open air) which are labelled R.9.1a & R.9.1b. In the explanation of the inputs, no units are given. I presume the following are correct (as these units were used in the earlier stored energy equation, R.8.2)...
  • E = joules
  • C = farads
  • V = volts

There is also another input, radius (r), which presumably corresponds to working distance and is therefore specified in millimetres or centimetres rather than inches?
Author:  Jim Phillips (brainfiller) [ Wed Sep 01, 2021 3:42 pm ]
Post subject:  Re: Units (for inputs) in IE calcs for capacitors (70E-2021)
This could have been written with more clarity.

It appears that C is total capacitance in farads as shown in equations such as R.8.2 and R.8.3c.

However for voltage V it depends on which equation is used. R.8.2 defines V as peak voltage where as R.8.3c defines V as phase-to-phase voltage rms.

Further information is found in R8.4 Voltage. For peak voltage, the voltage applied across the capacitors that would cause the maximum charge should be used. For disconnected capacitors in storage or for disposal, the capacitor rated voltage should be used.

I agree this is not clear and can be confusing as written.
Author:  w5m [ Mon Sep 13, 2021 8:07 am ]
Post subject:  Re: Units (for inputs) in IE calcs for capacitors (70E-2021)
Thanks for your reply Jim.

I'm not sure if you saw the question at the bottom of my initial post and are able to clarify units for Radius...

Radius (r) is another input in the Incident Energy equations [R.9.1a] and [R.9.1b], which presumably corresponds to working distance and is therefore specified in millimetres or centimetres rather than inches?
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