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 Post subject: DC Arc Flash Calculations for Battery Chargers/Rectifier
PostPosted: Tue Nov 17, 2020 11:01 pm 

Joined: Fri Apr 03, 2020 10:09 am
Posts: 2
Hi, I'm an engineer in training and this is my first time trying to perform an arc flash study on a DC System. I want to attach an AF label on a battery charger/rectifier unit. I'm using ETAP 20.0 I have a few stupid questions, kindly indulge please :

1. A battery charger/rectifier enclosure has an AC side and a DC Side - 480 V (AC)/ 50 A (AC)/3ph and 100 V (DC)/200 A (DC). My brain tells me that I calculate the worst case incident energy value on the ac side and the worst case incident energy value on the DC side and then attach a label with an IE value which would be the worst case out of the AC IE and DC IE. What would be your suggestions ? Would you calculate only the AC side for a rectifier ? If so, why ?

2. My second question is that on ETAP, if i fault a bus on the AC side of the charger, will there be any SC contribution to the fault from the DC side ? Is it possibly for a battery to feed a fault current on the AC side of the charger ? Conversely, if there a fault on the DC side, will the AC source be able to feed the fault on the DC side through the rectifier ?

3. Also, if possible, kindly let me know how to tackle an inverter unit !

thanks.


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 Post subject: Re: DC Arc Flash Calculations for Battery Chargers/Rectifier
PostPosted: Mon Nov 23, 2020 11:58 am 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 191
Location: Maple Valley, WA.
We calculate the AF energy for both the AC and DC sides of the battery charger/rectifier. We put both labels on the equipment. So chargers/rectifiers have separate compartments for the AC and DC sides. So an electrician needs to know the energy level on both the AC and DC sides so that they can determine what level of PPE to wear.

For second question, usually the electronics limit the SC current on the DC side. Also, the electronics will prevent DC current from flowing upstream to the AC side.

An inverter is similar. The SC current output is limited by the electronics.

For both rectifiers and inverters, find the instruction manual and search for current limits. You will usually see that it is limited to 500% or less of the full load amperes.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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 Post subject: Re: DC Arc Flash Calculations for Battery Chargers/Rectifier
PostPosted: Tue Nov 24, 2020 2:31 pm 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 540
As stated above, rectifier current is low and short duration. Battery current, however, is neither. A fault at the terminals has no protective device to interrupt it, and the magnitude is only limited by cell and intercell impedance. The charger cannot feed battery current to an AC fault unless it is designed to invert.


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