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 Post subject: Arc flash Study for 5 HP motors
PostPosted: Wed Oct 14, 2015 9:27 pm 
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Joined: Thu Jan 16, 2014 10:02 pm
Posts: 16
Location: pune
Why in Arc flash study the motors below 5HP are not considered? Is it there any standard for that?


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 Post subject: Re: Arc flash Study for 5 HP motors
PostPosted: Thu Oct 15, 2015 3:35 am 
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Joined: Wed May 07, 2008 5:00 pm
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Location: Rutland, VT
It is 50hp (37kW) motors not 5hp. This is noted in IEEE 1584-2002 Clause 4.2 and larger than 50hp motors need to be considered as they can contribute to the total fault current on a bus.

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 Post subject: Re: Arc flash Study for 5 HP motors
PostPosted: Thu Oct 15, 2015 8:26 pm 
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Large motors (over 50 HP) are inductors which store energy in their magnetic fields. During a fault, they become generators for a short period of time and increase the fault current. Small motors are too small to have enough energy to make a substantial contribution so they are ignored.

Second consideration and this is more of a practical one is that the overcurrent protection for a small motor will limit the incident energy to an acceptable level. Generally this holds true for breakers that are 100 A or smaller where the cable is less than 200-300 feet long and the voltage is 480 V. But I know of no detailed, published scientific study or engineering standard stating this.


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