VA Rating of MCB, Contactor and Overload Relay

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pkiranster Member Join Date: Jan 2011 Location: Bangalore Posts: 5	VA Rating of MCB, Contactor and Overload Relay 10/12/2011 6:04 AM
	How to calculate VA rating of MCB, contactor and overload relay.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#1 Re: VA rating of MCB, contactor and overload relay 10/12/2011 6:11 AM
	The rating is the maximum breaking voltage multiplied by the maximum passing current. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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giri.gtr2010 Participant Join Date: Oct 2011 Posts: 3	#2 Re: VA rating of MCB, contactor and overload relay 10/12/2011 6:20 AM
	HAI GOODDAY Normally the power is calculated by voltage * current * powerfactor if its VA means we calculate without powerfactor. so VA rating = Line Voltage * Line current Thanks & Regards GIRITHARAN.R CR4 Admin - email address removed From the CR4 Rules: Do not post phone numbers or email addresses. The CR4 Admin will delete all phone numbers and email addresses posted in threads or comments. You can share this information via the CR4 internal messaging system.
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kvsridhar Guru Join Date: Sep 2009 Location: Temporarily at Ashburn, VA Posts: 2744 Good Answers: 164	#3 Re: VA Rating of MCB, Contactor and Overload Relay 10/12/2011 7:50 AM
	An MCB's main function is protection against overcurrent...so the making/breaking capacity quoted for an MCB is as has been said. It has elements within it to sense overcurrents (usually bimetallic and magnetic protective elements) and trip the device. A contactor is a switching device whose main function is to switch on and off a motor (mainly) under normal and occassionally, abnormal conditions. There is no inherent intelligence within a contactor to sense an overcurrent. An overload relay is a purely overload sensing device mainly for motors. It usually can sense an unbalanced (and single-phase) condition also, which is unhealthy for the motor. It has NO current-breaking capacity. In the event of an overload, it interrupts the coil supply to the contactor, which trips and prevents the motor getting damaged. __________________ Nothing worthwhile can ever be taught, it can only be learnt.
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SHAMITHGAMBHIR1 Power-User Join Date: Feb 2010 Location: dubai,mangalore,bangalore..... Posts: 145 Good Answers: 8	#4 Re: VA Rating of MCB, Contactor and Overload Relay 10/12/2011 8:33 AM
	its easy, V x A. __________________ shamith gambhir, electrical engineer
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kvsridhar Guru Join Date: Sep 2009 Location: Temporarily at Ashburn, VA Posts: 2744 Good Answers: 164	#5 Re: VA Rating of MCB, Contactor and Overload Relay 10/12/2011 9:21 AM
	i should actually have said this before .... Transformers are rated in VA (kVA, MVA etc) indicating the power that they can handle. MCBs are rated in breaking capacity, in kA, and their current carrying capacity. Circuit Breakers used to be rated in MVA breaking capacity, but not any more. Contactors are rated in their power handling capacity, usually kW or HP, since most of them are used for motor control. They are not rated in kVA. (Special capacitor-switching contactors are rated in kVAR). Overload relays are rated in Amps. Nothing else (ok, trip classes are also specified sometimes, but no VA or anything like that) Why do you need VA of these things ? __________________ Nothing worthwhile can ever be taught, it can only be learnt.
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