REF vs Earth Fault Relay

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mohsinhamid Commentator Join Date: May 2011 Posts: 91	REF vs Earth Fault Relay 11/25/2011 11:39 PM
	Please some explain me why Restricted Earth fault protection relay is used in generating transformers alone..... I studied in book tat REF will operate only for 85% of the fault... then why we are using it...??? Why can't we use usual Earth fault protection in the generating transformers....????? Why we are using usual Earth fault protection in power as well as distribution transformers...????
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TonyS Guru Join Date: Aug 2010 Location: Resting under the Major Oak Posts: 4347 Good Answers: 181	#1 Re: REF vs Earth fault relay 11/26/2011 12:12 AM
	REF relays are used for many transformers not just generators. Personally I've never installed a power transformer with out one. By definition the relay protects a "restricted" area of a circuit, not the system in general. __________________ The reports of my death are greatly exaggerated.
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Jans Commentator Join Date: Feb 2009 Location: Tamil Nadu Posts: 66	#2 Re: REF vs Earth Fault Relay 11/28/2011 11:59 AM
	Earth fault relay is a must and REF relay is also generally not avoided for most of the transformers,generators,motors,reactors,capacitor banks. Amount of winding protected by ref relay is dependent on 1)The value of current being limited by NGR. 2) The line and neutral CT ratios. 3) CT characteristics. 4) Settings selected on the REF relay. If the available earth fault current is high, then the winding coverage can be nearly 100% With incorrect selections the coverage is 0%. Admittedly 85% coverage is typical but may be quite different in some instances. It is generally a straight forward application but can become involved for some types of resistance grounded systems. __________________ Better not count the cent if the risk is many many dollars.
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