Current Transformer - Sizing - CR4 Discussion Thread

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Selk Associate Join Date: Dec 2016 Posts: 48	Current Transformer - Sizing 02/09/2021 2:24 PM
	In general, it is typical to size the current transformer with +20% margin of Load current. Say, if the spec of the CT is 5P20, - It means for 20 times the CT primary, the error in the secondary will not exceed 5 percent. Since the accuracy is 5% for 20 times the rated CT primary, Why to size with a margin of 20%?
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SolarEagle Guru Join Date: Mar 2007 Location: by the beach in Florida Posts: 33392 Good Answers: 1817	#1 Re: Current Transformer - Sizing 02/09/2021 2:58 PM
	If you were designing a tightrope to walk across a great height, what would your margin of safety be, and why? https://resources.pcb.cadence.com/blog/2019-fmea-risk-assessment-for-pcb-component-selection __________________ All living things seek to control their own destiny....this is the purpose of life
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Stef Guru Join Date: May 2018 Location: Under the spreading Bunya Trees, South Burnett, Queensland, Australia Posts: 807 Good Answers: 65	#2 Re: Current Transformer - Sizing 02/10/2021 12:51 AM
	It has a lot to do with asynchronous currents causing saturation of the CT during fault conditions affecting the secondary current for protective devices operation. As we all know supply authorities always like to run the system above design load to squeeze just a bit more out for the peak demands and rely on the daily duty cycle, so 20% is acceptable. Even the curve of protective devices, say a CB, can have up to 25% overload current flowing before they will operate to trip in a predefined time. __________________ Hare today, goon tomorrow!
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raghun Power-User Join Date: Jan 2010 Location: Hyderabad Posts: 343 Good Answers: 22	#3 Re: Current Transformer - Sizing 02/12/2021 11:34 PM
	5P20 CT is allowed to have 5% error max at 20 times rated current and that is for protection system design engineers. 20% overload is for the operator to make sure that the CT can continuously be loaded to 120% without suffering any thermal damage, if and when required. When it comes to error in the operating load range, 5P20 CT can have max 1% error (not 5%). This as per IEC. __________________ Raghunath
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rwilliams Guru Join Date: Jul 2016 Posts: 1367 Good Answers: 105	#4 Re: Current Transformer - Sizing 02/13/2021 4:55 PM
	In addition to the GAs already posted, commercially & easily available instruments are designed with standard 5 ampere full scale. If you connect the 0-2000A meter with the 0-5A movement to your 2000:5 current transformer, any overload you might want to evaluate will be off the scale, telling you little about the nature of the problem, for instance how much over and for how long?
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raghun (1); rwilliams (1); SolarEagle (1); Stef (1)

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Current Transformer - Sizing