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What does the Relay Accuracy Class mean for CT's?

Published date: 11 June 2020

Accuracy Class rating on Current Transformers.

Product Line:
LV Transformers

Applies to Current Transformers (CT's) by SquareD/Schneider Electric

Many Current Transformers (CT's) are rated for both Relaying Accuracy and ANSI Metering Accuracy.  This FA is concerned with Relay Accuracy Class


A relaying accuracy class is designated by two symbols which effectively describe the capability of the transformer as follows:
C means the transformer ratio can be calculated, i.e. a window type current transformer with uniformly distributed windings. The C rating refers to a low-reactance design. The secondary terminal voltage rating is the voltage which the transformer will deliver to a standard burden at 20 times normal secondary current without exceeding 10% ratio error. Furthermore, the ratio error must be limited to 10% at any current from 1 to 20 times rated current at any lesser burden. For example, relay accuracy class C100 means that the ratio can be calculated and that the ratio error will not exceed 10% at any current if the burden does not exceed 1.0 ohms (1 ohm x 5 amperes X 20 times normal current = 100 volts). Note: Previous standards used the term 10L in place of C, such as 10L100, 10L400, etc. CSA Standard C13 used the term 10L in place of C.  For more information on instrument transformers, see the Instrument Transformer Catalog, document # 4210CT9701R6/01.

CTA-ID : 2009169

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