for National Electrical Manufacturers Association (NEMA) Standard 250 "Enclosures for Electrical Equipment (1000 Volts Maximum)" published in 1997
for more info. The only X appears at NEMA enclosure 4X to distinguish it from NEMA enclosure 4 since the NEMA 4X is corrosion resistant.
Also, visit
for International Electrotechnical Commission (IEC) Publication 529 "Classification of Degrees of Protection Provided by Enclosures"
for more info since the IEC enclosure and NEMA enclosures may be interchanged somewhat after a proper correspondence is established.
I beg your pardon.
Suggestion: Reference:
1. ANSI/IEEE C57.13-1978 "An American National Standard IEEE Standard Requirements for Instrument Transformers"
Reference 1 addresses CT Classes on page 19. There are different CT Classes:
a) 5.3 Standard Accuracy Classes that are reflected in Table 6, i.e. 0.3, 9.6, 1.2 for Metering CTs
b) 6.4 Accuracy Classes for Relaying. There are essentially two Classes "C" and "T"
Which type of CTs do you have in mind?
I don't have the relevant IEC standard to hand, but the Class X CT is defined there (and was in BS3938). Basically, it is a CT that has a stated knee point voltage and secondary resistance. This is distinct from the standard protection class CT s with ratings such as 5P10, 10VA.
The Class X CT is used for specific protection schemes, such as busbar protection, where a higher grade CT is required.
In Class X CT, u as the designer specify the knee point, to the manufacturer.
CT sec R, is also specicfed for the manufacturer.
He (or she) then generally processes all your class X CT orders form a single lot. This ensures that maximum similarity is maintained between the outputs of various CTs.
Generally Class X CTs are used in High Impedance Differential protection schemes. Where the fault behaviour of all CTs must be as exact as possible. esp in through fault conditions. In TFC the loaded CT tends to saturate, this is why the protection scheme designer 1st calculates the amount of through Fault current that is likely to flow and then works out a CT knee point that ensures linear range during max through fault conditions. In other words u try to ensure that your high impedance diff scheme will remain stable in case of Through Faults.
