S Class Protection CTs

[entropically]

I've got a few 1960s/70s 33kV CBs with 15VA S10 CTs fitted. Can anyone tell me how what the 'S10' means?
I'm guessing that it refers to a standard in use prior to the introduction ofuse of BS/IEC P class.

 

[Marmite]

Class S means current error at rated primary current plus or minus 3%. Composite error at accuracy limit primary current 5%.
CT's in class S are intended to apply to simple forms of protection such as overcurrent relays and trip coils, and of low imedance earth fault relays with time lag characteristics.
The class specifies limits of current error at rated primary current and limits of composite error at the accuracy limit primary current. The ratio between the accuracy limit primary current and the rated primary current is known as the rated accuracy limit factor, the standard factors being 5,10,15,20 & 30.

Regards
Marmite

 

[scottf]

Essentially the same as 5P10 - 15 VA.

 

[RRaghunath]

scottf,
5P has current error at rated primary current plus or minus 1%, to that extent the Class S seems to be differing with 5P.

 

[scottf]

Raghun...I've never seen a protection core that didn't have an accuracy of at least 1% at rated current. From a practical perspective, there's little difference.

For example, a CT rated 5P20 - 30 VA, is going to have very good accuracy at rated current, because the flux density at rated has got to be low enough that it won't saturate at 20 times rated current. Most normally constructed CTs with protection accuracy classes easily meet metering classes at rated currents.

 

[RRaghunath]

scottf,

I agree with you, what I wrote is from IEC60044 - 10P is said to conform to Class 3 in the load current range.

 

[entropically]

Thanks guys. Anyone know what standard covers these CTs?

 

[Marmite]

BS3938 1965.

Regards
Marmite