Core-1 , Core-2 of CTs

It is quite common to put multpiple CT's on one line and also common to put them in the same casting/casing/wrapping.

The various cores are magnetically isolated and the windings are electrically isolated so that they are totally independent of each other. They may be connected at the relay or burden end but care has to be taken with polarity etc.

Quite often they are both measuring, both protection or one (or more) of each.

They are distinguished by the numbering of the output terminals:

1S1 is the start of the winding on the first CT1, 1S2 is the first output tap, 1S3 the second output tap etc.

2S1 is the start of the winding on the second CT, 2S2 is the first output tap, 2S3 the second etc

The phasing must be such that 1S1 & 2S1 are of the same polarity.

The label on the CT must identify the function and rating of each core.

It is also common to have a spare CT (say CT3 with 3S1, 3S2 etc) if the CT's are in an inaccessible location so that further monitoring can be added later.

Further information can be obtained in the IEC and other standards IEC60044.1 AS/NZS60044.1 which are the current standards. However a standard that gives more information is Australian Standard AS1675 - 1986 (now officially non-current). Appendix B "Guidance in the application of current transformers." is well worth studying. It is now no longer current but still widely used.

If you can't get it I can give you details.

First, you must understand the difference in the functional requirements between a metering CT and a protection CT.

A metering CT is connected to measuring instruments, whose job is to measure current during normal conditions. Thus, it is enough if a metering CT maintains its accuracy only upto its rated current, may be few percentage more (120%, to be precise, as per the Standards). If the metering CT maintains its accuracy beyond this, then during fault conditions, the high fault current will be reflected to the meters connected to the CT Secondary. This increased current will have to be withstood by the instruments, till the fault is cleared. That is, the short time current withstand capacity of the instruments needs to be more, which will make the instruments bulky & costly. So, metering CTs are designed such that the core saturates for a low multiple of the primary current, generally 2 times. For example, if you have a 100/5A Metering CT with a Instrument Security Factor of 2, then for 100A Primary Current, this CT will give 5A Secondary Output and for 200A Primary Current, this CT will give 10A Secondary Current. Now, if the primary current is 300A, then this CT will NOT give a secondary current of 15A, but slightly more than 10A. If there is a fault in the primary and the fault current is 10, 000A, then also this CT will give a secondary output of 10A only. Thus, irrespective of the primary current magnitude, the instruments connected to this metering CT will have to bear a short time current of only 10A, thereby optimising on the size & cost of the instruments. It is the selection of the core material & the core design that makes this possible.

Alternately, a protection CT may or may not be accurate during normal conditions, but, must be accurate during fault conditions. Then only, the primary fault current will be reflected as it is to the protective relays connected to the protection CT Secondary, so that the relays will perform their intended operations. Thus, the protection CT has to maintain its accuracy for many multiples of rated current. For how many multiples, is decided by the Accuracy Limiting Factor. In your case, it is a 5P10 CT, meaning that this CT will maintain an accuracy of +/- 5% Composite Error, upto 10 times the rated current. This means that the CT shall not saturate upto 10 times the rated current. This is also made possible by proper selection & design of the core.