ALF and ISF in Current Transformers 1

[ravindranathan]

Both ALF(Accuracy Limit Factor) and ISF(Instrument Safaty Factor) for Current Tranformer mean the same.
Why do we have separet terminologies for these two?

 

[FreddyNurk]

That's because they don't really mean the same thing.
One is used for selection of CTs for protection purposes, the other for metering and measurement.
Whilst the function and behaviour of the CT is the same for both purposes, the requirements are different for the two applications, and the two terms reflect this.

EDMS Australia

 

[davidbeach]

Somehow it seems that Thermal Rating Factor covers both well enough.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[scottf]

To attempt to add some clarity:

ALF = Accuracy Limiting Factor is used for protection class CTs and it is the number times rated current that the stated accuracy is good up to. For example, 5P20...5 is 5% accuracy class and 20 is ALF. Meaning the CT will maintain 5% accuracy up to 20 times rated current with rated burden applied. In practice, it means that the CT won't saturate at 20 times rated current with rated burden applied.

ISF = Instrument Security Factory is used for metering applications. Normally something like 0.2Fs5 - 15 VA. That means that the CT is designed to saturate at 5 times rated current in order to protect the meter. This kind of rating only exists in the IEC-based world and not in ANSI/IEEE.

Continuous Thermal Rating Factor (often called Rating Factor) is the number of times rated current that the CT can be operated at and not exceed the temperature rise/thermal limits given in the standard. It also becomes the upper limit of the metering accuracy class.

Example CT, 200:5A, 0.3B0.5, RF2.0
For this CT, the CT can be operated continuously at 400A (200A * RF2.0) without exceeding temperature rise limits...in the IEEE world, unless stated otherwise, this is based on a 30C average ambient temp over a 24 hr period, not to exceed 40C peak temp.

Also, this means that the CT is guaranteed to be 0.3% accurate (in the class) from 200A to 400A primary (rated to rating factor) and 0.6% accurate (in the class) from 200A down to 20A (Rated down to 10% rated). This is perhaps the most misunderstood and misapplied rating that we see in CTs. In the IEEE world (and even in IEC world) CTs are meant to be / and should be operated above the rated current.