Instrument Security Factor (FS) of a Measuring CT

Short answer

The instrument security factor FS of a measuring CT says how many times the rated primary current makes the transformer start to saturate and stop passing the full current to the instrument. IEC 61869-2 defines the standard values FS5 and FS10, the instrument limit primary current IPL and an indirect test with the primary open, which avoids injecting currents of tens of kiloamperes.

Short answer

A measuring current transformer must be accurate at the normal circuit current. During a short circuit the opposite is wanted: it should saturate and limit the current it delivers, so the meter is not damaged. The instrument security factor FS measures that characteristic. IEC 61869-2 defines it as the ratio between the instrument limit primary current and the rated primary current, and sets the standard values FS 5 and FS 10.

The definitions in the standard

The standard uses three related terms:

Instrument limit primary current (IPL). The smallest primary current at which the composite error of the measuring CT is equal to or greater than 10 %, with the secondary burden equal to the rated burden (3.4.204).
Instrument security factor (FS). The ratio between IPL and the rated primary current (3.4.205).
Instrument limit secondary e.m.f. (EFS). The product of the FS factor, the rated secondary current and the vector sum of the rated burden and the secondary winding impedance (3.4.206).

For example, a 600/5 A CT with FS 10 reaches a composite error of 10 % when the primary current reaches 6,000 A (10 times 600 A) at most, with the rated burden connected. Beyond that it no longer reproduces the full current on the secondary.

The composite error is the r.m.s. value of the difference between the primary current and the secondary current multiplied by the rated ratio (3.4.203). It includes the distortion that appears when the core saturates.

Standard values and the rating plate

The standard allows the security factor to be specified on a measuring CT and gives FS 5 and FS 10 as standard values (5.6.201.6). The factor is written after the class, for example «15 VA cl. 0.5 FS 10» (6.13.202.2). If the CT also has an extended current rating, it goes between the class and the factor, as in «15 VA cl. 0.5 ext. 150 % FS 10».

Two notes in the standard explain how to read the value:

The safety of the apparatus supplied by the CT is greater when the rated FS is the lowest (note 2 of 3.4.205).
The real FS depends on the burden. If the burden is well below the rated one, the secondary delivers larger currents during a short circuit (note 1 of 3.4.205).

The practical conclusion is twofold. To protect a delicate meter, ask for a low FS. And the connected burden must be close to the rated one: a 15 VA CT with a 2 VA burden does not behave like a real FS 10.

How it is tested

The standard describes two ways to determine FS (7.2.6.202).

Indirect test. With the primary winding open, the secondary is supplied at rated frequency with a nearly sinusoidal voltage. The voltage is raised until the exciting current reaches 10 % of FS × Isr, where Isr is the rated secondary current. The resulting r.m.s. voltage at the terminals must be lower than the instrument limit secondary e.m.f. EFS.

The voltage is measured with an instrument whose response is proportional to the mean of the rectified signal but calibrated in r.m.s. The exciting current is measured with an r.m.s. instrument with a minimum crest factor of 3.

The standard notes that the EFS method gives a higher value than the real one. It was chosen to apply the same test method that is used for protection CTs.

Direct test. If the result of the indirect test is in doubt, a direct measurement is made with primary current (Annexes 2A.5 and 2A.6, which describe the circuits for measuring composite error). In that case the direct result governs.

The advantage of the indirect method is that no huge primary currents are needed. The standard gives as an example a 3,000 A primary CT with FS 10, which would require 30,000 A in a direct test and a burden able to carry 50 A. In the indirect test, the primary return conductor also has no influence. In service, that conductor can only increase the composite error, which favors the safety of the instrument.

As a sample test on each production series, the standard indicates that the type test is normally repeated with the indirect method (7.5.2).

What to check when receiving a measuring CT with FS

Read the output, class and FS on the plate. They must match the purchase specification.
Ask for the type test or sample test report that determines FS, with the method used.
Compare the rated burden with the real burden you will connect, cables and meter included.
If the CT will supply equipment with little capacity to withstand fault currents, confirm with the equipment manufacturer the FS it needs. That capacity is declared by the instrument manufacturer.

What not to do

Do not confuse FS with the accuracy limit factor ALF of a protection CT. One protects the instrument and the other keeps accuracy during the fault.
Do not pick a high FS «for margin» on a CT that supplies a meter: the margin needed is the opposite one.
Do not evaluate FS with a burden other than the rated one: the standard defines it with the rated burden.

The security factor is specified together with the accuracy class and complements the class S CTs. The 5P and 10P classes are used in protection, not in metering. The instruments being protected are described in energy meter; the CT in medium voltage. To check whether your CT saturates with a given burden, use our free CT burden and saturation tool.

When to call us

At Tiza Group we review the plates, reports and real burden of your current transformers, and we measure their accuracy and excitation on site. See our instrument transformer testing, in Colombia or elsewhere on request. If you are going to specify a metering system, our engineering design and studies service, remote and worldwide, defines the classes and security factors with you. Write to us for a quote.

References

IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, edition 1.0 (2012), clauses 3.4.203 to 3.4.206, 5.6.201.6, 6.13.202.2, 7.2.6.202, 7.5.2 and Annexes 2A.5 and 2A.6.

Frequently asked questions

Is FS5 or FS10 better?

To protect the connected instrument, the lower FS. IEC 61869-2 notes that the safety of the apparatus supplied by the CT is greater when the rated security factor is the lower one. With FS5 the core saturates sooner during a fault, and the current reaching the meter is smaller than with FS10.

Is the FS on the plate met with any burden?

No. FS is defined with the rated burden. If the real burden is much lower than the rated one, the CT takes longer to saturate and the instrument receives larger secondary currents during a fault. The standard states this expressly.

Can a measuring CT with FS 10 feed a protection relay?

That is not its purpose. FS describes when the CT saturates in order to protect an instrument, not the accuracy at fault currents. To feed a protection relay there are classes P, PR, PX and TP, each with its own limits.