IEC 61869-2 standardizes the primary currents of a CT (10 to 75 A and their decimal multiples or fractions) and the secondary currents (1 A or 5 A). In a multi-ratio CT, the standard distinguishes primary reconnection, whose requirements apply to all ratios, from secondary taps, whose requirements apply only to the highest ratio unless agreed otherwise.
Short answer
The primary current of a current transformer (CT) is not chosen freely: IEC 61869-2 gives a list of standard values. The same applies to the secondary, which is 1 A or 5 A. When a CT offers more than one ratio, there are two ways to achieve it and the standard treats them differently: reconnecting the primary or taking taps on the secondary.
Standard primary currents
Clause 5.201 sets the standard values of rated primary current:
10 - 12.5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 A, and their decimal multiples or fractions.
Multiplying by 10 or 100 gives, for example, 100, 125, 150, 200, 300, 400, 600 and 750 A, and the equivalent values in thousands of amperes. The standard also marks some of those values as preferred.
Standard secondary currents
Clause 5.202 sets two values: 1 A and 5 A. For protection CTs with transient performance (classes TPX, TPY and TPZ), the standard value is 1 A only.
The choice between 1 A and 5 A defines the burden the CT sees on the secondary, because the power consumed by a conductor is proportional to the square of the current. With 1 A, long cables weigh less on the burden. The standard does not recommend one or the other: the application calls for it. That burden is checked with the CT burden and saturation tool. For more on what primary injection validates that secondary injection does not: primary injection: what it validates and when.
Standard outputs and burdens
Clause 5.5.201 gives the rated output values for the metering, P and PR classes: 2.5 - 5 - 10 - 15 and 30 VA. Values above 30 VA can be chosen to suit the application. The standard allows other, non-standard outputs to be declared, provided they are associated with other standard classes.
For classes TPX, TPY and TPZ, clause 5.5.202 defines the rated resistive burden Rb. The standard values are 0.5 - 1 - 2 - 5 Ω, referred to a secondary current of 1 A. If the CT has another secondary current, those values are adjusted in inverse proportion to the square of the current. For example, 1 Ω with 1 A is equivalent to 0.04 Ω with 5 A.
Two ways to have several ratios
A multi-ratio CT lets one unit cover different load levels. The standard distinguishes the two solutions in clause 5.6.203.
| Solution | How the ratio changes | What the standard guarantees |
|---|---|---|
| Reconnectable primary | Primary sections are reconnected in series or in parallel | The accuracy requirements of all classes refer to all the specified reconnections (5.6.203.1) |
| Tapped secondary | A tap of the secondary winding is used | The requirements refer to the highest ratio, unless otherwise specified. If the purchaser requests it, the manufacturer reports the performance on the lower ratios (5.6.203.2) |
Table 208 of the standard shows how the terminals of each solution are marked. On the two-section primary, C1 and C2 appear in addition to P1 and P2. On the secondary with an intermediate tap, a third terminal, S3, appears.
What it means for protection classes
For classes TPX, TPY and TPZ, note 1 of Table 207 says that, in a CT with a tapped secondary, the accuracy requirements may be met for one ratio only. Note 2 says that in a CT with primary reconnection they may be met for different ratios. However, it warns that attention must be paid to the construction factor Fc, which can be affected by the configuration of the primary conductors.
Why a lower tap is more demanding
On a tapped secondary, each tap uses part of the turns of the winding. The voltage the secondary can deliver before saturating is proportional to the number of turns in use. So, on a lower tap, the CT has less margin: the same primary fault current produces a higher secondary current and the available voltage is lower. This is the usual technical reason why the standard guarantees the class on the highest ratio and leaves the lower ones to a separate report. With a reconnectable primary, the secondary keeps all its turns on every reconnection.
Continuous thermal current with multiple ratios
The rated continuous thermal current of a CT with several ratios is marked according to the solution chosen (6.13.202.1). The standard gives three examples.
- One core with secondary taps: 150 % of the primary current of each tap.
- Several cores of different ratios: the maximum continuous thermal current through all the cores, for example 450 A.
- Reconnectable primary, for example 4 x 300/1 A: 4 x 450 A, that is 450, 900 or 1800 A depending on the reconnection.
So, when reading the rating plate of a multi-ratio CT, check the continuous thermal current of each ratio and not only the rated primary.
How to specify a multi-ratio CT
- State how many ratios you need and which function each one serves.
- For protection functions, ask for the class and the accuracy limit factor to be guaranteed on the ratios you will use.
- If the solution is secondary taps, request the performance on the lower ratios, because the standard only guarantees the highest.
- Ask for the plate to show the continuous thermal current of each ratio.
- Check the secondary burden, cable included, with each ratio in service.
What not to do
- Do not use a lower ratio of a tapped CT for a protection function without the performance report for that ratio.
- Do not take the class on the plate as valid on all taps: it refers to the highest ratio, unless otherwise stated.
- Do not choose the 1 A or 5 A secondary current without checking the relay input and the cable to be connected.
- Do not choose the ratio only from the normal load current: the fault current also counts, and the CT must withstand it with its short-time thermal current (Ith) and its dynamic current (Idyn).
Related topics
The primary current is chosen together with the thermal and dynamic current and the terminal marking. The S classes, the safety factor and the 5P and 10P classes depend on the tap in use. The overview is in current transformer and medium voltage.
When to call us
At Tiza Group we define with you the ratio, class and burden of your current transformers, and we verify in the field the ratio of each tap before energizing. Learn about our engineering design and studies service, which we provide worldwide, remotely, and our testing and commissioning of current and voltage transformers in Colombia, or elsewhere on request. Write to us and we will send you a quote.
Related guides
- Ith and Idyn: thermal and dynamic current of a CT
- Terminal marking and rating plate of a current transformer
- Classes 0.2S and 0.5S and extended range of a CT
- Current transformer (CT): parts, functions and tests
- CT terminals, secondaries and wiring: P1, P2, S1, S2
- Primary injection: what it validates and when
References
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 5.201, 5.202, 5.5.201, 5.5.202, 5.6.203 and 6.13.202.1, with notes 1 and 2 of Table 207 and Table 208.
Frequently asked questions
Which primary currents does IEC 61869-2 standardize?
10, 12.5, 15, 20, 25, 30, 40, 50, 60 and 75 A, and their decimal multiples or fractions (clause 5.201). This gives, for example, 100, 150, 200, 300, 400, 600 and 750 A. The standard secondary currents are 1 A and 5 A (clause 5.202).
Does a CT with secondary taps keep its class on all ratios?
Not necessarily. On a tapped secondary, the accuracy requirements of all classes refer to the highest ratio, unless otherwise specified. If you request it, the manufacturer must report the performance on the lower ratios (clause 5.6.203.2).
And if the CT changes ratio by reconnecting the primary?
Then the accuracy requirements apply to all the specified reconnections (clause 5.6.203.1).