Ith is the highest primary current a current transformer withstands for a short time with the secondary short-circuited. Idyn is the highest peak value it withstands without electrical or mechanical damage. IEC 61869-2 standardizes the duration of Ith at 1 s and sets Idyn at 2.5 times Ith, unless another value is specified.
Short answer
A current transformer (CT) is in series with the line. When a short circuit occurs, the fault current flows through its primary. IEC 61869-2 defines two capabilities for that condition: the short-time thermal current (Ith), which measures the heating effect, and the dynamic current (Idyn), which measures the effect of the electromagnetic forces. Both are rated for the unit and proven with a type test. For more detail on when a similar transformer avoids the short-circuit test, read similar transformer and when no short-circuit test is needed.
What each current is
The standard defines them in clause 3.
- Rated short-time thermal current (Ith). The maximum value of the primary current that the transformer withstands for a specified short time without harmful effects, with the secondary winding short-circuited.
- Rated dynamic current (Idyn). The maximum peak value of the primary current that the transformer withstands without electrical or mechanical damage from the resulting electromagnetic forces, with the secondary winding short-circuited.
- Rated continuous thermal current (Icth). The current that can flow permanently through the primary. It is covered here because of its relationship with the two above.
Ith measures an energy effect: heating of the conductor during the fault. Idyn measures a force effect: the mechanical stress produced by the electromagnetic forces.
Standard values
| Quantity | Standard value | Clause |
|---|---|---|
| Duration of Ith | 1 s | 5.204.1 |
| Idyn | 2.5 times Ith | 5.204.2 |
| Continuous thermal current | The rated primary current | 5.203 |
| Higher continuous thermal current, preferred values | 120 %, 150 % or 200 % of the rated primary current | 5.203 |
The standard requires a value of Ith to be assigned to every transformer (5.204.1). If the fault current of your installation requires it, you can specify an Idyn different from 2.5 times Ith. In that case the rating plate shows it.
What the rating plate must show
Clause 6.13.202.1 requires the plate to show Ith, for example 40 kA. The rated dynamic current is shown only if it differs from 2.5 times Ith, for example Idyn = 85 kA. The continuous thermal current is shown if it differs from the rated primary current, with the examples 150 % for a core with secondary taps, 450 A for several cores of different ratios and 4 x 450 A for a reconnectable primary. These examples come from the standard.
How it is proven: the short-time test
Clause 7.2.201 specifies two type tests, both with the secondary winding short-circuited.
Thermal test. A current I’ is passed for a time t’, so that the product of I’ squared times t’ is not less than that of Ith squared times the specified duration t. The time t’ must be between 0.5 s and 5 s.
Dynamic test. A primary current is applied whose peak value is not less than Idyn, in at least one peak.
The two tests can be combined if the first major peak of the current in the thermal test is not less than Idyn.
Acceptance criteria
The CT passes if, after cooling to ambient temperature (between 10 °C and 40 °C), it meets four conditions.
- It has no visible damage.
- Its errors, after demagnetization, do not differ from those recorded before the test by more than half of the error limits of its accuracy class.
- It withstands the routine dielectric tests (7.3.1 to 7.3.4), with the voltages or currents reduced to 90 % of those stated.
- On examination, the insulation next to the conductor surface shows no significant deterioration, for example carbonization.
The examination in condition 4 is not required when the current density in the primary winding, corresponding to Ith, does not exceed 180 A/mm² in copper or 120 A/mm² in aluminium. The standard adds that experience shows that, in service, the thermal capability requirements are generally met with class A insulation if the current density does not exceed those values.
Temperature rise in continuous service
Clause 6.4.1 separately requires a temperature rise limit. The CT carries a primary current equal to the rated continuous thermal current. Its burden has unity power factor and corresponds to the rated output. The temperature rise must not exceed the values in Table 5 of IEC 61869-1. This is different from the short-circuit test: here the current is permanent and the burden is the normal one.
What the test report must show
- The rated Ith value and the duration to which it refers.
- The current I’ and the time t’ used in the thermal test.
- The peak value reached in the dynamic test, compared with Idyn.
- The errors before the test and after demagnetizing the CT.
- The results of the dielectric tests at 90 % of the routine values.
- The result of the insulation examination, or the current density that justifies not doing it.
How to choose Ith and Idyn in a project
- Obtain from the short-circuit study the maximum current at the installation point, with its X/R ratio.
- Choose Ith equal to or greater than the maximum symmetrical fault current, referred to the standard duration of 1 s.
- Check that the peak value of the fault does not exceed Idyn.
- Write both values in the specification, together with the continuous thermal current if the CT will operate under overload.
The guide on short-circuit peak factor and X/R ratio explains why the peak exceeds the symmetrical current. The short-circuit and per-unit calculator calculates the fault current of a transformer or a radial network.
What not to do
- Do not buy a CT with Ith equal to the rated line current. Ith responds to the fault, not to the load.
- Do not assume Idyn is 2.5 times Ith if the plate says another value. Read the plate.
- Do not test in the field with fault currents. Field tests are ratio, polarity, excitation and insulation; the short-circuit test is a laboratory test.
- Do not ignore the continuous thermal current if the CT will operate above its rated primary current.
Related topics
Ith and Idyn are set together with the primary current and the ratios, which are explained in multiple ratios. The terminal marking identifies the terminals. The short-circuit value comes from the short-circuit study and the peak factor. For high voltage, see high-voltage CT.
When to call us
At Tiza Group we calculate the short-circuit currents at the connection point and help you define Ith, Idyn and the rest of the specification of your current transformers. Learn about our engineering design and studies service, which we provide worldwide, remotely. If the CT is already installed, we carry out its commissioning in Colombia, or elsewhere on request. Write to us and we will send you a quote.
Related guides
- Terminal marking and rating plate of a current transformer
- Primary current and multiple ratios of a CT
- Current transformer (CT): parts, functions and tests
- Short-circuit study: contributions, IEC 60909 and ANSI/IEEE
- Short-circuit peak factor and X/R ratio
- Similar Transformer: When No Short-Circuit Test Is Needed
References
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 3.3.203 to 3.3.205, 5.203, 5.204, 6.4.1, 6.13.202.1 and 7.2.201.
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition, Table 5.
Frequently asked questions
What is the relationship between Ith and Idyn?
The standard value of the rated dynamic current, Idyn, is 2.5 times the rated short-time thermal current, Ith (IEC 61869-2, clause 5.204.2). The rating plate shows Idyn only if it differs from that value, for example Ith = 40 kA and Idyn = 85 kA.
For how long is Ith rated?
The standard value of the duration is 1 s (clause 5.204.1). The thermal test may be run with another duration between 0.5 s and 5 s, provided the product of the current squared and the time is at least that of Ith squared times 1 s.
What must the CT meet after the short-circuit test?
On cooling to ambient temperature, it must show no visible damage, its errors after demagnetization must not differ from those before by more than half of the limits of its class, it must withstand the dielectric tests at 90 % of the routine values and it must show no significant deterioration of the insulation next to the conductor (clause 7.2.201).