Rated Current and Temperature Rise of Switchgear

Short answer

The rated continuous current (Ir) is the r.m.s. value that switchgear carries without interruption in normal service. IEC 62271-1 takes it from the R10 series and verifies it with a temperature-rise test until the temperature stabilizes, with the main circuit resistance measured before the test as the reference for routine tests.

Short answer

The rated continuous current (Ir) is the r.m.s. value of current that switchgear can carry continuously under its service conditions (clause 5.5 of IEC 62271-1, 2017 edition). The type test that verifies it is called the continuous current or temperature-rise test: Ir is passed through the equipment until the temperature stabilizes, and it is checked that no part exceeds its temperature-rise limit.

A note to clause 5.5 clarifies the wording: the term «rated continuous current» in this edition is equivalent to «rated normal current» in the previous edition.

Ir values: the R10 series

The standard recommends taking Ir from the R10 series of IEC 60059: 1 – 1.25 – 1.6 – 2 – 2.5 – 3.15 – 4 – 5 – 6.3 – 8, and those numbers multiplied by 10 to the power n. This gives the usual nameplate values: 630 A, 1,250 A, 2,500 A, 4,000 A and similar.

What the temperature-rise test looks like

Clause 7.5 describes the conditions, which aim to make the result reproducible:

Test object. Clean contacts, filled with the appropriate gas or liquid at the minimum functional pressure (or density) for insulation (7.5.1).
Environment. Indoors and free of drafts, apart from those produced by the equipment’s own heat. In practice, it is considered met if the air speed does not exceed 0.5 m/s (7.5.2).
Mounting. As in service, with all normal covers, and in the least favorable position if the manufacturer allows several.
Phases. In principle, three-phase. It can be done on a single pole if the influence of the others is negligible, which is the general case for non-enclosed switchgear above 52 kV. For equipment up to 1,250 A the poles can be connected in series.
Current. Ir, practically sinusoidal: the r.m.s. value of the harmonics does not exceed 5% of the fundamental (7.5.3.1).
Duration. The time needed for the temperature rise to reach a stable value: change of up to 1 K in 1 h.

If the equipment is rated for 50 Hz and 60 Hz and has no ferromagnetic components next to the current-carrying parts, a 50 Hz test can cover both frequencies when the measured temperature rises do not exceed 95% of the maximum permitted. A 60 Hz test covers both with no additional condition.

Ambient temperature and measuring points

Ambient temperature is recorded with at least three thermometers or thermocouples spread around the equipment, about 1 m away and at the mean height of the current-carrying parts (7.5.4.1). It must be above 10 °C and equal to or below 40 °C, unless the manufacturer agrees otherwise. Within that range the temperature-rise values are not corrected.

The temperature of the parts with a limit is measured with thermometers, thermocouples or other sensitive sensors at the hottest accessible point. For coils the change-of-resistance method is normally used. The main circuit terminals and the temporary connections 1 m from them are also measured. The temperature-rise difference between the two must not exceed 5 K (7.5.4.2).

Main circuit resistance

Before the continuous current test, the main circuit resistance is measured with the equipment at ambient temperature (7.5.5). That value is used to compare the type-tested unit with the other units of the same type tested in routine.

In the routine test (8.4), the DC voltage drop or the resistance of each pole is measured under conditions as close as possible to those of the reference measurement. The measured resistance must not exceed 1.2 × Ru, where Ru is the value measured before the continuous current test. It is the same idea as a contact resistance test done in the field.

The rated current coexists with the short-time withstand current and peak withstand current: one is the continuous stress and the other the short-circuit stress. The normal service conditions for switchgear set the reference ambient temperature of the temperature-rise test, which is part of the type tests of switchgear.

To see how this fits in the rest of the IEC 62271 series, see the standards for primary equipment testing.

Acceptance criterion

Clause 7.5.6.1 considers the test passed if the temperature rise of the parts with a limit does not exceed the values of Table 14 of the standard. That table groups the limits by type of part, material and dielectric. Take the figures from Table 14 of the edition that applies to your equipment, not from memory.

Some rules of the standard that go with the table (clause 7.5.6.2):

If a part belongs to several categories because of its function, the lowest value governs.
For contacts with different coatings, the limits of the material with the lowest permitted value apply.
Non-oxidizing gases (SF6, N2, CO2 and CF4, pure or in a mixture) are treated differently from oxidizing ones (ambient air, dry air and any mixture that includes an oxidizing gas).
The values are valid even if the conductor connected to the terminal is bare.
The limits do not consider the influence of the cable or its termination on the insulation.

When the temperature-rise test is considered valid

Is the change in temperature rise 1 K or less in 1 h?
- No: Continue the test: the temperature has not yet stabilized
- Yes: Does any temperature rise exceed the limit for its part in Table 14?
  - Yes: Test not passed for that Ir
  - No: Test passed; the resistance measured earlier stays as the routine reference

What not to do

Do not specify an Ir outside the R10 series without confirming with the manufacturer that it exists: the standard recommends taking it from that series.
Do not overlook the test setup: the standard requires testing as in service, with normal covers and in the least favorable position.
Do not treat «normal current» and «continuous current» as different values: they are the same concept in two editions.
Do not ignore an increase in main circuit resistance: more than 1.2 times the reference value does not meet the routine requirement.

When to call us

At Tiza Group we measure main circuit resistance and carry out thermography of switchgear at commissioning and during maintenance. See our testing and commissioning service, in Colombia or elsewhere on request. Write to us and we will quote it.

References

IEC 62271-1, High-voltage switchgear and controlgear - Part 1: Common specifications for alternating current switchgear and controlgear, 2017 edition: clauses 5.5, 7.5.1 to 7.5.6, 8.4 and Table 14.
IEC 60059, R10 series of standard current ratings (cited by the standard).

Frequently asked questions

Which values can the rated current of a circuit breaker have?

IEC 62271-1, 2017 edition, recommends taking it from the R10 series: 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3 and 8, multiplied by 10 to the power n (clause 5.5). This gives values such as 630, 1,250, 2,500 and 4,000 A.

When is the temperature considered stable in the temperature-rise test?

When the change in temperature rise does not exceed 1 K in 1 hour. Per clause 7.5.3.1, that condition is usually reached after about five times the thermal time constant of the test object.

How much can the main circuit resistance increase in the routine test?

The resistance measured in the routine test must not exceed 1.2 times Ru, where Ru is the resistance measured before the continuous current test (clause 8.4 of IEC 62271-1).