Rated Voltage of Switchgear: Series I and II (Ur)

Short answer

The rated voltage Ur of switchgear is the maximum phase-to-phase voltage the equipment is designed for, not the nominal voltage of the network. IEC 62271-1 lists it in two series for 245 kV and below, Series I (international use) and Series II (US practice), and in a range II for above 245 kV.

Short answer

The rated voltage (Ur) is the first characteristic by which a circuit breaker, disconnector or switchgear assembly is identified. IEC 62271-1, 2017 edition, defines it as the phase-to-phase r.m.s. value equal to the maximum system voltage the equipment is designed for (clause 5.2.1). It is not the nominal voltage of the network: it is the ceiling the equipment is built and tested for.

What Ur indicates

Clause 5.2.1 says Ur marks the upper limit of the «highest voltage of the system» of the networks in which the equipment can be used. The standard refers to the definition of highest voltage for equipment (Um) in clause 3.7.3.

For a specification this has a direct consequence. A network has a nominal voltage and a maximum operating voltage. The equipment must have a Ur equal to or higher than that maximum voltage. Comparing only with the nominal voltage can leave the equipment short.

The note to the clause clarifies a difference in wording: the term «rated maximum voltage» used in most IEEE standards means the same as «rated voltage» in the IEC 62271 series.

Range I: 245 kV and below

Clause 5.2.2 gives two series of values.

Series I, the international series:

3.6 – 7.2 – 12 – 17.5 – 24 – 36 – 40.5 – 52 – 72.5 – 100 – 123 – 145 – 170 – 245 kV.

Series II, based on current practice in some countries, for example the USA:

4.76 – 8.25 – 15 – 15.5 – 25.8 – 27 – 38 – 48.3 – 72.5 – 123 – 145 – 170 – 245 kV.

Some values are in both series: 72.5, 123, 145, 170 and 245 kV. Below 72.5 kV the series do not overlap, which is why the series decides which equipment fits a project.

Range II: above 245 kV

Clause 5.2.3 lists the extra high voltage values: 300 – 362 – 420 – 550 – 800 – 1,100 – 1,200 kV. Their insulation levels are in their own tables (Tables 3 and 4) and the dielectric tests have a separate clause for above 245 kV (7.2.8).

Why 15 kV and 15.5 kV are not the same

Note 1 of clause 5.2.2 explains where the two values come from:

15 kV is used in the USA and in other countries. It was historically associated with metal-clad and metal-enclosed switchgear for mainly indoor applications, or outdoor ones where the insulation level is lower than that required for outdoor overhead lines.
For applications other than metal-clad or metal-enclosed switchgear, the standard prefers 15.5 kV.

Although both values look close, in the standard they are different rated values, with their own rows in the insulation level table. If a client asks for «a 15 kV switchgear assembly» without more detail, the specification must say which series it refers to.

Why 27 kV replaces 25.8 kV

Note 2 records a change in practice. The 25.8 kV value is still used in IEEE C37.04 as the rated voltage of circuit breakers and in some countries. In most equipment standards, 27 kV replaced it. For new applications and designs, the standard prefers 27 kV.

The case of 36 kV and 40.5 kV

Series I includes 36 kV and 40.5 kV. A note to the insulation level table clarifies that IEC 60038:2009 recognizes 40.5 kV, adding that unifying it with 36 kV is under study. The current insulation values for 40.5 kV come from IEC 60071-1:2006, Annex B.

Ur among the other common characteristics

Clause 5.1 lists the common characteristics assigned by the manufacturer:

Rated voltage (Ur) and insulation level (Ud, Up and, where applicable, Us).
Rated frequency and rated normal current.
Short-time withstand current, peak withstand current and duration of short circuit.
Supply voltage and frequency of auxiliary and control circuits.
Supply pressure of compressed gas, in controlled pressure systems.

The standard explains that these characteristics define what is needed to select and use the equipment in a network. Other important data, such as the minimum functional pressure for insulation, appear on the nameplate but are not rated characteristics.

The rated insulation level and the rated current and temperature rise go with Ur as common characteristics. The short-time withstand current and peak withstand current complete the set of rated values of the main circuit. For instrument transformers, see the highest voltage for equipment.

The map of standards for primary equipment testing places this part among the others of IEC 62271.

How to check a nameplate

How to read the rated voltage of a device

Is the rated voltage of the equipment in Series I or Series II?
- Series I (for example 12, 24, 36 or 72.5 kV): Compare Ur with the maximum voltage of the network
- Series II (for example 15, 15.5, 27 or 38 kV): Confirm that the project accepts the series and US practice
- Above 245 kV: Range II: use its insulation table and test requirements

After locating the series, check that the rated insulation level and the rated current correspond to that Ur.

What the nameplate and the data sheet say

The rated voltage should appear in three places that agree with each other: the purchase specification, the manufacturer’s offer and the equipment nameplate. A difference between them, for example 15 kV ordered and 15.5 kV offered, is a difference of series and insulation level, not a rounding detail. Before accepting the offer, confirm the series with the manufacturer and compare the withstand voltages of the offered equipment with those your project requires.

What not to do

Do not confuse Ur with the nominal voltage of the network: the equipment is chosen by the maximum voltage.
Do not swap a 15 kV device for a 15.5 kV one, or the other way round, without checking their insulation levels.
Do not mix values from the two series in one specification without saying which governs.
Do not design a new installation with 25.8 kV if the standard already prefers 27 kV.

When to call us

At Tiza Group we select with you the rated voltage and the other switchgear characteristics for your project and verify that the offered equipment meets them. See our primary engineering design service. 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 5.1, 5.2.1, 5.2.2 and 5.2.3, and the note to Table 1.
IEC 60038:2009 and IEC 60071-1:2006 (cited by the standard).

Frequently asked questions

Is the rated voltage of a switchgear assembly the nominal voltage of the network?

No. Per IEC 62271-1 (clause 5.2.1), the rated voltage Ur is the phase-to-phase r.m.s. value equal to the maximum system voltage the equipment is designed for. It sets the upper limit of the «highest voltage of the system» of the networks where it can be used.

What is the difference between 15 kV and 15.5 kV equipment?

They are different Series II rated values. The standard explains that 15 kV is used in the USA and other countries and was historically associated with metal-clad and metal-enclosed switchgear. For applications other than those, 15.5 kV is preferred (note 1 of clause 5.2.2).

Why do some devices carry 27 kV instead of 25.8 kV?

The 25.8 kV value still appears in IEEE C37.04 and in some countries, but most equipment standards have replaced it with 27 kV. For new designs and applications, the standard prefers 27 kV (note 2 of clause 5.2.2).