MV application guide for engineers to select and specify the right equipment

IEC MV Switchgear Rating Definitions

Standard IEC 62271-1 defines standard ratings for medium voltage switchgear. Full name of this standard is ‘High-voltage switchgear and controlgear – Part 1: Common specifications for alternating current switchgear and controlgear’. Ratings defined in this standard allow selection of equipment to match the electrical characteristics at the point of installation.

Voltage

Operating voltage, U (kV)

This is the system’s operating voltage at the point where the switchgear is installed. The operating voltage must always be less than or equal to the rated voltage of the switchgear equipment.

Rated voltage, U_r (kV)

This is the maximum rms voltage the switchgear equipment can continuously operate at, under normal conditions. The rated voltage is always higher than the systems operating voltage and determines the insulation levels of the equipment.

Medium voltage, metal-enclosed switchgear is defined for use on operating voltages from1 kV to 52 kV. Within this voltage range, IEC 62271-1 defines standard switchgear rated voltages as:

• Series I equipment (used in European 50 Hz installations): 3.6, 7.2, 12, 17.5, 24, 36, 52 kV
• Series II equipment (used in Non-European 60 Hz installations): 4.76, 8.25, 15, 15.5, 25.8, 27, 38, 48.3 kV

Insulation level voltages, U_d (kV rms 1 min) and U_p (kV peak)

The defined levels are stated for phase-to-earth and phase-to-phase limits under standardized ambient conditions. For installations above 1000 metres, these insulation levels must be derated.

Power frequency withstand voltage, U_d

This is the maximum rms voltage that the equipment can withstand at mains frequency for 1 minute. It simulates power surges originated from within a power system from such events as switching transients, resonance, etc.

Lightning impulse withstand voltage, Up

This is the peak transient voltage that the equipment can withstand from power surges originating from atmospheric conditions such as lightning. It is simulated using a standard voltage waveform.

Table 1 – Standard values for insulation level voltages

Current

Operating current, I (A)

This is the maximum rms current expected to flow through the equipment. The operating current must always be less than or equal to the rated current of the equipment.

Rated current, I_r (A)

This is the maximum rms current the equipment can continuously operate at, under normal conditions. This rating is based on an ambient operating temperature of 40°C, within an allowable maximum temperature rise. For temperatures above 40°C, switchgear rated current must be derated.

IEC 62271-1 specifies standard ratings as base 10 multiples of 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8.

Table 2 – Maximum permissible temperature rise

Peak withstand current, I_p (kA)

This is the peak current the equipment can withstand in the closed position from the first loop of a short circuit fault. This current contains a symmetrical AC component, superimposed on a decaying DC component.

NOTE – Switchgear peak withstand current rating is commonly referred to as rated short circuit making capacity.

Short-time withstand current, I_k (kA)

This is the level of symmetrical rms fault current the switchgear can carry in the closed position for a short time period (typically 1 second), without temperature rise exceeding predefined levels. IEC 62271-1 specifies standard ratings as base 10 multiples of 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8

Short-time withstand duration, t_k (seconds)

This is the period of time that the equipment is rated to carry the short-time withstand current. IEC 62271-1 specifies a standard rating of 1 second, although durations of 0.5, 2 and 3 seconds are allowed.

Frequency, fr (Hz)

This is the rated test frequency of the switchgear and must match the operating frequency of the installation. Two medium voltage mains supply frequencies are used globally:

• 50 Hz in European systems
• 60 Hz in American systems

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