Medium Voltage Application Guide

SWITCHGEAR
IEC Ratings
Disconnectors and earth switches are type tested to specific IEC standards. IEC 62271-1 provides standard ratings
and IEC 62271-102 details test methods and specific requirements for medium voltage disconnectors and earth
switches.
The nameplate label must show the manufacturer's name, equipment model and serial number, and certain rating
information. Many manufacturers also provide additional rating information.
Rated voltage, Ur (kV)
Maximum operating voltage (rms) the device can continuously withstand during normal operation. The rated
voltage must be greater than or equal to the system's operating voltage.
Standard values for U r : 3.6, 7.2, 12, 17.5, 24, 36 kV (source: IEC 62271-1)
MV switches tend to have a maximum U r of 36 kV.
Rated lightning impulse withstand rating, Up (kV)
This is the peak voltage the device can withstand for a 1.2/50 µs standard test wave.
Standard values for U p (source: IEC 62271-1):
U r (kV) 3.6 7.2 12 17.5 24 36
U p (kV) 40 60 75 95 125 170
Rated frequency, fr (Hz)
This rating must match the system's operating frequency. Rated frequency only has to be marked on the device
nameplate if it is not suitable for 50 Hz and 60 Hz operation.
Rated current, Ir (A)
This is the rms level of current which can continuously flow through a device without exceeding its maximum
allowable contact temperature rise.
Temperature rise limits are defined in IEC 62271-1, for an ambient temperature of 40 °C.
The rated current must be greater than the maximum expected load current, at the point of installation.
Standard values for I r : 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000 A (source: IEC 62271-1)
MV switches tend to have a maximum I r of 2500 A.
Rated short-time withstand current, Ik (kA)
This is the maximum rms symmetrical fault current the device can withstand, for a short time period, without risk of
damage. This rating must be higher than the prospective rms fault current at the point of installation.
I k
Where:
I k = short-time withstand current rating (kA)
I s = prospective rms fault current (kA)
S SC = system short circuit power (kVA)
U = system operating voltage (kV)
Standard values for I k : 6.3, 8, 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63 kA (source: IEC 62271-1)
MV switches tend to have a maximum I k of 31.5 A.
Rated short circuit duration, tk (s)
I
s
=
I s
S SC
3× U
This is the time the device can endure its rated short-time withstand current (I k ) without damage. This value must
be greater than the total expected clearing time of a fault at the point of installation.
Standard values for t k : 0.5, 1, 2, 3 seconds (source: IEC 62271-1)
MV switches tend to have a t k rating of 1 second.
Page 102 Medium Voltage Application Guide 710-12280-00A