Medium Voltage Application Guide

Fault current (kA)
SWITCHGEAR
Maximum permissible temperature rise
Material and dielectric medium
Maximum permissible
temperature (°C)
Temperature rise above
40 °C ambient (°C)
Contacts, in air
Bare copper, bare copper alloy or bare aluminium alloy 75 35
Silver or nickel coated 105 65
Tin-coated 90 50
Bolted connection (or equivalent), in air
Bare copper, bare copper alloy or bare aluminium alloy 90 50
Silver or nickel coated 115 75
Tin-coated 105 65
Source: derived from IEC 62271-1
Peak withstand current, Ip (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. Peak withstand
current is defined as 2.5 times the rated short-time withstand current for 50 Hz installations and 2.6 times the rated
short-time withstand current for 60 Hz installations.
The switchgear peak withstand current rating must be higher than the calculated peak dynamic current (I dyn )
expected if a short circuit fault occurred at the point of installation.
NOTE
Switchgear peak withstand current rating is commonly referred to as rated short circuit making capacity.
Peak withstand current
1
2
AC component
DC component
I p = 2.5 x Ik
(f = 50 Hz)
2 x I k
13936.A
Short-time withstand current, Ik (kA)
Time (ms)
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, tk (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:
45 ms
50 Hz in European systems
60 Hz in American systems
710-12280-00A Medium Voltage Application Guide Page 169