Ik” and Ip iec60909

[bspace123]

Hi

With fault analysis using iec60909 Ik” is used to determine the thermal withstand of switchboards, cables etc.

But Ik” does not consider the dc offset present for asymmetrical faults. So if Ik” is calculated as say 20kA, but the fault is asymmetrical with peak of 60kA, does this not imply that the rms whilst the dc offset is present will be higher than 20kA?

Shouldn’t the switchboard thermal rating take this into account? I’ve read a few places that the switchboard rating should just be based on Ik”.

So a manufacturer will spec the switchboard rating at Ik” for xx seconds (typically 1 or 3s). Is the increased RMS value whilst the dc offset is present considered negligible?

 

[ppedUK]

For switchboards in IEC land, an RMS and Peak value is assigned to a fault rated switchboard e.g. a switchboard built to EN 61439 and having a short time withstand rating of 50kA would have a peak (Ip) withstand rating of 105kA.